Question

I just recognized that imp.reload() does not delete old classes and functions if they were deleted from the module’s source file.

An Example:

:~$ python3
Python 3.2.3 (default, May  3 2012, 15:54:42) 
[GCC 4.6.3] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> print(open("test.py").read())
# empty file
>>> import test
>>> dir(test)
['__builtins__', '__cached__', '__doc__', '__file__', '__name__',
'__package__']
>>> print(open("test.py").read())                             
# new class A and B added
class A:                                                      
        pass                                                  

class B:                                                      
        pass

>>> import imp
>>> dir(imp.reload(test))
['A', 'B', '__builtins__', '__cached__', '__doc__', '__file__', '__name__',
'__package__']
>>> print(open("test.py").read())
# class A deleted
class B:
        pass

>>> dir(imp.reload(test))
['A', 'B', '__builtins__', '__cached__', '__doc__', '__file__', '__name__',
'__package__']
>>> import sys
>>> dir(sys.modules['test'])
['A', 'B', '__builtins__', '__cached__', '__doc__', '__file__', '__name__',
'__package__']
>>> sys.modules['test'].A
<class 'test.A'>

In the last lines you can see that there is a class object A although it was deleted from the modules source code. Why is this so? Is there a way to recognize such elements of a module?

Answer 1

According to the documentation:

If the new version of a module does not define a name that was defined by the old version, the old definition remains. This feature can be used to the module’s advantage if it maintains a global table or cache of objects — with a try statement it can test for the table’s presence and skip its initialization if desired:

try:
    cache
except NameError:
    cache = {}

So that’s why. If you don’t want those old objects there you can probably empty the module’s dictionary before reloading. For example, here I will import hashlib, empty its dictionary, and reload it.

import hashlib

for attr in dir(hashlib):
    if attr not in ('__name__', '__file__'):
        delattr(hashlib, attr)

hashlib = imp.reload(hashlib)

_{Poor hashlib.}