Python program's "main()" not run on invocation

Question

I'm new to learning Python and using Eclipse + PyDev. I can't seem to figure out why my program isn't running.

Here is my code:

def main():
    print("Testing")
    test1 = float(input("Test1: "))
    test2 = float(input("Test2: "))
    test3 = float(input("Test3: "))
    calculate_cost (test1, test2, test3)

def calculate_cost (test1, test2, test3):
    print("Testing")

I am assuming I am missing something in one or both of the functions to make this run properly.

You never called `main()` in your code. – Ashwini Chaudhary Sep 29 '13 at 15:13 — Ashwini Chaudhary, Sep 29 '13 at 15:13

score 6 · Accepted Answer · edited May 23 '17 at 10:25

6

This is Python, not C/C++, main() is not the entry point to your program, you have to invoke the function manually. Just call it:

# your code
# ...
main()

If you want to prevent that your code is called when importing from other place, then you want to use:

# your code
# ...
if __name__ == '__main__':
    main()

This question: What does if __name__ == “__main__”: do? explains this last block.

edited May 23 '17 at 10:25

Community

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answered Sep 29 '13 at 15:14

jabaldonedo

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It might be helpful to speak to *why* people do the `__name__ == '__main__'` bit, so it's not taken as a for-libraries-only thing. (For instance, being able to use static-analysis tools that import modules to analyze them). – Charles Duffy Sep 29 '13 at 15:17
Sure Charles, indeed I was looking for a SO question which explains it. – jabaldonedo Sep 29 '13 at 15:18
Yeah I am wondering that too. – user1804933 Sep 29 '13 at 15:19

score 3 · Answer 2 · edited May 23 '17 at 11:56

3

Unlike some other languages, there is no enforcement of the convention that a main function is called when a program is executed. In Python, you will have to do that manually. That means you will have to put a main() at the bottom of your script to make your main function execute.

A common pattern is to use the following:

if __name__ == '__main__`:
    main()

This will execute the main function when the script is run directly, but prevents its execution when the script is imported as a module somewhere else. See this question for more details on that part.

edited May 23 '17 at 11:56

Community

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answered Sep 29 '13 at 15:15

poke

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Awesome thanks I didn't realize this was necessary. – user1804933 Sep 29 '13 at 15:19

Python program's "main()" not run on invocation

2 Answers2