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An import gotcha in Python

Pankaj and I were stuck with this weird bug recently. We had a class A and a subclass B and an instance of B, b, was returning False for isinstance(b, A).

After some debugging, we found that it was a problem with imports, and the same module was being imported twice, using two different names – bar and baz.bar.

Here's a cooked-up example to demonstrate the "bug". We create a package baz, as shown below.

$ tree baz
baz
├── bar.py
├── foo.py
└── __init__.py

We have our two classes, A and B defined in the bar module.

class A(object):
    pass

class B(A):
    pass

We run foo.py and scratch our heads for a while, before figuring out what's wrong…

# We add the directory containing out package 'baz' to sys.path to be
# able to import using the package name.
import sys
sys.path.insert(1, '/tmp')

# We import A from bar module present in the same directory as foo
from bar import A
# We import B from bar, but refer to it, as a submodule of baz
from baz.bar import B

a = A()
b = B()

print "isinstance(b, A) -->", isinstance(b, A)

for cls in (A, B):
    print "%s -->" %cls, cls

for module in sys.modules:
    if 'bar' in module:
	print module, sys.modules[module]
isinstance(b, A) --> False
<class 'bar.A'> --> <class 'bar.A'>
<class 'baz.bar.B'> --> <class 'baz.bar.B'>
baz.bar <module 'baz.bar' from '/tmp/baz/bar.pyc'>
bar <module 'bar' from 'bar.pyc'>

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