Eric Snow added the comment:
A NamedTuple ABC doesn't have to define any API (so that part could wait?)[1].
I see it as most useful for isinstance checks. Here's a solution along those
lines:
class NamedTuple(Sequence):
@classmethod
def __subclasshook__(cls, C):
if cls is NamedTuple:
if any("_fields" in B.__dict__ for B in C.__mro__):
return True
return NotImplemented
def namedtuple(...):
...
NamedTuple.register(result)
return result
(or include NamedTuple in the bases in the template)
For structseq support:
class NamedTuple(Sequence):
@classmethod
def __subclasshook__(cls, C):
if cls is NamedTuple:
if any("_fields" in B.__dict__ or # for namedtuple
"n_fields" in B.__dict__ # for structseq
for B in C.__mro__):
return True
return NotImplemented
[1] I agree there is still a problem if we define an ABC here with no API and
then add some later. Then classes that inherit from NamedTuple would break
later when we add an API (just `_fields`?). Not sure how much that is a
concern though. Then again we could just close out issue1820 and actually
establish the API.
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<http://bugs.python.org/issue7796>
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