Question

In a big application I am working, several people import same modules differently e.g.
import x
or
from y import x
the side effects of that is x is imported twice and may introduce very subtle bugs, if someone is relying on global attributes

e.g. suppose I have a package mypakcage with three file mymodule.py, main.py and init.py

mymodule.py contents

l = []
class A(object): pass

main.py contents

def add(x):
    from mypackage import mymodule
    mymodule.l.append(x)
    print "updated list",mymodule.l

def get():
    import mymodule
    return mymodule.l

add(1)
print "lets check",get()

add(1)
print "lets check again",get()

it prints

updated list [1]
lets check []
updated list [1, 1]
lets check again []

because now there are two lists in two different modules, similarly class A is different
To me it looks serious enough because classes itself will be treated differently
e.g. below code prints False

def create():
    from mypackage import mymodule
    return mymodule.A()

def check(a):
    import mymodule
    return isinstance(a, mymodule.A)

print check(create())

Question:

Is there any way to avoid this? except enforcing that module should be imported one way onyl. Can’t this be handled by python import mechanism, I have seen several bugs related to this in django code and elsewhere too.

Answer 1

I can only replicate this if main.py is the file you are actually running. In that case you will get the current directory of main.py on the sys path. But you apparently also have a system path set so that mypackage can be imported.

Python will in that situation not realize that mymodule and mypackage.mymodule is the same module, and you get this effect. This change illustrates this:

def add(x):
    from mypackage import mymodule
    print "mypackage.mymodule path", mymodule
    mymodule.l.append(x)
    print "updated list",mymodule.l

def get():
    import mymodule
    print "mymodule path", mymodule
    return mymodule.l

add(1)
print "lets check",get()

add(1)
print "lets check again",get()


$ export PYTHONPATH=.
$ python  mypackage/main.py 

mypackage.mymodule path <module 'mypackage.mymodule' from '/tmp/mypackage/mymodule.pyc'>
mymodule path <module 'mymodule' from '/tmp/mypackage/mymodule.pyc'>

But add another mainfile, in the currect directory:

realmain.py:
from mypackage import main

and the result is different:

mypackage.mymodule path <module 'mypackage.mymodule' from '/tmp/mypackage/mymodule.pyc'>
mymodule path <module 'mypackage.mymodule' from '/tmp/mypackage/mymodule.pyc'>

So I suspect that you have your main python file within the package. And in that case the solution is to not do that. 🙂