Eugene Toder added the comment:

namedtuple._replace() actually doesn't call subclass' __new__. It calls 
tuple.__new__ directly, so it has the same problem as datetime classes.

Parameter and Signature are new in 3.3. I'm not sure if they're expected to be 
used as base classes.

@r.david.murray: is that contract specified anywhere? The doc says "Return a 
datetime with the same attributes, except for those attributes given new values 
by whichever keyword arguments are specified." This doesn't explicitly mention 
subclasses, but also doesn't mention the possibility of discarding any 
attribute values.

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Python tracker <rep...@bugs.python.org>
<http://bugs.python.org/issue20371>
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