Python Morsels: Making a main function in Python

Transcript

Let's talk about creating a main function in Python.

Many programming languages have the notion of a main function (or a main method), which acts as the entry point for a program. Python does not have main functions.

Python runs all the code in a Python script

We have a Python script called greet.py:

import random

salutations = ["Hello", "Hey", "Hi", "Hiya", "Howdy"]


def greet():
    """Print a salutation."""
    print(random.choice(salutations))


greet()

When we run greet.py from our system command-line, Python runs all of the code in this file:

$ python3 greet.py
Hi

But what if we import this Python file?

Python runs all the code in a Python module

If we import our greet module, Python also runs all of the code in our file:

>>> import greet
Hey

We're seeing "Hey" printed out at import time because Python ran our greet function (which we called at the bottom of our module):

greet()

It's very weird to see something printed out when we just import a module.

This greet.py file wasn't actually meant to be used as a module. This file wasn't meant to be imported; it was meant to be run as a script.

A Python module and a Python script all in one Python file

What if we wanted to make one .py file that could both be imported as a module and could be used as a Python script (by being run from the command-line)?

This version of our greet.py file can be run as a program to print out a random greeting:

$ python3 greet.py
Howdy
$ python3 greet.py
Hiya

But if we import it as a module, it doesn't do anything (except give us access to functions and whatever else is in that module):

>>> import greet
>>> greet.greet()
Hiya

How is this possible in Python?

Making a module/script hybrid by checking the __name__ variable

It turns out there's a way to ask, are we being run from the command-line. Or put another way, is our current Python module the entry point to our Python process? This question can be asked using the statement __name__ == "__main__".

This version of greet.py works as both a command-line script and an import-able module:

import random

salutations = ["Hello", "Hey", "Hi", "Hiya", "Howdy"]


def greet():
    """Print a salutation."""
    print(random.choice(salutations))


if __name__ == "__main__":
    greet()

Every module has a __name__ variable, and by default. The __name__ variable in each module is a string representing the name of that module:

>>> import greet
>>> greet.__name__
'greet'

That's the case if we're importing a module.

But what if we run that our greet.py file from the command-line?

In that case, __name__ is not going to be the name of that module. Instead, it's going to be "__main__". That's why running greet.py also runs the greet function:

$ python3 greet.py
Hiya

This block of code is really asking the question are we the entry point to our Python program (are we being run from the command-line rather than being imported as a module)?

if __name__ == "__main__":
    greet()

If we are being run from the command-line then we run our greet function.

The main function isn't special, it's just another name

You may sometimes see Python programs that have a main function:

import random

salutations = ["Hello", "Hey", "Hi", "Hiya", "Howdy"]


def greet():
    """Print a salutation."""
    print(random.choice(salutations))


if __name__ == "__main__":
    greet()

Python doesn't know about main functions, but there's nothing stopping us from making a function called main that we only call if we're being run from the command-line.

Summary

If you need to make a single Python file that can both be used as a module (being imported) and can be run as a Python script to do something, you can check the variable __name__ in your module to see whether it's equal to the string "__main__".

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