4.2 Structure of a Python Program

1.1 What Is the Structure of a Python Program?

The structure of a Python program means the way the code is organized in a file.

A small Python file may contain only one line:

print("Hello")

As the program becomes larger, organizing the code makes it easier to read, test, and update.

1.2 A Common Python File Layout

Many Python files follow this order:

1. Imports
2. Constants and configuration
3. Functions
4. Classes, when needed
5. The main program

1.3 Imports

An import loads code that you want to use. In this example, platform is a standard Python module:

import platform

print(platform.system())

Imports are usually placed at the top of the file so readers can quickly see what the program needs.

1.4 Constants and Configuration

Constants are values that normally do not change while the program runs. Python developers commonly write constant names in uppercase:

MAX_RETRIES = 3
DEFAULT_REGION = "us-east-1"

Do not store passwords, API keys, or other secrets directly in a Python file.

1.5 Functions

A function is a named group of instructions. It runs when you call it.

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


greet_user("Alex")

Defining a function does not run its instructions immediately. The function runs only when you call it.

1.6 The main() Function

The main() function usually contains the main steps of a script:

def main():
    print("Checking the server")


main()

Using a main() function keeps the starting point of the program clear.

1.7 The if __name__ == "__main__": Check

This common Python line means: run the following code only when this file is started directly.

def main():
    print("The script is running")


if __name__ == "__main__":
    main()

The check is useful because another Python file can import your functions without automatically starting the whole script.

1.8 How Python Runs the File

Python reads top-level code from top to bottom. It creates functions when it reaches their definitions, then runs the code that calls them.

print("1. Start")


def show_message():
    print("2. Inside the function")


print("3. Before the function call")
show_message()

The output is:

1. Start
3. Before the function call
2. Inside the function

1.9 Write Readable Python

Good structure helps other people understand your code.

• Use clear names such as server_count instead of x.
• Keep functions small and focused.
• Use four spaces for indentation.
• Put imports at the top.
• Keep configuration separate from the main logic.

1.10 Practice Exercise

Create a file named server_check.py:

import platform


def show_server_info():
    print(f"Operating system: {platform.system()}")
    print("Server check complete")


def main():
    show_server_info()


if __name__ == "__main__":
    main()

Run it with:

python3 server_check.py

On Windows, use:

python server_check.py

Quick Interview Answer

A Python file commonly contains imports, configuration values, functions, classes when needed, and a main program section. The if __name__ == "__main__": check runs the main function only when the file is executed directly. This allows the same file to work as both a script and an importable module.

Key Takeaways

• Organize Python code into clear sections.
• Put imports near the top of the file.
• Use functions to group reusable work.
• Use main() as the clear starting point of a script.
• Use the __main__ check to prevent code from running during import.
• Clear structure makes Python scripts easier to read and test.