Jason R. Coombs added the comment:
I recently ran into this error again. I was writing this class to provide
backward-compatible context manager support for zipfile.ZipFile on Python 2.6
and 3.1:
class ContextualZipFile(zipfile.ZipFile):
"""
Supplement ZipFile class to support context manager for Python 2.6
"""
def __enter__(self):
return self
def __exit__(self, type, value, traceback):
self.close()
def __new__(cls, *args, **kwargs):
"""
Construct a ZipFile or ContextualZipFile as appropriate
"""
if hasattr(zipfile.ZipFile, '__exit__'):
return zipfile.ZipFile(*args, **kwargs)
return super(ContextualZipFile, cls).__new__(cls, *args, **kwargs)
At the point where super is called, the author is unaware of the details of the
function signature for zipfile.ZipFile.__new__, so simply passes the same
arguments as were received by the derived class. However, this behavior raises
a DeprecationWarning on Python 2.6 and 3.1 (and would raise an error on Python
3.2 if the code allowed it).
What's surprising is that the one cannot simply override a constructor or
initializer without knowing in advance which of those methods are implemented
(and with what signature) on the parent class.
It seems like the construction (calling of __new__) is special-cased for
classes that don't implement __new__.
What is the proper implementation of ContextualZipFile.__new__? Should it use
super but omit the args and kwargs? Should it call object.__new__ directly?
Should it check for the existence of __new__ on the parent class (or compare it
to object.__new__)?
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Python tracker <[email protected]>
<http://bugs.python.org/issue1683368>
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