Structure of a Python script¶

There is a standard structure recommended for Python scripts:

imports
constants
functions and classes
__main__ block

Example¶

The following program contains most of these elements

from pprint import pprint

MESSAGE = "Good morning {}. Nice to see you!"

def hello(name):
    """Writes a hello world message"""
    msg = MESSAGE.format(name)
    pprint(msg)

if __name__ == '__main__':
    name = input("please enter your name: ")
    hello(name)

Imports¶

All import statements should be grouped together at the beginning of a Python file. If you have lots of imports, a standard for sorting them might be useful. The tools isort and black help with that.

Constants¶

In Python, constants do not exist as a syntax element. But it is a good practice to group variables “that the programmer does not intend to change”.

These constants should be written in capitals to distinguish them from variables that change during their lifetime:

PI = 3.14159

Function and class definitions¶

Functions and classes should be defined in nested-first-order. That means functions called by other functions go first, functions called from the main program only go last.

The main block¶

At the end of the program, there is a strange construct: The if __name__ == '__main__': block indicates the main program.

The __main__ block is only executed when you run the entire program with python my_program.py (or the equivalent in your editor). However, it is not executed when you import functions or classes from it. This way you can have a main program and reusable functions in the same file.

It you can keep an entire code block here, or only call a main() function. Sometimes the __main__ block is used for other things (e.g. test code).