Why Does Python Execute Code at Import Time, and How Can I Prevent It?

Understanding Python Module Execution at Import Time and Its Mitigation

You encounter a peculiar issue when executing your Python program using batch.py. Upon importing the main module, an error occurs. This question explores why Python executes code at import time and offers a solution to mitigate this behavior.

The Underlying Problem

Python interprets keywords like class and def as executable statements. Importing a module triggers the immediate execution of these statements. In the case of batch.py, importing main results in the execution of main's code, causing the aforementioned error.

Solution: Executing Code Only When Necessary

The idiomatic approach to address this issue is to segregate code execution. The following practices enable you to define functions and classes while preventing their execution unless the module is run directly:

# Code to be executed regardless of import status
# (e.g., class and function definitions)

def main():
    # Code to be executed only when the module is run as the main program

if __name__ == "__main__":
    main()

By placing your executable code within the main function, Python only executes it when the module is run directly through its script name, such as "python main.py". This prevents unintended code execution during module imports, such as in the case of "python batch.py".

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