Why does 'from file import function_name' fail in my interactive Python session but works in a script?

Question

I am moving common functions shared between a couple of Python files in to a third common_commands.py file. To test the functions I imported a couple of functions using:

from common_commands import func_one, func_two

But got this error:

ImportError: cannot import name 'func_two'

So I tried only importing func_one and that worked fine but then importing just func_two gives me the same error again! Why?! And to make things even confusing, the exact same import line from above works perfectly fine when I put it in the scripts I am refactoring.

What is causing this odd behavior?

Answer 1

TL;DR: I had renamed func_two since starting my interactive shell. Starting a new shell got things working.

What I learned:

I don't understand all the inner workings of the interactive shell and what happens when you call import, but after quitting and starting a new shell the exact same import call worked.

When I started the shell func_two was old_func_two but then I decided to rename it, and then I tried to import it with the new name, and that failed. After scratching my head and doing some google foo I found nothing that helped in my case and tried starting a new shell and it worked!

So I decided to do a little more experimenting before asking this question and learned that I could rename the function as much as I wanted after starting the shell but only until I first imported the file in some way.

That is to say, as soon as I called from common_commands import func_one I can no longer rename any functions and import them with the new name, since the file has already been imported. I can, however, still import old_func_two . I also tried changing the 'guts' of func_two after importing it and then importing it again and it kept the original behavior. So from what I can tell, the first time you import a file (not a function or class, but the whole file) it is cached and all future imports are run on the cached version, not the real file on disk.

So, even if you only import func_one ie from common_commands import func_one and then rename or change func_two and then import it, you'll have to use the original name of func_two and you'll get the original functionality as well, even though you didn't explicitly import it earlier.