Why is the value of __name__ changing after assignment to sys.modules[__name__]?

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This happens because you have overwrite your module when you did sys.modules[__name__] = _test() so your module was deleted (because the module didn’t have any references to it anymore and the reference counter went to zero so it’s deleted) but in the mean time the interpreter still have the byte code so it will still work but by returning None to every variable in your module (this is because python sets all the variables to None in a module when it’s deleted).

class _test(object): pass

import sys
print sys.modules['__main__']
# <module '__main__' from 'test.py'>  <<< the test.py is the name of this module
sys.modules[__name__] = _test()
# Which is the same as doing sys.modules['__main__'] = _test() but wait a
# minute isn't sys.modules['__main__'] was referencing to this module so
# Oops i just overwrite this module entry so this module will be deleted
# it's like if i did:
#
#   import test
#   __main__ = test
#   del test
#   __main__ = _test()
#   test will be deleted because the only reference for it was __main__ in
#   that point.

print sys, __name__
# None, None

import sys   # i should re import sys again.
print sys.modules['__main__']
# <__main__._test instance at 0x7f031fcb5488>  <<< my new module reference.

EDIT:

A fix will be by doing like this:

class _test(object): pass

import sys
ref = sys.modules[__name__]  # Create another reference of this module.
sys.modules[__name__] = _test()   # Now when it's overwritten it will not be
                                  # deleted because a reference to it still
                                  # exists.

print __name__, _test
# __main__ <class '__main__._test'>

Hope this will explain things.

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