Program Structure of a Python Program

Unlike C/Java, Python has no mandatory main function, no semicolons, and uses indentation instead of braces to define blocks.

# 1. Comments / documentation
"""
This program demonstrates the structure of a Python program.
"""

# 2. Import statements
import math

# 3. Global variable / constant
PI = 3.14159

# 4. Function definitions
def area_of_circle(radius):
    return PI * radius ** 2

# 5. Main logic / entry point
if __name__ == '__main__':
    r = 5
    print("Area:", area_of_circle(r))

Key structural rules

• Indentation defines blocks — typically 4 spaces. Inconsistent indentation raises IndentationError.
• No semicolons required — a newline ends a statement (semicolons are optional to put multiple statements on one line).
• Case sensitive — Value and value are different identifiers.
• if name == 'main': — the conventional entry point; code here runs only when the file is executed directly, not when imported as a module.

# Multiple statements on one line using semicolon (allowed but discouraged)
a = 1; b = 2; print(a + b)

# Indentation error example
if True:
print("This will raise IndentationError")   # missing indent

Comments in Python

# Single-line comment

"""
Multi-line comment /
docstring (also used for documentation)
"""

'''
Alternative multi-line
comment style with single quotes
'''