New submission from Josh Rosenberg <[email protected]>:
I don't really expect this to go anywhere until Python 4 (*maybe* 3.9 after a
deprecation period), but it seems like it would have been a good idea to make
NotImplementedType's __bool__ explicitly raise a TypeError (rather than leaving
it unset, so NotImplemented evaluates as truthy). Any correct use of
NotImplemented per its documented intent would never evaluate it in a boolean
context, but rather use identity testing, e.g. back in the Py2 days, the
canonical __ne__ delegation to __eq__ for any class should be implemented as
something like:
def __ne__(self, other):
equal = self.__eq__(other)
return equal if equal is NotImplemented else not equal
Problem is, a lot of folks would make mistakes like doing:
def __ne__(self, other):
return not self.__eq__(other)
which silently returns False when __eq__ returns NotImplemented, rather than
returning NotImplemented and allowing Python to check the mirrored operation.
Similar issues arise when hand-writing the other rich comparison operators in
terms of each other.
It seems like, given NotImplemented is a sentinel value that should never be
evaluated in a boolean context, at some point it might be nice to explicitly
prevent it, to avoid errors like this.
Main argument against it is that I don't know of any other type/object that
explicitly makes itself unevaluable in a boolean context, so this could be
surprising if someone uses NotImplemented as a sentinel unrelated to its
intended purpose and suffers the problem.
----------
messages: 333421
nosy: josh.r
priority: normal
severity: normal
status: open
title: Make NotImplemented unusable in boolean context
type: behavior
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Python tracker <[email protected]>
<https://bugs.python.org/issue35712>
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