Python Program Structure

Python Program Structure#

Note

Source: Adapted from the C# edition (data/programstructure.rst). The if __name__ == '__main__': pattern and indentation-as-syntax discussion are Python-specific original additions.

A Python program file — a .py file — is much simpler in structure than the equivalent in C# or Java.

No Required Wrapper#

In C#, every statement must live inside a class and a Main method:

// C# — not Python
using System;
class Hello {
    static void Main() {
        Console.WriteLine("Hello, world!");
    }
}

In Python, you simply write statements at the top level of the file:

print("Hello, world!")

There is no required class, no Main, no using directive.

The if __name__ == '__main__': Pattern#

When Python runs a file directly (python3 filename.py), it sets a special variable called __name__ to the string '__main__'. When the same file is imported by another file as a module, __name__ is set to the module’s name instead.

This lets you write files that work both as standalone programs and as reusable modules:

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

if __name__ == '__main__':
    greet("world")

  • If you run this file directly, __name__ == '__main__' is true and greet("world") is called.

  • If another file does import greet_module, the if block is skipped and only the greet function is made available.

This pattern is a best practice once you start writing functions you want to reuse. The earlier chapters do not require it, but you should adopt it as soon as your programs grow beyond a few lines.

Indentation#

Python uses indentation to show the structure of a program — which statements belong to a function, loop, or condition. Unlike C# which uses braces {}, Python requires consistent indentation.

def double(x):
    return x * 2      # indented — part of the function

print(double(5))      # not indented — not part of the function

The standard is 4 spaces per level. Never mix tabs and spaces.

A missing or wrong indentation level is a syntax error:

def double(x):
return x * 2   # IndentationError: expected an indented block

Comments#

A comment starts with # and runs to the end of the line. Python ignores comments completely:

HEIGHT = 8          # ceiling height in feet
area = length * width   # calculate floor area

Use comments to explain why, not what — the code itself should make what it does clear.

Statement Endings#

Python statements do not end with a semicolon. The end of a line ends the statement. If you need to continue a statement across lines, enclose the expression in parentheses:

result = (first_number
          + second_number
          + third_number)

You can also use a backslash \ to continue a line, but parentheses are preferred.