Unit 1: Algorithms

Unit 1, is subdivided into these parts:

1. Introduction to Algorithms
2. Binary Search
3. Asymptotic Notation

Week 0: Functions and Variables

Functions

A small program that allows you to perform some action in the desired Computer Programming Language.

print("Hello, World!")

When run, the following text will be printed on the screen:

Hello, World!

The text printed on the screen is termed as a Side Effect. It’s the side effect of running the function with the pre-determined input.

The built-in function print allows you to output some text to the screen in Python.

• You can change the underlying implementation of function as long as name and parameters remains same, nobody running the program will notice.

Bugs

An error/mistake in the code which stops the program from running normally.

• You make mistake while writing the code.
• You may forgot to close parenthesis, or forgot to close a quotation marks, or use some illegal (in-terms of programming of-course) input.

Comments

A note to yourself about the function of your code, so you don’t forget in the future, what’s really happening with your code.

# This code prints "Hello, World!" on the screen
print("Hello, World!")

If it’s a complex function, a comment will immediately remind you what’s this piece of code is actually doing inside the program.

Pseudocode

A plain statement in any human language which methodically and simply outlines what you want to build in the desired programming language. A comment starts with # symbol which tells the interpreter to ignore what comes after it.

# A program that asks for user name as an input
# It takes that input, and prints Hello to that user

Above lines are simply a pseudocode which explains the outcome we need from the actual code.

• No coding involved on this stage.
• Only plan english is used to list down the desired outcomes.
• It can act as a TODO list inside your code.
• It helps break down complex programs into simple workable chunks.

Parameters

Following are the types of parameters.

Positional Parameters

A parameter passed to the function will be printed at a position it’s on i.e., first, seconds etc.

print("Hello,", name)

First positional parameter is “Hello,”, will be printed first. The name variable is a second positional parameter, printed second.

Named Parameters

A named parameter is a built-in optional argument of the given function which is usually provided after the positional parameters.

print("Hello,", name, sep='?' )
#OR
print("Hello,", end="")
print(name)

Both sep=' ' and end="\n" (with defaults) are the Named parameters provided by the print function.

Strings str

An immutable sequences of Unicode characters used to store and manipulate textual data.

• In Python, anything inside '' or "" is called a string.
• The function input() only accepts string arguments.

Python f-string

A special of type string that provides a concise and efficient way to embed expressions inside string literals.

print(f'Hello, {name}')

Strings Methods

Python string methods are built-in functions attached to string objects that allow for text manipulation, such as changing case, searching, replacing, splitting, and formatting.

Because strings in Python are immutable, these methods don’t modify the original string; instead, they return a new string with the requested changes applied.

str.strip() method

Remove whitespace from around the strings:

str.strip()

str.capitalize() method

Change the first letter of the string to upper case.

str.capitalize()

str.title() method

Capitalize the first letter of all the words in the given string.

str.title()

str.join() method

The join() method takes all items in an iterable and joins them into one string. – W3School

Syntax = `string.join(iterable)

myDict = {"name": "John", "country": "Norway"}
mySeparator = "TEST"

x = mySeparator.join(myDict)
# OR separator string can be mentioned directly
x = ','.join(myDict)
print(x)

String Slicing

You can return a range of characters by using the slice syntax

Specify the start index and the end index, separated by a colon, to return a part of the string.

Example:

greet = "Hello, World!"

Slice From the Start

Get the characters from the start to position 5:

print(greet[0:5])
#OR
print(greet[:5])

When starting from index 0, we don’t need to mention it. The character at index [5] will not be included.

Slice From the End

By leaving out the end index, the range will go to the end:

print(greet[2:])

If we want to slice the last 6 characters:

print(greet[-6:])

Python Interactive Mode

The live Python interpreter, where you can write code and get the execution immediately, unlike you write your code in a file and then run that file with Python interpreter.

It removes the friction for some quick Python practice.

Integers int()

Integers are whole numbers without a decimal point i.e., -3, -2, -1, 0, 1, 2, 3 etc.

• In programming, ideal for counting or indexing

Floats float()

Floats (floating-point numbers) represent real numbers with decimal components.

• Ideal for a wider range of values and fractional precision, making them suitable for measurements and continuous data.

Function def()

A small reusable piece of program which can be defined once and used multiple times inside the code to reduce repeatability and increase the readability.

# All the code resides in the main
def main():
    return

# Define as many functions as you want
def func():
   return

# Call main() to start the execution
main()

Running without calling on the main() function, no execution will happen, as our main code resides in the main function and nobody calling it, when we call on the main() function, then the execution starts, and the rest of the user functions will also be executed one by one as they appear in the main().

Variable Scope

The variable only exists in the scope where you defined it.

• Variable defined inside the function, cannot be called on globally.
• Variable defined in the 2nd user function cannot be called in the first one.

Week 1: Conditionals

Conditionals

Ability to ask questions and answer those questions, in order to decide which line of code will be executed.

• To write conditional statements in Python, we can use if statement.
• The elif statement, ask a question, taking into account whether or not a previous question had a true or false answer.
• The else statement, used when the all the other options has bee exhausted and only logical answer is the last one. So, we don’t need to compare anything for it, because it’s the only answer left. It’s catch all statement, if everything else isn’t proved right, let’s assume this is the answer.

Some symbols used in Python, are:

Not better design (See compare.py)

A better designed program:

The final design:

• In the first version, program goes through all the code despite finding the correction answer earlier.
• In the second iteration, program will immediately terminate as it gets its answer.

Boolean Expression

A question that gives Yes or No answer, or simply True or False answer.

OR Returns True if at least one operand is True; it only returns False if both operands are false.

AND Returns True if both operands are true; otherwise, it returns False. It’s uses short-circuit evaluation, meaning if the first operand is false, the second is not evaluated.

Python Operators

match…case Statement

Provides structural pattern matching as a more readable and powerful alternative to if-elif-else chains. It evaluates an expression against successive patterns and executes the corresponding code block for the first match.

match name:
    case 'Harry':
        print('Gryffindor')
    case 'Hermione':
        print('Gryffindor')
    case 'Ron':
        print('Gryffindor')
    case 'Draco':
        print('Slytherin')
    case _: # Handle edge cases
        print('Who?')

More legible or better version:

match name:
  case 'Harry' | 'Hermione' | 'Ron':
    print('Gryffindor')
  case 'Draco':
    print('Slytherin')
  case _:
    print('Who?')

Week2: Loops

Loops

Ability in Python and other programming languages, to do something again and again.

while Loops

• Runs for forever unless certain conditions are met.

An infinite loop (design flaw):

i = 3
while i != 0:
    print('meow')

• This code will run for forever unless interrupted by the user (Ctrl + C).
• i != 0 will always remain True, we change the value of i inside the loop on each run.

Well designed while loop:

i = 3
while i != 0:
    print('meow')
    i = i - 1

• On each run, i = i - 1 reduces the value of i by one digit. Until i != 0 condition is no longer true, and the loop stops.

for loop

In Python, a for loop is a control flow statement used to iterate over a sequence (such as a list, tuple, string etc.) or any other iterable object.

for i in [0, 2, 3]:
    print("meow")
#OR using range() function
for i in range(3):
    print("meow")

We are not using the i variable in the code, though it has a use case for holding the range value.

The more Pythonic approach is to use _, when you don’t care about the variable used or not use it later in the code.

for _ in range(3):
    print("meow")

List []

A list in Python, is a ordered, and mutable data structure used to store a collection of items in a single variable.

• Defined using square brackets []
• Each list element is separated by commas
• Lists are heterogeneous, meaning they can contain elements of different data types (such as integers, strings, booleans or else) within the same structure.
• Each element has an index starting from [0].

len() function

It returns number of items (length or size) in an object, such as strings, lists, tuples, dictionaries, sets, etc.

students = ["Hermione", "Harry", "Ron"]
for i in range(len(students)):
  print(i + 1, students[i])

List Methods

To add an item to a list:

list.append("name")

The list.append() can also add/append another list to the particular list.

To add multiple items at once to the list

list.extend(["Silly", "Donkey King"])

The same list.append(["Silly", "Donkey King"]) would have added them as a list to the existing list.

Remove an item’s first instance from a list: For a single item:

list.remove("Silly")

You can remove a nested list:

list.remove(["Silly", "Donkey King"])

Add an item at the particular place in the list

list.insert([index], "Marty")

Reverse the order of a list

list.reverse()

Remove the last item from a list

list.pop()

To clear the whole list

list.clear()

List Comprehension

List comprehension offers a shorter syntax when you want to create a new list based on the values of an existing list. — W3School

newlist = [expression for item in iterable if condition == True]

list = []
for i in range(20):
    if i%2 == 0:
        list.append(i)
print(list)

Using list comprehension, the whole for loop can be condensed to a single line:

list = []
even_list = [i for i in range(20) if i%2==0]
print(even_list)

Dictionaries dict

A dictionary (or dict) is a built-in mutable mapping type that stores data as a collection of unique, hashable key-value pairs.

• Since Python 3.7, dictionaries are ordered by insertion, meaning they preserve the sequence in which keys were added.

Key characteristics:

• Keys must be immutable, while values can be any data type.
• Dictionaries are created using curly braces {} with colon-separated key-value pairs.

Dictionary Methods

Python dictionary methods are built-in functions designed to manipulate, access, and manage the key-value pairs stored in Python dictionaries.

If you want to get the value of specific key and don’t know if that exists:

dict.get("name", "Unknown")

If “name” is not a valid key, it will return “Unknown” instead of throwing error if we have tried to get it via dict.["name"].

To update the dictionary.

dict["name"] = "David"

Another method to update a dictionary is via dict.update(). It takes another dictionary as an arguments adds to the current dictionary.

dic.update({"name": "Dawood", "name": "Amin"})

To return all keys in the dict:

dict.keys()

To return all values in the dict:

dict.values()

Delete a particular key from the dictionary:

dict.pop("key")

To clear the whole dictionary key-value pairs:

dict.clear()

Return the key-value pairs as tuples in the dictionary

dict.items()

It returns the both key and value pairs as tuples, which we can access as:

for key, value in dict.items()
    print(key, value)

Dictionary Comprehension

Dictionary comprehension is used to create a dictionary in a short and clear way. It allows keys and values to be generated from a loop in one line. This helps in building dictionaries directly without writing multiple statements. — GeeksforGeeks

dict_comp = {key: value for key, value in iterable if condition}

Normal dictionary creation with a starting empty dict.

dict = {}
for i in range(10):
    if i%2 == 0:
        dict[i] = i ** 2
print(dict)

The dict comprehension:

dict = {}
dict_comp = {i:i**2 for i in range(10) if i%2==0}
print(dict_comp)

None

None is special built-in constant that represents the absence of a value or a null value. It’s the sole instance of the NoneType class and acts as a singleton, meaning there is only one None object in any Python interpreter session.

• None is distinct from 0, False, or an empty string "", though it evaluates to False in boolean contexts.
• Functions that don’t explicitly return a value automatically return None.

Tuples

Tuples are used to store multiple items in a single variable.

A tuple is a collection which is ordered and unchangeable. —W3School

• Tuples are written within parenthesis ().
• A tuple can contain different data-types at once
• A one item tuple can be created with ("item1", ), otherwise Python will not recognize it.
• The () will create an empty tuple.
• The len() can be used to find out the number of items in the tuple.

Tuples Items

Tuple items are ordered, unchangeable, and allow duplicate values.

Tuples items are indexed, the first item has index[0], the second item has index[1] etc.

Week 3: Exceptions

Python Exceptions

Exceptions are the problems in the Python or any other programming language code, that the programmer has to solve.

SyntaxError

A SyntaxError in Python occurs when code violates the grammatical rules of the language, making it impossible for the interpreter to parse and understand the structure. —TutorialsPoint.com

Common causes include:

• Missing punctuation
• Incorrect indentation
• Misspelled keywords
• Invalid string delimiters, such as mixing single and double quotes improperly

Value Error

A ValueError in Python is a built-in exception raised when a function or operation receives an argument of the correct data type but an inappropriate or invalid value. —RealPython.com

Unlike a TypeError, which occurs when the wrong data type is provided, a ValueError indicates that while the input is the right kind of object, its specific content is unsuitable for the operating being performed.

Common scenarios that trigger a ValueError include:

• Invalid Type Conversion: Attempting to convert a string that doesn’t represent a valid number into an integer or float
• Mathematical Constraints: Performing an operation that requires a specific range of values, such as taking the square root of a negative number or calculating the factorial of a negative integer
• List Operations: Trying to remove a value from a list that doesn’t exist within it.
• Unpacking Mismatches: Attempting to unpack a list or iterable into a different number of variables than there are items

NameError

The NameError exception occurs if you use a variable that is not defined. —W3School

You can handle the NameError in a try..except statement.

try and except

The try block lets you test a block of code for error.

The except block lets you handle the error.

The else block lets you execute code when there is no error.

The finally block lets you execute code, regardless of the result of the try and except blocks.

The raise keyword is used to raise an exception.

Let’s see a code:

try:
    x = int(input("What's the value of x: "))
except ValueError:
    print("x is not an integer.")
else:
    print(f"The value of x is: {x}")

The else part will only be executed if there are no error.

If you don’t want to print anything in the except block write pass, it will not raise/print anything to the user.

MemoryError

In Python, a MemoryError is a built-in exception that occurs when the interpreter cannot allocate enough memory for an operation, such as creating a large list, loading a huge file, or executing a recursive function without a base case.

This error signals that the program has attempted to use more memory (RAM or virtual address space) that the system can provide.

Common causes include:

• Unbounded Data Growth: Accumulating data in lists or dictionaries within infinite or large loops.
• Large Object Allocation: Attempting to create data structures (like NumPy arrays or Pandas DataFrames) that exceed available system memory.
• Excessive Recursion: Recursive functions that consume excessive stack space before hitting the recursion limit.
• Memory Leaks: Although less common as a direct cause, failing to release references to large objects can lead to exhaustion.

KeyError

A KeyError is an exception raised when you try to access a value in a dictionary (or other mapping) using a key that doesn’t exist within that collection.

It is a subclass of LookupError and serves as Python’s signal that the requested key cannot be found among existing keys.

Raise an Exception

As a programmer, you can choose to throw an exception if a condition occurs.

To throw an exception, use the raise keyword.

x = -1
if x < 0:
    raise Exception("Sorry, no numbers below zero")

You can define what kind of error to raise, and the text to print to the user.

x = "hello"
if not type(x) is int:
    raise TypeError("Only integers are allowed")

Debugging

Debugging is the process of identifying, analyzing, and resolving errors or bugs in code to ensure it runs correctly and produces expected results.

It involves detecting syntax errors (structural issues like typos) and semantic errors (logical issues where code runs but yields incorrect outputs).

The primary goal is to locate the root cause of unexpected behavior by examining the program’s state during execution. This is achieved through various techniques, including:

• Using Tracebacks: Analyzing error messages to pinpoint where exceptions occur.
• Print Statements: Inserting print() calls to track variable values and code flow.
• BreakPoints: Pausing execution at specific lines to inspect variables and state.
• Debuggers: Using tools like built-in pdb module or IDE debuggers (e.g., in Zed or VS Code) to step through code line-by-line and interactively inspect the execution environment.

Week 4: Libraries

Libraries

In programming, libraries are like ready-made tools that save time, reduce effort, and make development much easier.

A Python library is simply a collection of pre-written code that developers can use to perform specific tasks without having to write everything from scratch. —Antara Das via Medium

Basically, you can import the library into your code, and it comes with predefined functions and methods that help you get the gob done.

Module

A module in Python is just a library that typically or more functions or other features built into it.

Purpose of a Module or Library is a re-usability of a code when you find yourself using same lines of code over and over again from across your project’s code.

import

The keyword import allows to import some functions from modules in Python.

To import random library with all its functions:

import random
# Coin flip
coin = random.choice(["Heads", "Tails"])
print(coin)

from

The from keyword is primarily used to import specific functions, classes, or variables from a module into the current namespace, allowing direct access without prefixing the module name.

To import choice() function from random module:

from random import choice
# Coin flip
coin = choice(["Heads", "Tails"])
print(coin)

Command-Line Arguments

Command-Line arguments are parameters passed to a Python script upon execution from the terminal, allowing users to customize program behavior without modifying source code.

The one we use is using sys module feature sys.argv, where sys.argv[0] is the script name and subsequent indices are the provided arguments.

import sys
print("Hello, my name is", sys.argv[1])

Running with name as an argument:

python name.py Alex

If we replace with sys.argv[0], it will print the name of program as Hello, my name is name.py, and no CLI argument will be printed.

random Module

It’s a built-in Python module, that provides functions for generating pseudo-random numbers, selecting random elements, and shuffling sequences.

It’s not a separate package and doesn’t require installation; it’s used by simply adding import random to your code.

statistics Module

Python has a built-in module that you can use to calculate mathematical statistics of numeric data.

Statistics Methods (REF: W3School)

requests Module

The requests module allows you to send HTTP requests using Python. —W3School

The HTTP request returns a Response Object with all the response data (content, encoding, status, etc.).

Syntax:

requests.methodname(params)

Methods

Python json

Python has a built-in package called json, which can be used to work with JSON data. —W3School

There are many different methods json has, but for now we only see the json.dumps() method.

Format the Result

The json.dumps() prints a JSON string, but it is not very easy to read, with no indentations and line breaks.

The method has parameters to make it easier to read the results.

json.dumps(x, indent=4)

You can also define the separators, default value is `(", “, “: “), which means using a comma and a space to separate each object, and a colon and a space to separate keys from values:

json.dumps(x, indent=4, separators=(". ", "= "))

Order the Result

Use sort_keys parameter to specify if the result should be sorted or not:

json.dumps(x, ident=4, sort_keys=True)

PACKAGE

A package is a third-party library that we can install on our computer.

An external package maintainer is PyPI searchable via https://pypi.org or CLI.

You can install packages from PyPI via Python package manager called PIP.

Create a virtual environment inside your project:

python -m venv .myenv

Activate your virtual env:

source .myenv/bin/activate

Now install the module:

pip install cowsay

Now you can import it inside .py file.

After finishing your work, you can deactivate the env:

deactivate

Depending on your prompt, it will show you’re using a virtual environment with Python version number.

Create Your Own Package

Create a folder, put all your modules inside it. Create an empty file inside that folder:

touch __init__.py

Now you can import your package:

from foldername.module import function

APIs

API stands for Application Programming Interface.

An API is a set of rules, protocols, and specifications that allows different software applications to communicate with each other.

In context Python, APIs generally refer to two distinct concepts:

• Consuming External APIs: Using libraries like Requests to send HTTP requests to external services (e.g., weather data, social media) and parse the responses.
• Building Web APIs: Creating server-side endpoints using frameworks like Flask, Django or FastAPI that expose functionality and data to other applications via specific URLs and HTTP methods (GET, POST, etc.).

JSON

JSON stands for JavaScript Object Notation.

• JSON is a lightweight format for storing and transporting data.
• JSON is often used when data is sent from a server to a web page.
• JSON is “self-describing” and easy to understand

Though it’s related to JavaScript, but it’s itself typically used as a language agnostic format for exchanging data between computers.

__name__ REF: freeCodeCamp

When a Python interpreter reads a Python file, it first sets a few special variables. Then it executes the codes from the file.

One of those variables is called __name__.

So when the interpreter runs a module, the __name__ variable will be set as __main__ if the module is being run is the main program.

But if the code is importing the module from another module, then the __name__ variable will be set to that module name.

There is a really nice use case for the __name__ variable, whether you want a file that can be run as the main program or imported by other modules. We can use an if __name__ == "__main__" block to allow or prevent parts of code from being run when the modules are imported.

When the Python interpreter reads a file, the __name__ variable is set as __main__ if the module being run, or as the module’s name if it is imported. Reading the file executes all top level code, but not functions and classes (since they will only get imported).

Style

The standardized way of writing Python source code to ensure readability and maintainability.

The primary standard is PEP 8, which dictates rules for indentation (4 spaces), line length, naming conventions (snake_case for variables/functions, CamelCase for classes), and import ordering.

Adhering to these rules allows teams to collaborate efficiently and reduces bugs caused by inconsistent formatting.

REF: Python Style Basics - PEP8

• Indentation: Indent code by 4 spaces and be consistent.
• Tab/Spaces: Early on python, some people use 2 spaces or tabs. Those practices have died out, and now 4 spaces is the standard.
• Max Lines Length: In the old days of relatively small computer displays, projects would frequently have a rules that no line in the code could be wider than 80 or 100 characters, so that the code would fit on the display. This sort of rule has becomes less common. Often in Python, the simplest thing to do is just let a long line be long.
  • However if a line so long that it’s hard to read or work with, break it into shorter, separate lines.
• Blank Lines: PEP8 requires 2 blank lines before each def in a file. This is one of the weaker PEP8 rules.

Week 5: Unit Tests

Unit Tests

Unit testing is a software testing technique in which individual components or units of a software application are tested independently from the rest of the application. —freeCodeCamp.org

assert

The assert keyword is used when debugging code.

The assert keyword lets you test if a condition in your code returns True, if not, the program will raise an AssertionError.

You can write a message to be written if the code returns False:

x = "welcome"
#if condition returns False, AssertionError is raised:
assert x != "welcome", "x should not be 'welcome'"

pytest module

The pytest is popular testing framework for Python that makes it easy to write and run tests. Unlike unittest, pytest’s simple syntax allows developers to write tests directly as functions or within classes. This lets you write clean, readable code without complexities. —freeCodecCamp.org

The benefits of using pytest (REF: freeCodeCamp):

• Flexibility: It provides flexibility in test structure by supporting tests for functions, classes, and modules.
• Detailed test output: It provides a detailed and readable test output, making it easy to understand test failures and errors.
• Automatic test discovery: It automatically discovers tests by looking for files that start with “test_” or end with “_test.py”. This eliminates the need for manually specifying test files.
• Parameterization: It supports parameterized tests, which allow you to run a single test functions with multiple sets of inputs.
• Fixtures: It provide setup and tearDown methods that help prevent code repetition. This enables you to set up baseline conditions for your tests and also delete them after each test.
• Plugins and extension: It has a rich set of plugins and extensions to add extra functionalities.
• Compatibility: It’s compatible with other testing frameworks like unittest, allowing you to migrate tests from different testing frameworks and run them seamlessly on it.

Tests Folder

Make folder name test inside your project directory:

mkdir test

Create an empty file named __init__.py:

touch __init__.py

Create the test files like test_[PROGRAM-NAME] inside the test folder.

An empty __init__.py will tell the pytest module that test/ is a package. A package is a python module or multiple modules organized inside a folder.

Now run the pytest module on the whole folder:

pytest test/

 def test_addition():
     assert 1 + 1 == 2

Raise an Error

If your function raises an error like ZeroDivisionError when some condition is met, you want to make sure your unit test, catches that:

import pytest

def test_zero_division:
    with pytest.raises(ZeroDivisionError):
        my_func(1/0)

Here my_func() should raise a ZeroDivisionError and crash the unit test, but with pytest.raises(ZeroDivisionError) tells pytest, “I expect this block of code to through exactly this error”.

This test only passes if this error actually happens when my_func(1/0) runs, if no error occurs, the test fails.

So, it confirms, the my_func() correctly propagates the error instead of silently failing and returning something wrong.

Week 6: File I/O

File Handling (I/O)

(REF: Python File Open)

When programming, you have likely encountered a situation where all data is lost once your script finishes running. This happens because the data is stored in the memory, and when the program ends, the memory is cleared.

To solve this issue, Python offers a feature called File I/O, which allows you to read from or write to files, ensuring that data persists even after your program has ended.

File I/O refers to the ability of a program to take a file as input or create a file as output. This is essential when you want to store data for later use, process large datasets, or manage configurations. —Dilan Nawarathne via Medium

The key function for working with files in Python is the open() function.

The open() function takes two parameters; filename, and mode.

There are four different methods (modes) for opening a file:

"r" - Read — Default Value. Opens a file for reading, error if the file doesn’t exist.

"a" - Append — Opens a file for appending, creates the file if it doesn’t exist.

"w" - Write — Opens a file for writing, creates the file if it doesn’t exist or overwrites the existing file.

"x" - Create — Creates the specified file, returns an error if the file exists.

In addition you can specify if the file should be handled as binary or text mode:

"t" - Text — Default value. Text mode

"b" - Binary — Binary mode (e.g., images)

Syntax

To open a file for reading it is enough to specify the name of the file:

file = open("demofile.txt")

The code above is the same as:

file = open("demofile.txt", "rt")

As "r" for read, and "t" for text are the default values, you don’t need to specify them.

Python Read File

Using the with statement

You can also use the with statement when opening a file:

with open("demofile.txt") as f:
  print(f.read())

Then you don’t have to worry about closing your files, the with statement takes care of that.

Close Files

It is a good practice to always close the file when you are done with it.

If you’re not using the with statement, you must write a close statement in order to close the file:

f = open("demofile.txt")
print(f.readline())
f.close()

Read Only Parts of the File

By default the read() methods returns the whole text, but you can also specify how many characters you want to return:

with open("demofile.txt") as f:
  print(f.read(5))

Read Lines

You can return one line by using the readline() method:

with open("demofile.txt") as f:
  print(f.readline())

By calling readline() two times, you can read the two first lines:

with open("demofile.txt") as f:
  print(f.readline())
  print(f.readline())

By looping through the lines of the file, you can read the whole file, line by line:

with open("demofile.txt") as f:
    for line in f:
        print(line)

Python File Write

Write to an Existing File

To write to an existing file, you must add a parameter to the open() function:

"a" - Append — will append to the end of the file

"w" - Write —- will overwrite any existing content

with open("demofile.txt", "a") as f:
  f.write("Now the file has more content!")

# open and read the file after appending
with open("demofile.txt") as f:
  print(f.read())

Create a New File

To create a new file in Python use the open() method, with one of the following parameters:

"x" - Create — will create a file, returns an error if the file exists

f = open("myfile.txt", "x")

Result: A new empty file is created.

csv Module

The csv module reads and writes tabular data in CSV(Comma Separated Values) format.

pillow Module

REF: Python:Pillow

Pillow is a Python library used for working with images. It is a user friendly and actively maintained fork of the original Python Imaging Library(PIL). It not only supports various image formats like JPEG, PNG, GIF, TIFF, and BMP, but also offers capabilities such as resizing, cropping, rotating, and color adjustments.

Use cases of Pillow

Pillow is used for automating and customizing image-related tasks, particularly when graphical interfaces aren’t ideal. Here are some compelling use cases:

• Resize and crop images
• Format conversion: Convert between file formats like JPEG, PNG, GIF, BMP.
• Image Enhancement: Adjust brightness, apply filters, or add visual effects.
• Batch Processing: Automate processing of large volumes of images.
• Image Generation: Create custom graphics, visualization, or CAPTCHAS.
• Meme or Thumbnail Creation: Combine Pillow with text or shape drawing for custom graphics.

Installing Pillow

Install the official release of Pillow:

pip install pillow

General Workflow of Pillow

Pillow operations typically follow a clear, intuitive workflow:

• Import the necessary classes from the PIL module
• Open an image using Image.open()
• Apply transformations like resizing, rotating, or filtering
• Save the final output using Image.save()

Week 7: Regualr Expressions

Regular Expressions

A RegEx, or Regular Expression, is a sequence of characters that forms a search pattern. — W3School

RegEx can be used to check if a string contains the specified search pattern.

Python has a built-in packaged called re, which can be used to word with Regular Expressions.

Import the re module:

import re

Metacharacters

Metacharacters are characters with a special meaning:

(): It groups the pattern and stores the matched text so you can retrieve it later.

(?P<group_name>): If there many capture groups in the match pattern, you can use match.group("group_name") instead of match.group(n) where n is the group number starting from 1.

(?:): It groups the pattern but stores nothing

Flags

You can add flags to the pattern when using regular expressions.

Special Sequences

A special sequence is a \ followed by one of the characters in the list below, and has a special mean:

Sets

A set is a set of characters inside a pair of square brackets [] with a special meaning:

Assignment Expression/Walrus Operator :=

The := operator was introduced in Python 3.8. It allows you to assign a value to a variable as part of a larger expression, returning the assigned value unlike standard assignment(=), which doesn’t return a value.

Syntax

The operator is written as variable := expression. It evaluates the right-hand side, assigns it to the variable, and returns the value for use in the surrounding context.

Parentheses

In most contexts, such as top-level statements or certain conditionals, the assignment expression must be enclosed in parentheses to avoid syntax error (e.g., (x := 5)).

# Without walrus operator
data = get_data()
if data is not None:
    print(data)

# With walrus operator
if (data := get_data()) is not None:
    print(data)

re Module Functions (via w3schools)

The findall() function

The findall() function returns a list containing all matches.

Print a list of all matches:

import re

txt = "The rain in Spain"
x = re.findall("ai", txt)
print(x)

The list contains the matches in the order they are found.

If no matches are found, an empty list is returned.

The search() Function

It searches the string for a match, and returns a Match Object if there is a match.

If there is more than one match, only the first occurrence of the match will be returned:

Search for the first white-space character in the string:

import re

txt = "The rain in Spain"
x = re.search(r"\s", txt)

print("The first white-space character is located in position:", x.start())

If no matches are found, the value None is returned.

The split() Function

The split() function returns a list where the string has been split at each match.

Split at each white-space character:

import re

txt = "The rain in Spain"
x = re.split(r"\s", txt, MAXSPLIT)
print(x)

You can control the number of occurrences by specifying the optional maxsplit parameter.

The sub() Function

It replaces the matches with the text of your choice.

Replace every white-space character with number 9:

import re

txt = "The rain in Spain"
x = re.sub(r"\s", "9", txt, COUNT)
print(x)

You can control the number of replacements by specifying the optional count parameter.

Match Object

A Match Object is an object containing information about the search and the result.

Do a search that will return a Match Object:

import re

txt = "The rain in Spain"
x = re.search("ai", txt)
print(x) #this will print an object

The Match Object has properties and methods used to retrieve information about the search, and the result.

• .span() returns a tuple containing the start, and end positions of the match.
• .string returns the string passed into the function
• .group() returns the part of the string where there was a match.

Week 8: Object-Oriented Programming

What is OOP?

Object-Oriented Programming (OOP) is a programming paradigm that organizes software design around object rather than functions and logic.

Let’s breakdown the OOP into its reusable building block:

• Classes: These are user-defined data types that act as the blueprint for objects, attributes, methods.
• Objects: These are instances of a class with specifically defined data. When a class is defined initially, the description is the only object that is defined.
• Methods: These are functions that are defined inside a class that describe the behavior of an object. They are useful for re-usability or keeping functionality encapsulated inside one object at a time. Code re-usability is a great benefit when debugging.
• Attributes: These are defined in the class template and represent the state of an object. Objects contain data stored in the attribute field.

OOP stands for Object-Oriented Programming.

Python is an object-oriented language, allowing you to structure your code using classes and objects for better organization and re-usability.

Principles of OOP

In order for use to write good OOP code, we need to understand the 4 pillars of OOP we should adhere to:

1. Encapsulation: Wrapping data and functions into a single unit(object).
2. Abstraction: Hiding the implementation details of a class and only showing the essential features to the user.
3. Inheritance
4. Polymorphism (Operator overloading): When the same operator is allowed to have different meaning according to the context.

Advantages of OOP

• Provides a clear structure to programs
• Makes code easier to maintain, reuse, and debug
• Helps keep your code DRY (Don’t Repeat Yourself)
• Allows you to build reusable applications with less code

What are Classes and Objects?

Classes and objects are the two core concepts in OOP.

A class defines what an object should look like, and an object si created based on that class. For example:

When you create an object from a class, it inherits all the variables and functions defined inside that class.

Attributes

There are two types of attributes/variable in the classes.

1. Class Attributes: Python variables that belong to a class itself, and are shared between all class instances/objects.

2. Instance Attributes: Python variables that belong to one object/instance and are only accessible within the object; they are unique to each object and are not shared between class instances.

Python Classes and Objects

Python is an object-oriented programming language.

Almost everything in Python is an object, with its properties and methods.

A Class is like an object constructor, or a “blueprint” for creating objects.

Create a Class

To create a class, use the keyword class.

Create a class named MyClass, with a property named x:

class MyClass:
    x = 5

Create Object

Now we can use the class named MyClass to create objects.

Create an object named p1, and print the value of x:

p1 = MyClass()
print(p1.x)

del keyword

Used to delete object properties or object itself

You can delete objects by using the del keyword.

Delete the p1 or its name property object:

del p1
#OR
del p1.name

Multiple Objects

You can create multiple objects from the same class:

Create three objects from the MyClass class:

p1 = MyClass()
p2 = MyClass()
p3 = MyClass()

print(p1.x)
print(p2.x)
print(p3.x)

The pass Statement

class definitions cannot be empty, but if you for some reason have a class definition with no content, put in the pass statement to avoid getting an error.

class Person:
    pass

Python __init__() Method

The __init_() Method

All classes have a built-in method called __init__(), which is always executed when the class is being initiated.

The __init__() method is used to assign values to object properties, or to perform operations that are necessary when the object is being created.

Example

Create a class named Person, use the __init__() method to assign values for name and age:

class Person:
    def __init__(self, name, age):
        self.name = name
        self.age = age

p1 = Person("Email", 36)

print(p1.name)
print(p1.age)

Why Use __init__()?

Without the __init__() method, you would need to set properties manually for each object:

class Person:
    pass

p1 = Person()
p1.name = "Tobias"
p1.age = 25

print(p1.name)
print(p1.age)

Using __init__() makes it easier to create object with initial values.

Default Values in __init__()

You can also set default values for parameters in the init() method.

class Person:
    def __init__(self, name, age=18):
        self.name = name
        self.age = age

p1 = Person("Emil")
p2 = Person("Tobias", 25)

print(p1.name, p1.age)
print(p2.name, p2.age)

Multiple Parameters

The __init__() method can have as many parameters as you need:

class Person:
  def __init__(self, name, age, city, country):
    self.name = name
    self.age = age
    self.city = city
    self.country = country

p1 = Person("Linus", 30, "Oslo", "Norway")

print(p1.name)
print(p1.age)
print(p1.city)
print(p1.country)

Python Self Parameter

The self parameter is a reference ot the current instance of the class.

It is used to access properties and methods that belong to the class.

Use self to access class properties:

class Person:
  def __init__(self, name, age):
    self.name = name
    self.age = age

  def greet(self):
    print("Hello, my name is " + self.name)

p1 = Person("Emil", 25)
p1.greet()

Why Use self?

Without self, Python would not know which object’s properties you want to access.

The self parameter links the method to the specific object:

class Person:
    def __init__(self, name, age):
        self.name = name
        self.age = age

    def greet(self):
        print("Hello, my name is " + self.name)


p1 = Person("Emil", 25)
p1.greet()

self Does Not Have to Be Named “self”

It doesn’t have to be named self, you can call it whatever you like, but it has to be the first parameter of any method in the class.

Calling Methods with self

You can also call other methods within the class using self.

Call one method from another method using self:

class Person:
  def __init__(self, name):
    self.name = name

  def greet(self):
    return "Hello, " + self.name

  def welcome(self):
    message = self.greet()
    print(message + "! Welcome to our website.")

p1 = Person("Tobias")
p1.welcome()

Python Magic Methods

Magic methods are special methods whose names start and end with double underscores, like __init__() and __str__(). Because of the double underscores, they are also called Dunder methods (short for “double underscore”).

You don’t call magic methods directly. Python calls them for you, automatically, in certain situations - like when an object is created, printed, or compared to another object.

List of Magic methods

The __str__() Method

The __str__() method is a special method that controls what is returned when the object is printed.

Without the __str__() method, printing an object shows its memory address:

class Person:
  def __init__(self, name, age):
    self.name = name
    self.age = age

p1 = Person("Emil", 36)
print(p1)

With the __str__() method, you can control what is shown when an object is printed:

class Person:
  def __init__(self, name, age):
    self.name = name
    self.age = age

  def __str__(self):
    return f"{self.name} ({self.age})"

p1 = Person("Tobias", 36)
print(p1)

Class method

A class method is bound to the class and receives the class as an implicit first argument.

In Python, @classmethod is a built-in decorator used to define a method that is bound to the class itself rather than a specific object instance.

Unlike regular instance methods, a class method doesn’t have access to self(the instance). Instead, it implicitly receives the class object as its first argument, which is conventionally named cls.

class Student:
    @classmethod #decorator
    def college(cls):
        pass

Property Decorator

We use @property decorator on any method in the class to use the method as a property

Inheritance

Inheritance allows us to define a class that inherits all the methods and properties from another class.

Parent class is the class being inherited from, also called base class.

Child class is the class that inherits from another class, also called derived class.

Exceptions in Python are an example of class inheritance.

Super Method

super() method is used to access methods of the parent class.

class Car:
    def __init__(self, type):
        self.type = type

    @staticmethod
    def start():
        print("Car is started..")

class ToyotaCar(Car):
    def __init__(self, name):
        super().__init__(type)
        self.name = name
        super().start() #call parent method with super()

Operator Overloading

Magic methods can control what happens when you use an operator, like +, on your own objects. This is called operator overloading.

Operator overloading refers to the ability to redefine the behavior of built-in operators for user-defined classes. In other words, you can “teach” Python how to evaluate expressions like a+b when a and b are instances of a custom class. —Leapcell.io

Python achieves this by mapping operators to special methods, also known as dunder methods (short for “double underscores). For example:

• + corresponds to __add__
• - corresponds to __sub__
• * corresponds to __mul__
• / corresponds to __truediv__
• == corresponds to __eq__
• < corresponds to __lt__

By defining these methods in your class, you can control how objects behave when used with those operators.

Best Practices

1. Keep operations intuitive: Only overload operators where it makes logical sense.
2. Return new instances: Avoid mutating existing instances unless that’s the intended behavior.
3. Ensure compatibility: Always handle edge cases and validate types when performing operations.

The __add__() Method

Use __add__() to add the age of two Person objects together:

class Person:
    def __init__(self, name, age):
        self.name = name
        self.age = age

    def __add__(self, other):
        return self.age + other.age

p1 = Person("Emil", 22)
p2 = Person("Tobias", 19)

print(p1+p2)

References

• OOP Meaning – What is Object-Oriented Programming? via freeCodeCamp.org
• Python OOP via W3School
• Understanding Operator Overloading in Python via Leapcell.io
• Lecture 8 : OOPS in Python | Object Oriented Programming | Classes & Objects | Python Full Course [Urdu/Hindi] via Shradha Khapra
• Lecture 9 : OOPS Part 2 | Object Oriented Programming | Python Full Course [Urdu/Hindi] via Shradha Khapra
• Python Object Oriented Programming (OOP) - For Beginners [English] via Tech with Tim
• Python Attributes: Class vs. Instance Explained via builtin.com

Week 9: Et Cetera

Set set()

Sets are used to store multiple items in a single variable.

Set is one of 4 built-in data types in Python used to store collections of data, the other 3 are list(), tuple(), and dict(), all with different qualities and usage.

A set is a collection which is unordered, unchangeable, and unindexed.

• Sets are written with curly brackets.
• They don’t allow duplicate values.
• Set items can appear in a different order every time, and cannot be referred to by index or key.
• The values True and 1 are considered the same value in sets, and are treated as duplicates.
• The values False and 0 are considered the same value in sets, and are treated as duplicates.

Get the Length of a Set

To determine how many items a set has, use len() func.

fruits = {"Apple", "Kiwi", "Banana"}
print(len(fruits))

Set Items — Data Types

Set items can be of any data type.

String, int, and boolean data types:

fruits = {"Apple", "Kiwi", "Banana"}
nums = {1, 2, 3, 4}
bool = {True, False, True, False}

A set can contain different data types.

The set() Constructor

It is also possible to use the set() constructor to make a set.

fruits = set(("apple", "banana", "kiwi"))
print(fruits)

Remove Item

To remove an item in a set, use the remove() or the discard() method.

• You can also use the pop() method to remove an item, but this method will remove a random item, so you cannot be sure what item gets removed.
• The clear() method empties the set.
• The del keyword will delete the set completely.

fruits = {"Apple", "Kiwi", "Banana"}
fruits.remove("Kiwi")
print(fruits)

The global Variable

Variables that are created outside of a function are known as global variables

Global variables can be used by everyone, both inside of functions and outside.

• If you create a variable with the same name inside a function, this variable will be local, and can only be used inside the func. The global variable with the same name will remain as it was, global and with original value.

balance = 0 #global var

def current_balance():
    balance = 1000 #func local var
    print("Balance:", balance)

current_balance()
print("Balance:", balance)

The global keyword

Normally, when you create a var inside a func, that var is local, and can only be used inside that func.

To create a global var inside a function, you can use global keyword

To change a value of a global variable inside a func, refer to the variable by using global keyword.

balance = 0 #global var

def current_balance():
    global balance
    balance = 1000 #changes global var value to 1000
    print("Balance:", balance)

current_balance()
print("Balance:", balance)

Constants

One have some constant value somewhere in the code, in Python and other programming languages, a convention is to bring them towards the start.

• Capitalize the whole CONSTANT=<value> word, to indicate to other programmers working on the code, they shouldn’t touch it.

MEOWS = 3
for _ in range(3):
    print("meow")

• But nothing is stopping other programmers to re-assign the constant’s value to something else unlike other programming languages, Python operates on “honor” system.
• The same applies to the class variables.

class Cat:
    MEOWS = 3

    def meow(self):
        for _ in range(Cat.MEOWS):
            print("meow")

cat = Cat()
cat.meow()

Type hints

• Python is a dynamically typed language, you don’t need to mention if your variable value is str, int, or float. Python figures this out own its own.
• Unlike other languages like C, C++, Java etc., the programmers have to specify sometimes what type of variable you want something to be.
• Type hints help the programmer find out bugs more easily.
• Python has type hints too, but they are not strictly enforced by the language.

def meow(n: int):  # type hint
    for _ in range(n):
        print("meow")

number: int = int(input("Number: "))  # type hint
meow(number)

The mypy module provides support for type hints in Python code.

pip install mypy

Run the mypy on meows.py:

mypy meows.py

To check, if function returns something:

def meow(n: int) -> str:  # type hint: func should return a str
    return "meow\n" * n

number: int = int(input("Number: "))  # type hint
meows: str = meow(number) # type hint

print(meows, end="")

docstrings

Python provides docstrings to add comments to your code.

• Python has documentation generation tools, which can generate a PDF/md from your docstrings inside your code.

def meow(n: int) -> str:  # type hint
    """
    Meow n times.

    :param n: Number of times to meow
    :type n: int
    :raise TypeError: if n is not int
    :return: A string of n meows, one per line
    :rtype: str
    """
    return "meow\n" * n

number: int = int(input("Number: "))  # type hint
meows: str = meow(number)

print(meows, end="")

The argparse module

The argparse module makes it easy to build user-friendly command-line interfaces.

It parses arguments and options, generates help and usage messages automatically, and provides errors when users give the program invalid arguments.

unpacking

In Python, unpacking lets you assign or pass multiple values at once by expanding an iterable into individual items.

You’ll see it in assignments for parallel name binding in expressions and function calls via the iterable unpacking(*) and dictionary unpacking (**).

The single asterisk (*) in Python is used for two main way:

1. Unpacking Lists (or Tuples)
2. Unpacking in Function calls

The Double asterisk (**) is used for unpacking dictionaries as keyword arguments (key-value pairs).

1. This passes the dictionary entries as keyword arguments to the function.
2. You can also use ** to combine dictionaries in a clean way.

defaults = {"debug": False}
overrides = {"debug": True, "verbose": True}

final_config = {**defaults, **overrides}
print(final_config)
# Output: {'debug': True, 'verbose': True}

In this case:

• The overrides dictionary updates values from defaults.
• The result is a merged dictionary where later entries take priority.

In assignments, the right-hand side must be an iterable, and the left-hand side is a comma-separated sequence of assignment targets, often written as a tuple of names. The number of values and targets must match. Alternatively, you can use a single starred target(*rest) to collect multiple values into a list.

In function calls, *iterable expands an iterable into positional arguments, while **dictionary expands into keyword arguments.

When to Use * and **

• * —> Capture or expand multiple values (lists, args)
• ** —> Capture or expand keyword pairs(dicts)
• Perfect for flexible functions, cleaner data handling

Why use * and **?

• They make your code more flexible by allowing dynamic argument passing.
• They help you avoid repetition and make code more concise and readable.
• Especially useful in functional programming, API calls, and clean config handling.

Examples

Parallel assignment and value swapping:

>>> a, b = 1, 2
>>> a, b
(1, 2)

>>> a, b = b, a
>>> a, b
(2, 1)

Starred targets:

>>> first, *middle, last = [10, 20, 30, 40]
>>> first
10
>>> middle
[20, 30]
>>> last
40

You can also use * in other positions:

>>> *start, end = [1, 2, 3]
>>> start = [1, 2], end = 3

>>> first, *rest = [10, 20, 30]
>>> first = 10, rest = [20, 30]

Function calls:

>>> def greet(greeting, name):
...     return f"{greeting}, {name}!"
...
>>> args = ("Hello", "Pythonista")
>>> greet(*args)
'Hello, Pythonista!'

>>> def connect(host, port, timeout=10):
...     return host, port, timeout
...
>>> conn_params = {"host": "localhost", "port": 8000}
>>> connect(**conn_params)
('localhost', 8000, 10)

Unpacking in expressions:

>>> numbers = [0, *range(1, 4), 4]
>>> numbers
[0, 1, 2, 3, 4]

*args and **kwargs

By default, a function must be called with the correct number of arguments.

However, sometimes you may not know how many arguments that will be passed into your function.

*args and **kwargs allow functions to accept an unknown number of arguments.

Arbitrary Arguments — *args

If you don’t know how many arguments will be passed into your function, add a * before the parameter name. This way, the function will receive a tuple of arguments and can access the items accordingly.

Example:

Using *args to accept any number of arguments:

def my_function(*kids):
  print("The youngest child is " + kids[2])

my_function("Emil", "Tobias", "Linus")

What is *args?

The *args parameter allows a function to accept any number of positional arguments.

Inside the function, args becomes a tuple containing all the passed arguments.

Using *args with Regular Arguments

You can combine regular parameters with *args.

Regular parameters must come before *args:

def my_function(greeting, *names):
  for name in names:
    print(greeting, name)

my_function("Hello", "Emil", "Tobias", "Linus")

In this example, "Hello" is assigned to greeting, and the rest are collected in names.

Practical Examples with *args

*args is useful when you want to create flexible functions:

def my_function(*numbers):
  total = 0
  for num in numbers:
    total += num
  return total

print(my_function(1, 2, 3)) # Output: 6
print(my_function(10, 20, 30, 40)) # Output: 100
print(my_function(5)) # Output: 5

Find the max value:

def my_function(*numbers):
  if len(numbers) == 0:
    return None
  max_num = numbers[0]
  for num in numbers:
    if num > max_num:
      max_num = num
  return max_num

print(my_function(3, 7, 2, 9, 1)) # Output: 9

Arbitrary Keyword Arguments — **kwargs

If you don’t know how many keyword arguments will be passed into your function, add two asterisks ** before the parameter name.

This way, the function will receive a dictionary of arguments and can access the items accordingly.

Example:

Using **kwargs to accept any number of keyword arguments.

def my_function(**kid):
  print("His last name is " + kid["lname"])

my_function(fname = "Tobias", lname = "Refsnes")

What is **kwargs?

The **kwargs parameter allows a function to accept any number of keyword arguments.

Inside the function, kwargs becomes a dictionary containing all the keyword arguments.

Using **kwargs with Regular Arguments

You can combine regular parameters with **kwargs.

Regular parameters must come before **kwargs:

def my_function(username, **details):
  print("Username:", username)
  print("Additional details:")
  for key, value in details.items():
    print(" ", key + ":", value)

my_function("emil123", age = 25, city = "Oslo", hobby = "coding")

Combining *args and **kwargs

You can use both *args and **kwargs in the same function.

The order must be:

1. Regular parameters
2. *args
3. **kwargs

def my_function(title, *args, **kwargs):
  print("Title:", title)
  print("Positional arguments:", args)
  print("Keyword arguments:", kwargs)

my_function("User Info", "Emil", "Tobias", age = 25, city = "Oslo")

References

• Python Sets via W3Schools
• Python — Global Variables via W3Schools
• Python argparse Module via W3Schools
• Python — unpacking via Real Python
• Unpacking in Python, Simply Explained via Ivan R | Medium.com
• Python — *args and **kwargs via W3Schools

Section 1: Python Basics

Everyone in their life, spent a lot of time on repetitive tasks, which can be automated through a simple script.

Automate the boring stuff with Python uses Python 3.

How to get help?

Being stuck while coding is a normal happening, but not asking for help isn’t.

When you go online to ask for help, make sure:

• Explain what you are trying to do, not just what you did.
• If you get an error message, specify the point at which the error happens.
• Copy and paste the entire body of the error message and your code to a Pastebin site like Pastebin.com or GitHub Gist.
• Explain what you’ve already tried to do to solve your problem.
• List the version of Python you’re using.
• Say whether you’re able to reproduce the error every time you run the program or whether it happens only after you perform certain actions. If the latter, then explain what those actions are.
• Specify what Operating System you’re on, what version of that OS you’re using.

Basic Terminology and using an IDLE

IDLE stands for Integrated Development and Learning Environment.

There are different programming text editors available:

• Visual Studio Code

• Sublime Text

• PyCharm

• Expressions = Values + Operators

In python, these expressions always evaluate to a single result. Arithmetic Operators are:

Data Types

• Integers — “ints” (1,2,3…)
• Floating point — “floats” (1.0, 1.1…)
• Strings (“Hello World”)
  • Strings Concatenation: When two strings are joined together using a + symbol. (“Hello " + “World”)
  • String Replication: A string can be replicated by using * operator. (3 * “Hello World!”)
  • Both These operations can be combined like this "Hello World" + "!" * 5
  • Both concatenation and replication accepts strings values only.

Variables

Variable can store different values, like a box:

A too generic name given to a variable is a bad practice, which can create headache down the line while interacting with your code.

• If a python instruction evaluates to a single value, it’s called an expression.
• If it doesn’t evaluate to a single value, it’s called a statement.

We can update the variable value by calling it down the line in the program:

>>> spam = 'Hello'  
>>> spam  
'Hello'  
>>> spam = 'Goodbye'  
>>> spam  
'Goodbye'

Just like the box, we can remove the old item with the new one.

Variable Names

You can name your variable anything, but Python does have some restrictions too:

• It can be only one word with no spaces.
• It can use only letters, numbers, and the underscores (_) character.
• It can’t begin with a number.
• Var names are case-sensitive too.

Though Spam is a valid var, but it is a Python convention to start var name with a lowercase letter.

camelCase for variables can be used though Python PEP8 style guide instead recommends the use of underscores like this camel_case.

Though PEP8 guide itself says:

Consistency with the style guide is important. But most importantly: know when to be inconsistent—sometimes the style guide just doesn’t apply. When in doubt, use your best judgment.

Writing Our First Program

• Python ignore comments starting with #.
• It also skips the blank lines.
• Functions — They are like mini-programs in Python.

print("Hello World!")

# Ask for their name
yourName = input("Type your name: ")
print("It is good to meet you, " + str(yourName))
print("Your name length is: " + str(len(yourName)))

# Ask for their age
print("What is your age?")
yourAge = input("Type your age: ")
print("You will be " + str(int(yourAge) + 1) + " in a year.")

• len(): It prints out the total number of characters in a string.
• input() function always returns a string value, so you may have to convert it according to your need to float(), int() etc.
• You can not concatenate str() and int() together, you will need to convert int() to str(int()), to concatenate them.

hello.py Evaluation steps look like this:

Extras (BOOK)

Python round(number, ndigits=None) Function

Return number rounded to ndigits precision after the decimal point. If ndigits is omitted or is None, it returns the nearest integer to its input.

>>> round(23)  
23  
>>> round(23.0)  
23  
>>> round(23.3)  
23  
>>> round(23.345)  
23  
>>> round(23.5)  
24  
>>> round(23.49)  
23
>>> round(32.35, 1)  
32.4  
>>> round(32.355, 2)  
32.35

Section 2: Flow Control

Flow Charts and Basic Flow Control Concepts

A flowchart starts at the start box, and you follow the arrow at the other boxes until you reach the end box. You take a different path depending on the conditions.

Based on how expression evaluate, a program can decide to skip instructions, repeat them, or choose one of several instructions to run. In fact, you almost never want your programs to start from the first line of ode and simply execute every line, straight to the end.

Flow control statements can decide which Python instructions to execute under which conditions.

These flow control statements directly correspond to the symbols in a flowchart.

In a flowchart, there is usually more than one way to go from the start to the end. The same is true for lines of code in a computer program. Flowcharts represent these branching points with diamonds, while the other steps are represented with rectangles. The starting and ending steps are represented with rounded rectangles.

Boolean Values

Boolean Data Type has only to values True and False.

How to represent YES and NO values:

• Boolean Values
• Comparison Operators
• Boolean Operators
• When entered as Python code, the Boolean always starts with a capital T or F, with the rest of the word in lowercase.

(Boolean is capitalized because the data type is named after mathematician George Boole)

➊ >>> spam = True  
   >>> spam  
   True  
➋ >>> true  
   Traceback (most recent call last):  
     File "<pyshell#2>", line 1, in <module>  
       true  
   NameError: name 'true' is not defined  
➌ >>> True = 2 + 2  
   SyntaxError: can't assign to keyword

Like any other value, Boolean values are used in expressions and can be stored in variables ➊. If you don’t use the proper case ➋ or you try to use True and False for variable names ➌, Python will give you an error message.

Comparison Operators

They also called relational operators, compare two values and evaluate down to a single Boolean value.

• These operators evaluate to True or False depending on the values you give them.
• The == and != operators can actually work with values of any data type.

>>> 'hello' == 'hello'  
   True  
   >>> 'hello' == 'Hello'  
   False  
   >>> 'dog' != 'cat'  
   True  
   >>> True == True  
   True  
   >>> True != False  
   True  
   >>> 42 == 42.0  
   True  
➊ >>> 42 == '42'  
   False

An integer or floating point value will always be unequal to a string value. There 42 == '42'➊ evaluates to False because Python considers the integer 42 to be different from the string '42'.

The <, >, <=, and >= operators, on the other hand, work properly only with integer and floating-point values.

Boolean Operators

The three Boolean operators (and, or, and not) are used to compare Boolean values. Like comparison operators, they evaluate these expressions down to a Boolean value.

Binary Boolean Operators

The and and or operators always take two Boolean values (or expressions), so they’re considered binary operators.

and Operator: It evaluates to True only if both Boolean values are True.

or Operator: It evaluates to True if one of the Boolean values is True.

The not Operator

It has only one Boolean value (or expression)

Mixing Boolean and Comparison Operators

Since the comparison operators evaluate to Boolean values, you can use them in expressions with the Boolean operators.

>>> (4 < 5) and (5 < 6)  
True  
>>> (4 < 5) and (9 < 6)  
False  
>>> (1 == 2) or (2 == 2)  
True

You can also use multiple Boolean operators in an expression, along with the comparison operators:

>>> 2 + 2 == 4 and not 2 + 2 == 5 and 2 * 2 == 2 + 2  
True

The Boolean operators have an order of operations just like the math operators do. After any math and comparison operators evaluate, Python evaluates the not operators first, then the and operators, and then the or operators.

Elements of Flow Control

Flow control statements often start with a part called the condition and are always followed by a block of code called the clause.

Conditions

The Boolean expressions you’ve seen so far could all be considered conditions, which are the same thing as expressions; condition is just a more specific name in the context of flow control statements.

Conditions always evaluate down to a Boolean value, True or False. A flow control statement decides what to do based on whether its condition is True or False, and almost every flow control statement uses a condition.

Blocks of Code

Lines of Python code can be grouped together in blocks.

There are 3 rules for block:

1. Blocks begin when the indentation increases.
2. Blocks can contain other blocks.
3. Blocks end when the indentation decreases to zero or to a containing block’s indentation.

  name = 'Mary'  
  password = 'swordfish'  
  if name == 'Mary':  
    ➊ print('Hello, Mary')  
       if password == 'swordfish':  
        ➋ print('Access granted.')  
       else:  
        ➌ print('Wrong password.')

You can view the execution of this program at https://autbor.com/blocks/. The first block of code ➊ starts at the line print(‘Hello, Mary’) and contains all the lines after it. Inside this block is another block ➋, which has only a single line in it: print(‘Access Granted.’). The third block ➌ is also one line long: print(‘Wrong password.’).

If, Else, and Elif Statements

The statements represent the diamonds in the flowchart. They are the actual decisions your programs will make.

if Statements

If this condition is true, execute the code in the clause. if statement, consists of the following:

• The if keyword
• A condition (that is, an expression that evaluates to True or False)
• A colon
• Starting on the next line, an indented block of code (called the if clause)

else Statements

An if clause can optionally be followed by an else statement. The else clause is executed only when the if statement’s condition is False.

An else statement doesn’t have a condition. In code, an else statement always consists of the following:

• The else keyword
• A colon
• Starting on the next line, an indented block of code (called the else clause)

elif Statements

While only one of the if or else clauses will execute, you may have a case where you want one of many possible clauses to execute.

The elif statement is an “else if” statement that always follows an if or another elif statement. It provides another condition that is checked only if all the previous conditions were False.

In code, an elif statement always consists of the following:

• The elif keyword
• A condition (that is, an expression that evaluates to True or False)
• A colon
• Starting on the next line, an indented block of code (called the elif clause)

if name == 'Alice':  
    print('Hi, Alice.')  
elif age < 12:  
    print('You are not Alice, kiddo.')

The elif clause executes if age < 12 is True and name == 'Alice' is False. However, if both of the conditions are False, then both of the clauses are skipped. It is not guaranteed that at least one of the clauses will be executed. When there is a chain of elif statements, only one or none of the clauses will be executed. Once one of the statements’ conditions is found to be True, the rest of the elif clauses are automatically skipped.

name = 'Carol'
age = 3000
if name == 'Alice':
    print('Hi, Alice.')
elif age < 12:
    print('You are not Alice, kiddo.')
elif age > 2000:
    print('Unlike you, Alice is not an undead, immortal vampire.')
elif age > 100:
    print('You are not Alice, grannie.')

• The program vampire.py has 3 elif statements. If any of the three, is found True program execution will stop.
• The order of elif statements is also important.
• Optionally, you can have an else statement after the last elif statement. In that case, it is guaranteed that at least one (and only one) of the clauses will be executed. If the conditions in every if and elif statement are False, then the else clause is executed.

For example, let’s re-create the Alice program to use if, elif, and else clauses.

age = 3000  
if name == 'Alice':  
    print('Hi, Alice.')  
elif age < 12:  
    print('You are not Alice, kiddo.')  
else:  
    print('You are neither Alice nor a little kid.')

When you use if, elif, and else statements together, remember these rules about how to order them to avoid bugs like the one in Figure 2.7. First, there is always exactly one if statement. Any elif statements you need should follow the if statement. Second, if you want to be sure that at least one clause is executed, close the structure with an else statement.

name = 'Carol'  
age = 3000  
if name == 'Alice':  
   print('Hi, Alice.')  
elif age < 12:  
   print('You are not Alice, kiddo.')  
elif age > 100:  
   print('You are not Alice, grannie.')  
elif age > 2000:  
   print('Unlike you, Alice is not an undead, immortal vampire.')