Anything Else

In python a module can be imported multiple times, and only at the first import will it be really imported, and subsequently the same module will be returned. This plays a very important role in python, as all kind of singleton class patter or module level initializations that are to be guaranteed to executed only once, relies on this behavior. But this breakdowns when a module can be referenced via two different path. Here is a demonstration:

>>> from path import path
>>> path(".").abspath()
path(u'C:\\Documents and Settings\\amitu')
>>> path("t").mkdir()
>>> import os, sys
>>> os.chdir("t")
>>> path(".").abspath()
path(u'C:\\Documents and Settings\\amitu\\t')
>>> sys.path += [path("..")]
>>> file("t2.py","w").write("""
... print "loading module t2.py"
... """)
>>>
>>> import t2
loading module t2.py
>>> file("__init__.py", "w").write("")
>>> import t.t2
loading module t2.py
>>> from t import t2
>>> sys.modules["t2"]
<module 't2' from 't2.py'>
>>> sys.modules["t.t2"]
<module 't.t2' from 'C:\Documents and Settings\amitu\t\t2.pyc'>
>>>

As you can see the module is loaded twice, with different names. I discovered this when my signal handler was getting called twice in a django project. Further discussion here.  

To avoid this situation, be consistent when importing a module about its path. Either always use package_name.module_name or module_name. Or just make sure pythonpath never contains both a folder and its descendent.