🐍 Python Basics — Enhanced Teaching Material (Parts 1–6)

1. Introduction to Python

What is Python?
Python is a high-level, interpreted, and general-purpose programming language designed for readability and simplicity. Created by Guido van Rossum in 1991, it emphasizes code clarity and uses English-like syntax. Unlike compiled languages (like C++), Python code is executed line-by-line by an interpreter, making it ideal for rapid development and prototyping.

Key Features of Python

• Dynamic Typing: You don’t declare variable types. Python infers them at runtime.
• Automatic Memory Management: Garbage collector reclaims unused memory.
• Multi-paradigm: Supports procedural, object-oriented, and functional styles.
• Extensible: Can integrate C/C++ libraries via modules like ctypes or Cython.
• Interpreted: Code runs without compilation — write and run immediately.

How Python Executes Code

When you run a Python script:

1. Source code (.py file) is compiled into bytecode (.pyc files).
2. Bytecode is executed by the Python Virtual Machine (PVM).
3. This makes Python faster than pure interpreters but slower than compiled languages like Java or C++.

Real-World Examples

# Script to automate file renaming
import os
for filename in os.listdir("."):
    if filename.endswith(".txt"):
        os.rename(filename, filename.replace("old_", "new_"))

# Simple web server (one line!)
python -m http.server 8000

# Data analysis with pandas (requires pip install pandas)
import pandas as pd
data = pd.read_csv("sales.csv")
print(data.head())

2. Installing Python and Setting Up the Environment

To begin programming in Python, you need to install the Python interpreter and choose a suitable development environment.

Installing Python

1. Go to https://www.python.org/downloads/
2. Download the latest stable release (e.g., Python 3.12.x)
3. CRITICAL STEP: On Windows, check the box Add Python to PATH before clicking Install.
   Without this, you cannot run python from the command line.
4. Verify installation by opening terminal (cmd / PowerShell / Terminal) and typing:

python --version

If you see Python 3.12.x, you’re good!

Virtual Environments (Best Practice)

Python projects often depend on different versions of libraries. To avoid conflicts, use virtual environments:

# Create a virtual environment
python -m venv myproject_env

# Activate it (Windows)
myproject_env\Scripts\activate

# Activate it (macOS/Linux)
source myproject_env/bin/activate

# Now install packages safely
pip install requests

Deactivate with: deactivate

Choosing an Editor/IDE

Running Python Code

Interactive Mode: Type python in terminal → you enter REPL (Read-Eval-Print Loop):

>>> print("Hello!")
Hello!
>>> 2 + 3
5
>>> exit()

Script Mode: Save code in a file (e.g., hello.py):

name = input("Enter your name: ")
print(f"Welcome, {name}!")

# Run in terminal:
python hello.py

Example: Writing Your First Script

Create a file called calculator.py:

# calculator.py
num1 = float(input("Enter first number: "))
num2 = float(input("Enter second number: "))
print(f"Sum: {num1 + num2}")
print(f"Product: {num1 * num2}")

Run it:

python calculator.py
Enter first number: 5.5
Enter second number: 3
Sum: 8.5
Product: 16.5

3. Python Syntax and Structure

Python’s syntax is intentionally minimalistic and readable. Unlike C, Java, or JavaScript, Python uses indentation instead of braces to define code blocks.

Indentation — The Heart of Python

Python uses whitespace (spaces or tabs) to indicate blocks. This enforces clean, readable code.

Correct Example

if x > 10:
    print("x is greater than 10")
    y = x * 2
    print(y)
print("This runs regardless")

Incorrect Example

if x > 10:
print("x is greater than 10")  # ❌ IndentationError: expected an indented block

Comments

Single-line comments start with #:

# This is a comment
x = 5  # This comment explains the variable

Multi-line strings are often used as docstrings (documentation) for functions, classes, or modules:

"""
This is a multi-line string
used as a docstring.
It does not cause an error,
but it's ignored unless assigned.
"""

Line Continuation

Long lines can be broken using:

• Explicit continuation: Backslash \
• Implicit continuation: Parentheses (), brackets [], or braces {} (preferred)

# Explicit (avoid if possible)
total = 1 + 2 + 3 + \
        4 + 5 + 6

# Implicit (recommended)
total = (1 + 2 + 3 +
         4 + 5 + 6)

# Also works with lists
fruits = [
    "apple",
    "banana",
    "cherry"
]

Semicolons

Optional in Python. You can write multiple statements on one line using semicolons:

x = 5; y = 10; z = x + y
print(z)  # Output: 15

Reserved Keywords

These words have special meaning and cannot be used as variable names:

False, None, True, and, as, assert, async, await, break, class, continue, def, del, elif, else, except, finally, for, from, global, if, import, in, is, lambda, nonlocal, not, or, pass, raise, return, try, while, with, yield

Case Sensitivity

Python is case-sensitive:

name = "Alice"
Name = "Bob"
NAME = "Charlie"

print(name)  # Alice
print(Name)  # Bob
print(NAME)  # Charlie

Whitespace Matters Beyond Indentation

Spaces around operators improve readability:

# Good
result = a + b
if x > 5:

# Avoid (but valid)
result=a+b
if x>5:

Example: Real-World Syntax

# Function definition with proper structure
def calculate_area(length, width):
    """Calculate area of rectangle."""
    area = length * width
    return area

# Call function
area = calculate_area(10, 5)
print(f"Area: {area}")

# Conditional logic
if area > 50:
    print("Large room")
elif area >= 20:
    print("Medium room")
else:
    print("Small room")

if x > 5:
print("x is big")

class = 10

a = 10
A = 20
print(a + A)

4. Variables and Data Types

In Python, variables are labels that point to objects in memory. You don’t declare types — Python infers them dynamically.

Dynamic Typing

x = 5          # x is int
x = "hello"      # Now x is str
x = [1, 2, 3]    # Now x is list
x = None         # Now x is NoneType

Core Built-in Data Types

Type Checking

print(type(42))        # 
print(type("Hi"))        # 
print(type(3.14))        # 
print(type(True))        # 
print(type(None))        # 

Variable Naming Rules

• Must start with a letter or underscore
• Can contain letters, digits, underscores
• Case-sensitive
• Cannot be a keyword

valid_name     ✅
_my_var        ✅
name123        ✅
123name        ❌ (starts with digit)
class          ❌ (keyword)
first-name     ❌ (hyphen not allowed)

Type Conversion (Casting)

num_str = "123"
num_int = int(num_str)       # → 123
num_float = float(num_str)   # → 123.0

text = str(456)              # → "456"
boolean = bool(0)            # → False
boolean = bool(1)            # → True
boolean = bool("")           # → False (empty string)
boolean = bool(" ")          # → True (non-empty)

Memory Reference Example

a = 10
b = a        # b points to same object as a
print(id(a)) # Same memory address as id(b)
print(id(b))

a = 20       # Now a points to new object
print(b)     # Still 10 — b unchanged

Multiple Assignment

# Simultaneous assignment
x, y, z = 1, 2, 3
print(x, y, z)  # 1 2 3

# Swapping values
a, b = 5, 10
a, b = b, a
print(a, b)     # 10 5

Example: Real-World Usage

# Student grade tracker
name = "Maria"
age = 19
gpa = 3.8
is_enrolled = True
graduation_year = None

print(f"Student: {name}, Age: {age}, GPA: {gpa}")
print(f"Enrolled: {is_enrolled}, Grad Year: {graduation_year}")

# Convert user input
user_input = input("Enter your age: ")
age_as_int = int(user_input)  # Converts string to integer
print(f"You will be {age_as_int + 1} next year.")

x = "5"
y = 3
print(x + y)

a = 10
b = a
a = 20
print(b)

5. Operators

Operators perform operations on variables and values. Python supports arithmetic, comparison, logical, assignment, identity, and membership operators.

Arithmetic Operators

Operator Precedence

Python follows PEMDAS:

1. Parentheses ()
2. Exponentiation **
3. Unary plus/minus +x, -x
4. Multiplication, division, floor division, modulus * / // %
5. Addition, subtraction + -

result = 2 + 3 * 4 ** 2
# Step-by-step: 4**2 = 16 → 3*16 = 48 → 2+48 = 50
print(result)  # 50

Comparison Operators

==   Equal
!=   Not equal
>    Greater than
<    Less than
>=   Greater than or equal
<=   Less than or equal

x = 5
y = 10
print(x == y)   # False
print(x != y)   # True
print(x < y)    # True

Logical Operators

age = 18
has_license = True

print(age >= 18 and has_license)      # True
print(age < 18 or has_license)        # True
print(not has_license)                # False

Assignment Operators

x = 10
x += 5   # x = x + 5 → 15
x -= 3   # x = x - 3 → 12
x *= 2   # x = x * 2 → 24
x /= 4   # x = x / 4 → 6.0
x //= 2  # x = x // 2 → 3.0
x %= 2   # x = x % 2 → 1.0

Identity Operators

Check if two variables refer to the same object in memory.

a = [1, 2]
b = a
c = [1, 2]

print(a is b)   # True — same object
print(a is c)   # False — different objects, even though content is same
print(a == c)   # True — content equality

Membership Operators

Check if a value exists in a sequence (list, string, tuple, etc.)

fruits = ["apple", "banana", "cherry"]
print("apple" in fruits)     # True
print("grape" not in fruits) # True

text = "Hello World"
print("World" in text)       # True
print("w" in text)           # False (case-sensitive)

Example: Full Operator Demo

# Calculate BMI
weight_kg = 70
height_m = 1.75
bmi = weight_kg / (height_m ** 2)

print(f"BMI: {bmi:.2f}")

if bmi < 18.5:
    category = "Underweight"
elif bmi < 25:
    category = "Normal"
elif bmi < 30:
    category = "Overweight"
else:
    category = "Obese"

print(f"Category: {category}")

# Check eligibility
is_adult = age >= 18
has_id = True
can_vote = is_adult and has_id
print(f"Can vote: {can_vote}")

x = 3
y = 2
z = x ** y + 1
print(z)

6. Input and Output

Input/output (I/O) allows interaction between your program and the user or external data sources.

Output: print() Function

The print() function outputs text to the console.

print("Hello, World!")                      # Basic
print("Age:", 25)                             # Multiple args → space separated
print("Name:", name, "Score:", score, sep="-") # Custom separator
print("First", end=" ")                       # No newline
print("Second")                               # Output: "First Second"

# F-strings (formatted strings) — MOST COMMON IN MODERN PYTHON
name = "Alice"
age = 25
print(f"My name is {name} and I am {age} years old.")
print(f"Pi ≈ {3.14159:.2f}")                  # Format to 2 decimals
print(f"{1000:,}")                            # Comma separator → 1,000

Input: input() Function

Always returns a string. You must convert manually.

name = input("Enter your name: ")           # Returns string
age_str = input("Enter your age: ")
age = int(age_str)                              # Convert to integer

# One-liner version
age = int(input("Enter your age: "))

Common Pitfall: Type Errors

# ❌ Wrong
num1 = input("Number 1: ")  # "5"
num2 = input("Number 2: ")  # "3"
print(num1 + num2)          # Output: "53" — string concatenation!

# ✅ Correct
num1 = float(input("Number 1: "))
num2 = float(input("Number 2: "))
print(num1 + num2)          # Output: 8.0

Formatted Output — Legacy Methods

# .format()
print("Hello {}, you are {} years old.".format(name, age))

# %-formatting (old style — avoid in new code)
print("Hello %s, you are %d years old." % (name, age))

Example: Interactive Calculator

# calculator.py
print("=== Simple Calculator ===")
num1 = float(input("Enter first number: "))
op = input("Enter operator (+, -, *, /): ")
num2 = float(input("Enter second number: "))

if op == "+":
    result = num1 + num2
elif op == "-":
    result = num1 - num2
elif op == "*":
    result = num1 * num2
elif op == "/":
    if num2 == 0:
        print("Error: Cannot divide by zero!")
        exit()
    result = num1 / num2
else:
    print("Invalid operator!")
    exit()

print(f"Result: {result}")

Reading from Files (Advanced Preview)

# Read a text file
with open("data.txt", "r") as f:
    content = f.read()
    print(content)

# Write to a file
with open("output.txt", "w") as f:
    f.write("Hello from Python!\n")
    f.write("This is line 2.\n")

Handling Invalid Input Gracefully

while True:
    try:
        age = int(input("Enter your age: "))
        break
    except ValueError:
        print("❌ Please enter a valid integer.")

print(f"You entered: {age}")

print("Hello", "World", sep="---")

x = 5
y = 10
print(f"The sum of {x} and {y} is {x+y}.")

number = input("Enter a number: ")
print(number * 3)