float() can return not exact float instance

[serhiy-storchaka]

BPO	26983
Nosy	@mdickinson, @serhiy-storchaka
Dependencies	bpo-26995: Add tests for parsing float and object arguments
Files	float_exact.patch float_exact2.patch issue26983.diff

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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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']

[serhiy-storchaka]

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]

Proposed patch makes float() always returning exact float.

[mdickinson]

+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]

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]

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]

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]

Can you please attach the new patch?

[serhiy-storchaka]

Oh, sorry, how could I miss it?

[serhiy-storchaka]

Ping.

[mdickinson]

float_exact2.patch LGTM

[python-dev]

New changeset 050e5f803999 by Serhiy Storchaka in branch 'default':
Issue bpo-26983: float() now always return an instance of exact float.
https://hg.python.org/cpython/rev/050e5f803999

[serhiy-storchaka]

Thank you for your review Mark.