Why is Python running my module when I import it and how do I stop it

Question

I have a Python program I m building that can be run in either of 2 ways  the first is to call  python main py  which prompts the user for input in a friendly manner and then runs the user input through the program   The other way is to call  python batch py -file-  which will pass over all the friendly input gathering and run an entire file s worth of input through the program in a single go   The problem is that when I run  batch py  it imports some variables methods etc from  main py   and when it runs this code   import main   at the first line of the program  it immediately errors because it tries to run the code in  main py    How can I stop Python from running the code contained in the  main  module which I m importing

User · Accepted Answer

Because this is just how Python works - keywords such as class and def are not declarations. Instead, they are real live statements which are executed. If they were not executed your module would be .. empty :-)

Anyway, the idiomatic approach is:

# stuff to run always here such as class/def
def main():
    pass

if __name__ == "__main__":
   # stuff only to run when not called via 'import' here
   main()

See What is if __name__ == "__main__" for?

It does require source control over the module being imported, however.

Happy coding.

User · Answer

There was a Python enhancement proposal PEP 299 which aimed to replace if   name         main     idiom with def   main     but it was rejected  It s still a good read to know what to keep in mind when using if   name        main

User · Answer

Use the if   name         main    idiom --   name   is a special variable whose value is    main    if the module is being run as a script  and the module name if it s imported  So you d do something like    imports   class function definitions if   name         main           code here will only run when you invoke  python main py

User · Answer

Due to the way Python works  it is necessary for it to run your modules when it imports them   To prevent code in the module from being executed when imported  but only when run directly  you can guard it with this if   if   name         main           this won t be run when imported   You may want to put this code in a main   method  so that you can either execute the file directly  or import the module and call the main    For example  assume this is in the file foo py   def main        print  Hello World   if   name         main         main     This program can be run either by going python foo py  or from another Python script   import foo       foo main

User · Answer

Unfortunately  you don t  That is part of how the import syntax works and it is important that it does so -- remember def is actually something executed  if Python did not execute the import  you d be  well  stuck without functions   Since you probably have access to the file  though  you might be able to look and see what causes the error  It might be possible to modify your environment to prevent the error from happening

User · Answer

Try just importing the functions needed from main py  So   from main import SomeFunction   It could be that you ve named a function in batch py the same as one in main py  and when you import main py the program runs the main py function instead of the batch py function  doing the above should fix that  I hope

User · Answer

Put the code inside a function and it won t run until you call the function   You should have a main function in your main py   with the statement   if   name         main       main     Then  if you call python main py the main   function will run   If you import main py  it will not   Also  you should probably rename main py to something else for clarity s sake

User · Answer

You may write your  main py  like this      usr bin env python    all     somevar    do something    somevar     def do something        pass  blahblah  if   name       main         do something

User · Answer

Although you cannot use import without running the code  there is quite a swift way in which you can input your variables  by using numpy savez  which stores variables as numpy arrays in a  npz file  Afterwards you can load the variables using numpy load    See a full description in the scipy documentation  Please note this is only the case for variables and arrays of variable  and not for methods  etc

[python] Why is Python running my module when I import it, and how do I stop it?

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