Nick Coghlan <ncogh...@gmail.com> added the comment:
NotImplemented is a speed and maintainability hack - the runtime cost and
additional code complexity involved in doing the same operator signalling via
exceptions would be prohibitive (check Objects/abstract.c in the CPython source
if you want the gory details).
As far as an implementation of @total_ordering that correctly handles
NotImplemented goes, yes, I absolutely agree we should do this correctly. The
fact that it is *hard* is an argument in *favour* of us getting it right, as
there is a decent chance that manually written comparison operations will also
stuff it up.
That said, I don't think sane_total_ordering quite gets the semantics right,
either.
Some helper functions in the closure would let the existing lambda functions be
updated to do the right thing (and I believe the semantics I have used below
are the correct ones for handling NotImplemented in @total_ordering). (I
haven't actually run this code as yet, but it should give a clear idea of what
I mean)
def not_op(op, other):
# "not a < b" handles "a >= b"
# "not a <= b" handles "a > b"
# "not a >= b" handles "a < b"
# "not a > b" handles "a <= b"
op_result = op(other)
if op_result is NotImplemented:
return op_result
return not op_result
def op_or_eq(op, self, other):
# "a < b or a == b" handles "a <= b"
# "a > b or a == b" handles "a >= b"
op_result = op(other)
if op_result:
# Short circuit OR, as op is True
# NotImplemented is also passed back here
return op_result
return self.__eq__(other)
def not_op_and_not_eq(op, self, other):
# "not (a < b or a == b)" handles "a > b"
# "not a < b and a != b" is equivalent
# "not (a > b or a == b)" handles "a < b"
# "not a > b and a != b" is equivalent
op_result = op(other)
if op_result:
# Short circuit AND, as not_op is False
# NotImplemented is also passed back here
if op_result is NotImplemented:
return op_result
return not op_result
return self.__ne__(other)
def not_op_or_eq(op, self, other):
# "not a <= b or a == b" handles "a >= b"
# "not a >= b or a == b" handles "a <= b"
op_result = op(other)
if op_result is NotImplemented:
return op_result
if op_result:
return self.__eq__(other)
# Short circuit OR, as not_op is True
return not op_result
def op_and_not_eq(op, self, other):
# "a <= b and not a == b" handles "a < b"
# "a >= b and not a == b" handles "a > b"
op_result = op(other)
if op_result is NotImplemented:
return op_result
if op_result:
return self.__ne__(other)
# Short circuit AND, as op is False
return op_result
The conversion table then looks like:
convert = {
'__lt__': [
('__gt__',
lambda self, other: not_op_and_not_eq(self.__lt__, self, other)),
('__le__',
lambda self, other: op_or_eq(self.__lt__, self, other)),
('__ge__',
lambda self, other: not_op(self.__lt__, other))
],
'__le__': [
('__ge__',
lambda self, other: not_op_or_eq(self.__le__, self, other)),
('__lt__',
lambda self, other: op_and_not_eq(self.__le__, self, other)),
('__gt__',
lambda self, other: not_op(self.__le__, other))
],
'__gt__': [
('__lt__',
lambda self, other: not_op_and_not_eq(self.__gt__, self, other)),
('__ge__',
lambda self, other: op_or_eq(self.__gt__, self, other)),
('__le__',
lambda self, other: not_op(self.__gt__, other))
],
'__ge__': [
('__le__',
lambda self, other: not_op_or_eq(self.__ge__, self, other)),
('__gt__',
lambda self, other: op_and_not_eq(self.__ge__, self, other)),
('__lt__',
lambda self, other: not_op(self.__ge__, other))
]
}
----------
nosy: +ncoghlan
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<http://bugs.python.org/issue10042>
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