[FIXED] Python program importing itself

Issue

I made a file named challenge.py with this code in it:

import challenge

def main():
     print('Circular much...')

challenge.main()

From this I was expecting python to raise an error due to the circular import of importing the file which is running but I found that on python 3.7 & 3.8 this file runs and prints out Circular much... twice. I would understand once as that would mean that the rest of the file when it is importing itself isn't running and I would understand a recursion error as it ran challenge.main() infinitely down the stack but I don't understand why it prints it twice and stops?

Solution

Tracing this through:

import challenge

Ok, we'll import challenge.py. Here we go...

    import challenge

We're already importing challenge.py, so we won't do it again.

    def main():
         print('Circular much...')

Defined the function main() in the namespace challenge. Cool.

    challenge.main()

Now call the function main() in the namespace challenge. That prints Circular much.... There's your first print.

Now we're back in the main module again.

def main():
     print('Circular much...')

This defines the function main() in the global namespace (which never gets called).

challenge.main()

This calls the function main() in the namespace challenge, again printing Circular much....

And we're done. Two prints of your message.

Does that help?

Answered By - Fred Larson