The Main Program

Describing Methods for File Execution

• There are two way in which a source file can execute:

  1. Executing code in its own namespace

     • This is executed as a library module
     • This refers to executing import statement

  2. Executing code as the main program

     • This occurs when we supply the program as the script name to the interpreter
     • This refers to executing python3 spam.py

Describing Method 2: Executing as __main__

• Each module defines a variable __name__
• This variable contains the module name
• Programs can examine this variable to determine the module in which they are executing

• The top-level module of the interpreter is named __main__

  • Programs specified on the command line run inside the __main__ module

# Check if running as a program
>>> if __name__ == '__main__':
...     # Yes
...     statements
... else:
...     # No, I must have been imported as a module
...     statements

• Typically, libraries use this technique for including optional testing or example code
• For example, suppose we're developing a a module
• We can put code for testing the features of our library inside the if block
• This code won't run for users who import our library

References

• Python Essential Reference