Your First Python Program

Writing your first Python program introduces you to the fundamental workflow: write code in a .py file, then execute it with the Python interpreter. This topic covers script creation, execution, command-line arguments, and the script vs. module pattern.

Creating and Running a Script

Python scripts are plain text files with the .py extension. You can create them with any text editor or IDE.

• Create a file named hello.py and write your code.
• Run it from the terminal: python3 hello.py.
• On Unix systems, add a shebang line (#!/usr/bin/env python3) at the top and make it executable with chmod +x hello.py.

The print() Function

print() is the most basic output function. It converts arguments to strings and writes them to stdout.

• print("Hello") — print a string
• print("A", "B", "C") — print multiple values separated by spaces
• print("A", "B", sep="-") — custom separator
• print("Hello", end="!") — custom line ending instead of newline
• print(f"Name: {name}") — f-string formatting

Command-Line Arguments

Scripts can accept arguments from the command line using the sys.argv list or the more powerful argparse module.

The if __name__ == '__main__' Pattern

This is one of Python's most important patterns. When a Python file is run directly, __name__ is set to '__main__'. When imported as a module, it's set to the module name. This allows a file to serve as both a script and a reusable module.

Common Pitfalls

• Forgetting the file extension: Python files must end with .py to be recognized by the interpreter.
• Running from wrong directory: Make sure you're in the same directory as your script, or provide the full path.
• Not using if __name__ == '__main__': Code that runs on import can cause unexpected side effects when the module is imported elsewhere.