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Make NotImplemented unusable in boolean context #79893
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MojoVampire commented
on Jan 10, 2019 MojoVampiremannequinMannequinAuthorI 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.
the canonical __ne__ delegation to __eq__ for any class should be implemented as something like
I disagree that your code snippet is the canonical way to write __ne__. I'd write it like this:
def __ne__(self, other): return not (self == other)
Or just not write it at all. Python will automatically call __eq__ going back to Python 2.4 (and possibly older).
Delegating to __eq__ directly is the wrong way to do it.
NotImplemented is a sentinel value that should never be evaluated in a boolean context
I don't see why not. I can't think of any use-cases where I would want to *specifically* use NotImplemented in a boolean context, but I see no good reason why I would want it to fail if one happened to do so.
This is a common mistake. Even the default implementation of object.__ne__ had a bug, fixed only 4 years ago in bpo-21408. There were several errors in stdlib. I am sure there is a lot of occurrences of this errors in a third party code.
So I like this idea. This can help to fix hidden errors in existing code. But I share also Josh's concerns.
There is related common mistake. In C code, the result of PyObject_IsTrue() often is treated as just a boolean, without checking for errors. Fortunately, in the current CPython code it is handled properly, but I am sure this error still is occurred in third-party extensions.
When these two errors (using NotImplemented in the boolean context and ignoring the error in PyObject_IsTrue() in the C code) meet, this can lead to manifestation of more weird bugs than treating NotImplemented as true: from a crash in debug build to raising an exception in the following unrelated code.
I suggest to start emitting a DeprecationWarning or a FutureWarning in NotImplemented.__bool__.
I agree.
On Thu, Jan 10, 2019 at 11:24 PM Serhiy Storchaka <report@bugs.python.org>
wrote:Serhiy Storchaka <storchaka+cpython@gmail.com> added the comment:
This is a common mistake. Even the default implementation of object.__ne__
had a bug, fixed only 4 years ago in bpo-21408. There were several errors
in stdlib. I am sure there is a lot of occurrences of this errors in a
third party code.So I like this idea. This can help to fix hidden errors in existing code.
But I share also Josh's concerns.There is related common mistake. In C code, the result of
PyObject_IsTrue() often is treated as just a boolean, without checking for
errors. Fortunately, in the current CPython code it is handled properly,
but I am sure this error still is occurred in third-party extensions.When these two errors (using NotImplemented in the boolean context and
ignoring the error in PyObject_IsTrue() in the C code) meet, this can lead
to manifestation of more weird bugs than treating NotImplemented as true:
from a crash in debug build to raising an exception in the following
unrelated code.I suggest to start emitting a DeprecationWarning or a FutureWarning in
NotImplemented.__bool__.----------
nosy: +gvanrossum, serhiy.storchaka
Python tracker <report@bugs.python.org>
<https://bugs.python.org/issue35712\>
--
--Guido (mobile)I consider it a nice feature of Python that all builtin objects, and, AFAIK (and Josh, apparently), all stdlib class instances, have a boolean value. (I am aware of numpy's element-wise behavior.) I hate to give this up. This is part of Python's general avoidance of singular exceptions and exceptions to exceptions. This proposal would be the latter: "An object is truthy, unless its class makes it false, unless it is NotImplemented and a TypeError."
If this exception is made, I expect that there will be proposals to extend the exception to other objects, such as Ellipsis.
MojoVampire commented
on May 8, 2019 MojoVampiremannequinMannequinAuthorI've submitted a PR for this. I did discover a use case for boolean evaluation while doing this in the functools.total_ordering helpers. While it's legitimate, it *looks* wrong (the code looks like it didn't consider the possibility of NotImplemented even though it did), and really only applies to the case total_ordering handles on behalf of most folks (implementing one comparator in terms of two others).
The specific case was:
def _le_from_lt(self, other, NotImplemented=NotImplemented): 'Return a <= b. Computed by @total_ordering from (a < b) or (a == b).' op_result = self.__lt__(other) return op_result or self == other
(with a similar implementation for _ge_from_gt). It happens to work because the return value of __lt__ is used directly for both True and NotImplemented cases, with only False delegating to equality. It's not at all clear that that's correct at first glance though, and the fix to avoid the warning is simple, matching the other 10 comparator helpers by explicit checking for NotImplemented via:
if op_result is NotImplemented: return NotImplemented
That's about the strongest case I can find for "legitimate" use of NotImplemented in a boolean context, so I figured I'd point it out explicitly so people knew it existed.
If that makes an eventual TypeError a non-starter, I'd still like that usage to emit a warning (e.g. a RuntimeWarning) simply because the utility of that one little shortcut pales in comparison to the damage done by the *many* misuses that occur.
MojoVampire commented
on Jul 26, 2019 MojoVampiremannequinMannequinAuthorMoving to 3.9 target, per Serhiy's request. PR has been rebased against master, including updating the What's New info to be in the 3.9 version, not 3.8.
MojoVampire commented
on Aug 7, 2019 MojoVampiremannequinMannequinAuthorIn the docs for my PR, I mention using NotImplemented in a boolean context is deprecated, raising DeprecationWarning, and will raise TypeError in a future version of Python. Serhiy has suggested the end state might be RuntimeWarning instead of TypeError. I'm in favor of it ending up as an error, but not rigid on it. Does anyone else have a strong opinion among the following options?
- DeprecationWarning, documented as eventual TypeError (current PR)
- DeprecationWarning, documented as eventual RuntimeWarning
- Immediate RuntimeWarning, no DeprecationWarning period (since a warning only causes stderr output by default, it doesn't prevent working code from working unless warnings are explicitly configured to become exceptions)
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I am sad that such code (as well as my former code in total_ordering) no longer works, but this is just a clever trick, and the code can be written in less clever but more explicit way.
On other hand, the warning helps to catch common mistakes like
not self.__lt__(other)
self.__lt__(other) or self.__eq__(other)
super().__eq__(other) and self.x == other.xEven non-beginners often make such kind of mistakes, and it is hard to catch them if you have not trained yourself specially. I suggest you to include lessons about writing complex comparison methods in your course for advanced users.
The following code now leads to a
DeprecationWarning, but I am unclear why it should.>>> from enum import Enum >>> >>> class CardColour(Enum): ... """Enumeration of the two colours in a pack of cards.""" ... ... BLACK = 'black' ... RED = 'red' ... ... def other_colour(self): ... """Given one colour, get the other one.""" ... return next(filter(self.__ne__, CardColour)) ... >>> CardColour.BLACK.other_colour() <input>:5: DeprecationWarning: NotImplemented should not be used in a boolean context <CardColour.RED: 'red'>If I change the last line of
other_colourto eitherreturn next(colour for colour in CardColour if colour != self)orreturn next(colour for colour in CardColour if colour is not self), the warning goes away.Is this intended behaviour?
vedgar commented
on Sep 20, 2021 vedgarmannequinMannequinPlease see the message https://bugs.python.org/issue35712#msg349303. Filtering with those dunder sesqui-dispatch methods really is a bug magnet.
Thanks, Vedran. I read https://bugs.python.org/issue35712#msg349303 before adding my message, but am not quite clear why my snippet is the same situation.
next(filter((2).__eq__, 'text'))surely returns 't' because(2).__eq__('t')returnsNotImplemented, andNotImplementedis truthy. (Apologies if my understanding is wrong here.)I'm unclear, however, why
x.__ne__(y)should ever returnNotImplemented(or even have the possibility of returningNotImplemented) if it is known that bothxandyare members of the sameEnum. The documentation forEnums clearly states that equality comparisons between members of the same enum are defined (https://docs.python.org/3/library/enum.html#comparisons).If the argument is "filter should never be used with a predicate that could return
NotImplementedin some situations", then I think that could usefully be added to the documentation forfilter. Moreover, if that is the argument, then I don't understand why the following does not raise a DeprecationWarning:>>> next(filter((2).__eq__, (2, 3, 4))) 2Again, apologies if I'm missing something here.
Interesting, it is because object().__eq__(object()) returns NotImplemented instead of False.
object.__eq__ could return False if arguments have same type (or in some other cases). I think it would not break anything, and it would fix your case. But I am not sure that it is worth to change this. Using bound methods __eq__ and __ne__ is an antipattern, and we should not encourage this, even if it is safe as in your case. If you want to discuss this more, it is better to do this on a mailing-list or Discuss.
Your code can be rewritten as:
def other_colour(self): for other in CardColour: if self != other: return other assert False, "not reachable"
Thanks, Serhiy, that makes sense. I'll consider raising this elsewhere, as you suggest.
I've opened https://discuss.python.org/t/should-bool-notimplemented-become-an-error/51342 to discuss whether we should go through with the deprecation.
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