What's in a __name__?

Overview

Teaching: 10 min
Exercises: 10 min

Questions

• How do modules know when they are executing as a program, or being imported?

• What does if __name__ == "__main__" do?

Objectives

• Understand the meaning and use of __name__.

• Be able to use if __name__ == "__main__" when required in your programs.

__name__ is a string containing the module name

Recall from the modules episode that modules are just Python files. Every module in Python has the special attribute __name__. When a module is imported, __name__ for that module will be set to the module name. Let’s have a look at this in action.

Print the __name__ of the math and sys modules

In an interactive Python interpreter, import the math and sys modules and print their __name__ strings.

Try importing a module with an alias and then print that name string. Does it show the alias name?

Solution

$ python3
Python 3.6.7 (default, Oct 22 2018, 11:32:17) 
[GCC 8.2.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>> import math
>> print(math.__name__)
math
>> import sys
>> print(sys.__name__)
sys
>> import csv as c
>> print(c.__name__)
csv

Note that in each case, the value of __name__ is the same as the module name. The alias does not change this.

There’s Always an Exception to the Rule

Printing the Exception to the Rule

Create a new Python script called “name.py”. In it, print the value of __name__. When you run the script, do you see the module (file) name?

Solution

No. You should see the special value “main”.

There is one special condition where __name__ does not equal the module name. For Python files (recall that a Python file is a module) directly executed by Python, __name__ gets a special value of “main”, indicating that the current module (file) is the main program.

So, about that if __name__ == "__main__"?

In short, if __name__ == "__main__" is used for code that should only run when a Python script is running as the main module.

How is this helpful?

Create and run a short program called “my_useful_program.py” with the following code in it:

def my_useful_function():
    return "is my_useful_function useful?"

print(my_useful_function())

After using this program for a while, you decide that my_useful_function() is so useful that you want to import it into another program to use there. To emulate this, open a Python interpreter and import the function:

from my_useful_program import my_useful_function

Can you explain what just happened?

Solution

When a module is imported, all the code gets executed once. So the code in the file that makes sense when running “my_useful_program.py” as a program still runs when the function is imported. A lot of the time this is not what we want.

Key Points

• __name__ is a string containing the module name.

• For files directly executed by Python, __name__ gets the special value “main”, indicating that this is the main file.

• The if __name__ == "__main__" expression prevents code from running when the module is being imported.