GeezerVison is showing github.com — float() can return not exact float instance · Issue #71170 · python/cpython
float() can return not exact float instance #71170Labels interpreter-core(Objects, Python, Grammar, and Parser dirs)(Objects, Python, Grammar, and Parser dirs)type-bugAn unexpected behavior, bug, or errorAn unexpected behavior, bug, or error Description
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GitHub fields:
assignee = 'https://github.com/serhiy-storchaka'
closed_at = <Date 2016-06-06.10:01:54.560>
created_at = <Date 2016-05-09.13:46:33.061>
labels = ['interpreter-core', 'type-bug']
title = 'float() can return not exact float instance'
updated_at = <Date 2016-06-06.10:01:54.558>
user = 'https://github.com/serhiy-storchaka' bugs.python.org fields:
activity = <Date 2016-06-06.10:01:54.558>
actor = 'serhiy.storchaka'
assignee = 'serhiy.storchaka'
closed = True
closed_date = <Date 2016-06-06.10:01:54.560>
closer = 'serhiy.storchaka'
components = ['Interpreter Core']
creation = <Date 2016-05-09.13:46:33.061>
creator = 'serhiy.storchaka'
dependencies = ['26995']
files = ['42807', '42815', '43256']
hgrepos = []
issue_num = 26983
keywords = ['patch']
message_count = 15.0
messages = ['265191', '265267', '265295', '265297', '265298', '265299', '265316', '265318', '267096', '267101', '267129', '267130', '267521', '267523', '267524']
nosy_count = 4.0
nosy_names = ['mark.dickinson', 'SilentGhost', 'python-dev', 'serhiy.storchaka']
pr_nums = []
priority = 'normal'
resolution = 'fixed'
stage = 'commit review'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue26983'
versions = ['Python 3.6'] Activityserhiy-storchaka commented on May 9, 2016 The float constructor can return an instance of float subclass.
>>> class FloatSubclass(float):
... pass
...
>>> class BadFloat:
... def __float__(self):
... return FloatSubclass(1.2)
...
>>> type(float(BadFloat()))
<class '__main__.FloatSubclass'> Comparing with other types, complex() always returns complex:
>>> class ComplexSubclass(complex):
... pass
...
>>> class BadComplex:
... def __complex__(self):
... return ComplexSubclass(1.2, 3.4)
...
>>> type(complex(BadComplex()))
<class 'complex'> And int() can return an instance of int subclass, but this behavior is deprecated:
>>> class BadInt:
... def __int__(self):
... return True
...
>>> int(BadInt())
__main__:1: DeprecationWarning: __int__ returned non-int (type bool). The ability to return an instance of a strict subclass of int is deprecated, and may be removed in a future version of Python.
True May be we should either deprecate __float__ returning non-float (as for int), or convert the result to exact float (as for complex).
The constructor of float subclass always returns an instance of correct type.
>>> class FloatSubclass2(float):
... pass
...
>>> type(FloatSubclass2(BadFloat()))
<class '__main__.FloatSubclass2'> serhiy-storchaka commented on May 10, 2016 Proposed patch makes float() always returning exact float. mdickinson commented on May 11, 2016 +1 for the idea, and the patch LGTM.
I was initially a bit confused by the wording of the warning: presumably, we're not going to change Python to make returning an instance of a strict float subclass from __float__ illegal (I don't really see how that would be possible); we're just going to check the return type at the internal call sites to __float__ and raise if we get something other than an exact float. That is, I'd still expect this code to work on future versions of Python:
>>> class MyFloat(float): pass
...
>>> class A:
... def __float__(self): return MyFloat()
...
>>> a = A()
>>> a.__float__()
0.0 But these would both become an error in Python 3.7 (say):
>>> float(a)
0.0
>>> math.sqrt(a)
0.0 Does that match your thinking?
We should probably add issues for checking and fixing other places that __float__ is used internally (like the math module). serhiy-storchaka commented on May 11, 2016 The warning message is copied from __int__. As I understand, the conclusion of the Python-Dev discussion is that __int__, __float__, __complex__, etc should return exact int, float, complex, not subclasses. I have another patch that adds a warning for __complex__, and I am working on patches for other special methods. If my understanding is wrong, we should remove the deprecation warning for __int__. mdickinson commented on May 11, 2016
As I understand, the conclusion of the Python-Dev discussion is that __int__, __float__, __complex__, etc should return exact int, float, complex, not subclasses.
Agreed; that matches my understanding. I'm only observing that it would be difficult (and likely undesirable) to actually enforce that __int__ itself always returns an exact int: I'm not sure where that check would/could take place, and I'd expect that a direct user-level call to my_object.__int__ probably wouldn't be subject to such a check. What we can do is check that the result of the nb_int call is always an int in the places we use it.
Or we could even go further, and write custom slot_nb_float and slot_nb_int functions in Objects/typeobject.c that incorporates those checks. What I don't think we can do is check that a direct call to __int__ or __float__ always returns something of the exact type. serhiy-storchaka commented on May 11, 2016 Here is updated patch. Differences:
- Deprecation warning is now emitted in functions accepting floats (like math.sqrt).
- Improved error messages. Now reported wrong type name and a type of wrong __float__ method.
- float() now returns the same object for exact float argument.
Could you propose better warning message? mdickinson commented on May 11, 2016 Can you please attach the new patch? serhiy-storchaka commented on May 11, 2016 Oh, sorry, how could I miss it? serhiy-storchaka commented on Jun 3, 2016 mdickinson commented on Jun 3, 2016 python-dev commented on Jun 3, 2016 serhiy-storchaka commented on Jun 3, 2016 Thank you for your review Mark. SilentGhost commented on Jun 6, 2016 test_format resulted in semi-failure due to this change. The attached patch fixes the issue. python-dev commented on Jun 6, 2016 serhiy-storchaka commented on Jun 6, 2016 My failure. Thank you for your report and patch SilentGhost.
MetadataMetadatainterpreter-core(Objects, Python, Grammar, and Parser dirs)(Objects, Python, Grammar, and Parser dirs)type-bugAn unexpected behavior, bug, or errorAn unexpected behavior, bug, or error DevelopmentNo branches or pull requestsIssue actions
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