[issue26983] float() can return not exact float instance

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

My failure. Thank you for your report and patch SilentGhost.

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resolution:  -> fixed
status: open -> closed

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[issue26983] float() can return not exact float instance

[Roundup Robot]

Roundup Robot added the comment:

New changeset 6216fb8afa53 by Serhiy Storchaka in branch 'default':
Issue #26983: Fixed test_format failure.
https://hg.python.org/cpython/rev/6216fb8afa53

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[issue26983] float() can return not exact float instance

[SilentGhost]

SilentGhost added the comment:

test_format resulted in semi-failure due to this change. The attached patch 
fixes the issue.

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assignee:  -> serhiy.storchaka
nosy: +SilentGhost
resolution: fixed -> 
stage: resolved -> commit review
status: closed -> open
Added file: http://bugs.python.org/file43256/issue26983.diff

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[issue26983] float() can return not exact float instance

[Serhiy Storchaka]

Changes by Serhiy Storchaka :


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resolution:  -> fixed
stage: patch review -> resolved
status: open -> closed

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[issue26983] float() can return not exact float instance

[Roundup Robot]

Roundup Robot added the comment:

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

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nosy: +python-dev

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[issue26983] float() can return not exact float instance

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

Thank you for your review Mark.

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[issue26983] float() can return not exact float instance

[Mark Dickinson]

Mark Dickinson added the comment:

float_exact2.patch LGTM

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[issue26983] float() can return not exact float instance

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

Ping.

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[issue26983] float() can return not exact float instance

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

Oh, sorry, how could I miss it?

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Added file: http://bugs.python.org/file42815/float_exact2.patch

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[issue26983] float() can return not exact float instance

[Mark Dickinson]

Mark Dickinson added the comment:

Can you please attach the new patch?

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[issue26983] float() can return not exact float instance

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

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?

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[issue26983] float() can return not exact float instance

[Serhiy Storchaka]

Changes by Serhiy Storchaka :


--
dependencies: +Add tests for parsing float and object arguments

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[issue26983] float() can return not exact float instance

[Mark Dickinson]

Mark Dickinson added the comment:

> 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.

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[issue26983] float() can return not exact float instance

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

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__.

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[issue26983] float() can return not exact float instance

[Mark Dickinson]

Mark Dickinson added the comment:

+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).

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[issue26983] float() can return not exact float instance

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

Proposed patch makes float() always returning exact float.

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keywords: +patch
stage:  -> patch review
Added file: http://bugs.python.org/file42807/float_exact.patch

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[issue26983] float() can return not exact float instance

[Serhiy Storchaka]

New submission from 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()))


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()))


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()))


--
components: Interpreter Core
messages: 265191
nosy: mark.dickinson, serhiy.storchaka
priority: normal
severity: normal
status: open
title: float() can return not exact float instance
type: behavior
versions: Python 3.6

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