Python 101 a Comprehensive Guide

Introduction

Python is an interpreted, high-level programming language that was developed by Guido van Rossum and released in 1991.

It is a widely used language in areas such as Web development, Machine learning, machine learning and Artificial Intelligence , Automation and scripting

Getting Started

To get started, download the latest version for your operating system from the official website here

Once that’s done, set it up in VScode using this guide

Writing and executing a python program

To write a python program, you’ll need an IDE(Integrated development environment) or a text editor

Common IDE’s and editors include

• PyCharm
• Text editors: VScode, sublime text
• Google Colab

To execute a python file, you can use a terminal such as

• PowerShell
• Command Prompt (CMD)
• Terminal (Linux/macOS)

Example:

Create a folder for your python projects (Python-projects), add a file say hello.py

Write the following line of code and save

print("Hello, World!")

To execute the file, open your terminal and type the following command

python3 hello.py

You should get this output

Hello,World!

Variables and Data Types

A variable is a named container used to store data

Variables can store different types of data types

x = 10      # integer
y = 10.1    # float
name = "Ruth" # string
is_student = True  # Boolean

Data types defines the kind of values a variable holds

Type	Example
Integer	1, -5, 30
float	3.14, 4.1, 0.7
string	“Hi”, “Python”
Bool	True, False

Variable Naming Conventions

1. Variable names cannot start with a number

1name = "John" ❌
name1 = "John" ✅

1. Variable names can contain:
   • Letters
   • Numbers
   • Underscores (_)
2. Variable names cannot contain spaces

first name = "Ruth" ❌
first_name = "Ruth" ✅

1. Variable names are case-sensitive

age = 20
Age = 30

Data structures

Defines how data is organized and stored so that it can be accessed and modified efficiently

Types

1.Lists - are ordered and mutable collections of elements and are defined using brackets []

Example

fruits = [ 'mangoes', 'bananas', 'apples', 'kiwi']

lists can also be defined using list() method

students = list(('mary', 'jane', 'wandia'))

Accessing values in a list using indexing

students[0]

output:

mary

2.Tuples - are ordered collection of elements which can be of different data types, unlike lists tuples are immutable i.e. elements cannot be changed once defined

Tuples are defined using parenthesis `()`

```python
my_tuple = (1, 2, 3)
```

3.Dictionary - Store data in key-value pairs are mutable and unordered (in older Python versions) and are defined using curly braces {}

user = {
    'name': 'Ambogo',
    'age': 13,
    'city': 'Nairobi'
}

4.Sets - Unordered collections of unique elements which do not allow duplicates. sets are defined using curly braces {}

my_set = {1, 2, 3}

Formatted string(f string)

An f-string in Python is a string literal that allows expressions to be embedded inside curly braces {}.

Example:

name = 'Ruth'
age = 15
print(f'Hello, my name is {name}, I am {age} years old.')

Output:

Hello, my name is Ruth, I am 15 years old.

Type conversion

Type conversion is the process of converting a value from one data type to another

Example:

height = "20"
converted_height = int(height)
print(converted_height)

Output

20

Common Type Conversion Functions

Function	Use
int(x)	converts x to integer
float(x)	converts x to float
str(x)	converts x to string
bool(x)	converts x to bool

Control flows

Conditional statements define the order in which statements are executed in a program based on certain conditions.

In Python, the main conditional statements are:

• if
• elif
• else

if Statement

The if statement checks a condition and executes a block of code if the condition is True.

elif Statement

The elif statement checks another condition if the previous if condition is False.

else Statement

The else statement executes a block of code when all preceding conditions are False.

Example:

age = 18
if age < 21:
    print('You are still young for college')
elif age == 21:
    print('Welcome to college')
else:
    print('Try getting a job maybe')

Output

You are still young for college

Operators

Operators are symbols used to perform operations on variables and values in Python.

Types of Operators

Category	Operators	Example
Arithmetic	+, -, *, %, /, //	4 + 1 = 5
Comparison	==, ≠, >, <, ≥, ≤	4 > 1 = False
Logical	and, or, not	True and False = True
Assignment	=, +=, -=
Membership	in, not in

Loops

Loops are used to repeatedly execute a block of code in Python.

Python provides two main types of loops:

• for loops
• while loops

for Loop

A for loop is used to iterate over a sequence such as a list, string, or range of numbers.

Syntax:

for item in iterable:
    # code block

Example:

fruits = ['Mango', 'Orange', 'Banana', 'Grapes']
for fruit in fruits:
    print(fruit)

Output:

Mango
Orange
Banana
Grapes

---

while Loop

A while loop executes as long as a condition remains True.

Syntax:

while condition:
    # code block

Example:

count = 1

while count <= 5:
    print(count)
    count+=1

Output:

1
2
3
4
5

Functions

A function is a named block of reusable code that perform a specific task.

Functions help make code:

• Reusable
• Organized
• Easier to maintain

Defining a Function

Functions are defined using the def keyword.

Syntax:

def function_name():
    # code block

Example:

def greet():
print("Hello World")

greet()

output

Hello World

Function Parameters

Parameters allow functions to accept input values.

Example:

def greet(name):
print(f"Hello{name}")

greet("Ruth")

Output

Hello Ruth

Return Statement

The return statement sends a value back from a function.

Example:

def add_numbers(a,b):
return a+b

result=add_numbers(5,3)
print(result)

Output

8

Built-in Functions

Python provides many built-in functions.

Common examples include:

• print()
• input()
• len()
• type()
• range()

Example:

name="Python"

print(len(name))

File I/O

Why File Handling?

File handling in Python is important because it allows you to work with files stored on your computer.

It is used to:

• Save data permanently
• Create logs
• Store configurations
• Read and process data from files

Opening a file

The open() function is used to open a file.

It takes two parameters:

• File name (including extension)
• Mode (how the file should be used)

It returns a file object.

syntax

open("filename.txt", "mode")

Common File Modes

• "r" → Read (default mode)
• "w" → Write (overwrites file)
• "a" → Append (adds to file)
• "x" → Create new file

Example

with open('data.txt', 'r') as f:
    content = file.read()
    print(content)

Writing to a file

In Python, you can write data into a file using the "w" mode.
Example:

with open('output.txt', 'w') as f:
    file.write('Hello, writing into files')

w→ write mode creates a new file or overwrites an existing file

Appending to a File

Appending means adding new content without deleting existing data.

Use "a" mode.

Example:

with open("data.txt","a") as f:
    f.write("\nThis is a new line.")

Exception Handling

Errors and exceptions are problems that occur during execution of a program

Causes of errors and exception can vary from

• Incorrect syntax
• Invalid data types
• Attempting to access an undefined variable

Common exceptions include:

• Division by zero (ZeroDivisionError)
• Type errors (TypeError)
• File not found (FileNotFoundError)

Handling Exceptions

Exceptions can be handled using built-in keywords or constructs

When exceptions are raised, they can be handled using these clauses

1. try - Code that may cause an error
2. except- Handles the error
3. else - Runs if no error occurs
4. finally → Always executes, regardless of errors

Example: Division by Zero Error

try:
    result = 10 / 0
    print(result)
except ZeroDivisionError:
    print("You cannot divide by zero!")

The output will be

You cannot divide by zero!