Trouble importing files in Python

Question

I'm just finding out now that when importing a module, it seems to run through ALL the code, instead of just the one function that I want it to go through. I've been trying to find a way around this, but can't seem to get it. Here is what is happening.

#mainfile.py    
from elsewhere import something_else
number = 0
def main():
    print('What do you want to do? 1 - something else')
    donow = input()
    if donow == '1':
        something_else()
while 1:
    main()

#elsewhere.py
print('I dont know why this prints')
def something_else():
    from mainfile import number
    print('the variable number is',number)

Now, although this code KIND OF works the way I want it to, the first time when I initiate it, it will go to the main menu twice. For example: I start the program, press one, then it asks me what I want to do again. If I press one again, then it will print "the variable number is 0".

Once I get this working, I would like to be importing a lot of variables back and forth. The only issue is,if I add more import statements to "elsewhere.py" I think it will just initiate the program more and more. If I put "from mainfile import number" on line 1 of "elsewhere.py", I think this raises an error. Are there any workarounds to this? Can I make a different file? What if I made a class to store variables, if that is possible? I'm very new to programming, I would appreciate it if answers are easy to read for beginners. Thank you for the help.

Winston Ewert · Accepted Answer · 2014-06-27 01:49:02Z

2

As Jan notes, that's what import does. When you run import, it runs all of the code in the module. You might think: no it doesn't! What about the code inside something_else? That doesn't get run! Right, when the def statement is executed it creates a new function, but it doesn't run it. Basically, it saves the code for later.

The solution is that pretty much all interesting code should be in a function. There are a few cases which make sense to put at the top-level, but if in doubt, put it inside a function. In your particular case, you shouldn't be printing at the top level, if you need to print for some reason, put that into a function and call it when you need it. If you care when something happens, put it in a function.

On a second node, don't import your primary script in other scripts. I.e. if your mainfile.py directly, don't import that in other files. You can but it produces confusing results, and its really best to pretend that it doesn't work.

Don't try to import variables back and forth. Down that path lies only misery. You should only be importing things that don't change. Functions, classes, etc. In any other case, you'll have hard time making it do what you want.

If you want to move variables between places, you have other options:

Pass function arguments
Return values from a function
Use classes

I'll leave it is an exercise to the reader to learn how to do those things.

answered Jun 27, 2014 at 1:49

Winston Ewert

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2 Comments

Kyle Me Over a year ago

Thank you. I appreciate all the notes. I'm trying to make a game, but I'm having trouble keep track of the variables throughout different files. I'm using the "global" statement, but that isn't helping me very much either. I can just make the game in one file, but it's easy to get lost in the code. Function arguments and return values are a bit of a pain for me. I will try to learn classes to see if this fixes my issue.

Kyle Me Over a year ago

I think I fixed my issue by using classes. Thank you

Jan Vlcinsky · Accepted Answer · 2014-06-27 01:30:59Z

1

`import` executes imported code

import simply takes the Python source file and executes it. This is why it prints, because that instruction is in the code and with import all the instructions get exectued.

To prevent execution of part of imported package/module, you shall use the famous:

if __name__ == "__main__":
    print("I do not print with `import`")

Note, that this behaviour is not new in Python 3, it works the same way in Python 2.x too.

answered Jun 27, 2014 at 1:30

Jan Vlcinsky

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