Make NotImplemented unusable in boolean context #79893
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assignee = None closed_at = <Date 2020-03-03.18:52:02.603> created_at = <Date 2019-01-10.21:55:06.172> labels = ['type-feature', '3.9'] title = 'Make NotImplemented unusable in boolean context' updated_at = <Date 2021-09-20.16:34:34.784> user = 'https://github.com/MojoVampire'
activity = <Date 2021-09-20.16:34:34.784> actor = 'AlexWaygood' assignee = 'none' closed = True closed_date = <Date 2020-03-03.18:52:02.603> closer = 'serhiy.storchaka' components =  creation = <Date 2019-01-10.21:55:06.172> creator = 'josh.r' dependencies =  files =  hgrepos =  issue_num = 35712 keywords = ['patch'] message_count = 28.0 messages = ['333421', '333426', '333433', '333440', '333483', '333508', '341909', '348529', '349203', '349215', '349303', '363293', '363956', '363961', '363969', '363973', '364065', '380596', '380603', '380604', '380605', '380613', '380614', '402246', '402248', '402253', '402256', '402259'] nosy_count = 9.0 nosy_names = ['rhettinger', 'terry.reedy', 'steven.daprano', 'ethan.furman', 'serhiy.storchaka', 'josh.r', 'veky', 'xtreak', 'AlexWaygood'] pr_nums = ['13195'] priority = 'normal' resolution = 'fixed' stage = 'resolved' status = 'closed' superseder = None type = 'enhancement' url = 'https://bugs.python.org/issue35712' versions = ['Python 3.9']
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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.
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.
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__.
On Thu, Jan 10, 2019 at 11:24 PM Serhiy Storchaka <firstname.lastname@example.org>
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.
I'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.
In 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?
I do not have a strong opinion, so I suggest to not mention neither TypeError, nor RuntimeWarning. After some time of using a DeprecationWarning we can have have some information about whether there is a code which can't be fixed if bool(NotImplemented) will raise an error.
I know I'm late to the party, but if
def __ge__(self, other): return not self.__lt__(other)
if __lt__ returns then __gt__ returns
Correct code (which checks for NotImplemented) would still work, and buggy code (which just returns the bool() of NotImplemented), would then be correct.
First, it is impossible. nb_bool and PyObject_IsTrue() can return only three value: 1 for true, 0 for false, and -1 for error. It is not possible to represent NotImplemented without breaking all extensions.
if not a: b() else: c()
is equivalent to
if a: c() else: b()
If a is NotImplemented, what branch be executed in every case?
Second, it would not help. Because real-world examples are not always so trivial as "return not self.__lt__(other)". It may be a part of more complex expression, e.g.:
So it may help in some examples, but make bugs in other examples even more mystical.
Huh. How is -1 interpreted? Does it become a TypeError?
Side-stepping to other __dunder__ methods for a moment: if, for example, both __add__ and __radd__ return NotImplemented then the interpreter will convert that into a TypeError.
So my thinking is that when the interpreter gets the
I don't see the problem -- breaking it down:
return NotImplemented and ...
So to make that work,
The reason I would prefer this solution is that if it could behave as I described above, the
class NoAdd: # def __add__(self, other): return NotImplemented # def __radd__(self, other): # fake a NotImplemented TypeError from bool(NotImplemented) raise TypeError("cannot boolean NotImplemented")
# what we should see, since the error is in the users' code --> NoAdd() + 7 Traceback (most recent call last): File "<stdin>", line 1, in <module> TypeError: unsupported operand type(s) for +: 'NoAdd' and 'int' # what we will see -- a leaky implementation detail --> 7 + NoAdd() Traceback (most recent call last): File "<stdin>", line 1, in <module> File "<stdin>", line 6, in __radd__ TypeError: cannot boolean NotImplemented
It is interpreted as error. It requires an exception be set. If you return -1 without setting an exception you will usually get a SystemError or crash. Oh, and this may happen during executing other code, so you will search a bug in wrong place.
You cannot change this without breaking the C and Python API in hard way.
So you literally want NotImplemented raising a TypeError in boolean context except the "not" operator, and redefine
You can do this, but this is a different issue, and I doubt that it will solve many problems.
One of the idioms for removing None no longer works:
>>> L = [0, 23, 234, 89, None, 0, 35, 9] >>> list(filter(None.__ne__, L)) Warning (from warnings module): File "<pyshell#1>", line 1 DeprecationWarning: NotImplemented should not be used in a boolean context [0, 23, 234, 89, 0, 35, 9]
... as it probably should: look at https://bugs.python.org/msg349303
Yes, filtering comprehensions are a frequently used niche, and too long in the "official" parlance. I seem to recall that
(instead of triple-x version) was rejected because it was too hard to parse. Maybe now we can really implement it?
Not really, but it does come up and I've seen it in customer code more than once.
I do show people this:
>>> data = [10.5, 3.27, float('Nan'), 56.1] >>> list(filter(isfinite, data)) [10.5, 3.27, 56.1] >>> list(filterfalse(isnan, data)) [10.5, 3.27, 56.1]
The question does arise about how to do this for None using functional programming. The answer is a bit awkward:
>>> from operator import is_not >>> from functools import partial >>> data = [10.5, 3.27, None, 56.1] >>> list(filter(partial(is_not, None), data)) [10.5, 3.27, 56.1]
From a teaching point of view, the important part is to show that this code does not do what people typically expect:
>>> data = [10.5, 0.0, float('NaN'), 3.27, None, 56.1] >>> list(filter(None, data)) [10.5, nan, 3.27, 56.1]
FWIW, this issue isn't important to me. Just wanted to note that one of the idioms no longer works. There are of course other ways to do it.
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
Even 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
If I change the last line of
Is this intended behaviour?
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.
I'm unclear, however, why
If the argument is "filter should never be used with a predicate that could return
Again, 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"