What Does `if __name__ == '__main__':` Do in Python and Why Is It Important?

What Does name == "__main__": Do and Why Is It Necessary?

When a Python script is executed as the main program, the interpreter assigns the special variable __name__ to "__main__". If the script is imported as a module by another program, __name__ is instead set to the name of the importing module.

To understand why this is important, consider the following example:

# Suppose this is foo.py.

print("before import")
import math

print("before function_a")
def function_a():
    print("Function A")

print("before function_b")
def function_b():
    print("Function B {}".format(math.sqrt(100)))

print("before __name__ guard")
if __name__ == '__main__':
    function_a()
    function_b()
print("after __name__ guard")

Execution Flow:

1. Special Variables Setup:

   • name is set to "__main__" since foo.py is run as the main program.

2. Code Execution:

   • The script executes all of its code line by line:

     • Prints "before import", loads the math module, etc.
     • Defines the functions function_a and function_b.
   • The name check is evaluated: since it matches "main", all code within the if block is executed (printing "Function A" and "Function B 10.0").
   • The code below the name check is executed (printing "after name__ guard").

Importance of name Check:

This check is crucial because:

• Protection from Accidental Invocation: If foo.py doesn't have the name check, importing it as a module would trigger the script to run using the importing script's command line arguments, which is usually undesired.
• Pickle File Issue: If foo.py has custom classes saved to a pickle file, unpickling it in another script would import foo.py and execute its code without the name check, leading to the above issues.

Additional Notes:

• You can have multiple name checks in a script, but it's uncommon.
• Running the script foo2.py with the name checks included results in "a1", "m1", "a2", "b", "a3", "m2", "t2", while removing the name check results in "a1", "a2", "b", "a3", "t2".
• Running foo3.py as a script results in "t1", "a1", "a2", "b", "a3", "t2", while importing it as a module results in nothing being executed.
• Setting __name__ to "__main__" in foo4.py (even when imported as a module) means the script will always run when imported, as __name__ == __main__ will always evaluate to True.

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