[issue26975] Decimal.from_float works incorrectly for non-binary floats

[Stefan Krah]

Stefan Krah added the comment:

Let's close it then.

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resolution:  -> not a bug
stage:  -> resolved
status: open -> closed

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[issue26975] Decimal.from_float works incorrectly for non-binary floats

[Raymond Hettinger]

Raymond Hettinger added the comment:

I vote for ignoring this and calling it not-a-bug.  In a way, it is no more 
interesting than a dict like object defining __hash__ to return a random number.

The intention for from_float() is to convert binary floats.  It was not meant 
to be generalized to handle arbitrary fractions.  I concur with Mark's comment, 
"Note that by overriding as_integer_ratio, you're breaking its "contract"".

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[issue26975] Decimal.from_float works incorrectly for non-binary floats

[Mark Dickinson]

Mark Dickinson added the comment:

> I see following ways to resolve this issue:

I don't think 4. is an option: for input of exact type float, from_float should 
continue to produce the exact value of the float, rather than an approximation 
to that exact value. There's code out there that depends on this behaviour. 
Options 1 through 3 all seem reasonable to me.

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[issue26975] Decimal.from_float works incorrectly for non-binary floats

[Stefan Krah]

Stefan Krah added the comment:

As Mark hinted at, many people would say there is no issue at all.
Subclassing like that often breaks the Liskov Substitution Principle.

For more information, see e.g.:

  http://okmij.org/ftp/Computation/Subtyping/


"Alas, LSP when considered from an OOP point of view is undecidable. You cannot 
count on a compiler for help in pointing out an error. You cannot rely on 
regression tests either. It's manual work -- you have to see the problem"

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[issue26975] Decimal.from_float works incorrectly for non-binary floats

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

Once I was going to propose to make as_integer_ratio() returning a pair (n, d) 
with minimal positive d that n/d (as float) is exactly equal to the original 
float. I think this is legitimate implementation and doesn't contradict the 
documentation.

I see following ways to resolve this issue:

1. Ignore it.

2. Ignore methods overriding:
2a) either convert float subclass to exact float with known behavior;
2b) or directly call static float.__abs__() and float.as_integer_ratio() 
instead of resolving dynamic methods.

3. Check that the denominator is a power of 2 and raise error otherwise.

4. Implement different different algorithm. Returns decimal n / 10**k with 
minimal k that after converting to float is exactly equal to the original float.

Most of these solutions will automatically fix issue26974, so it doesn't need 
special fix.

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[issue26975] Decimal.from_float works incorrectly for non-binary floats

[Stefan Krah]

Stefan Krah added the comment:

It seems to work correctly here for non-binary floats:

>>> from _pydecimal import Decimal
>>> Decimal.from_float(Decimal("1.2"))
Traceback (most recent call last):
  File "", line 1, in 
  File "/usr/local/lib/python3.6/_pydecimal.py", line 739, in from_float
raise TypeError("argument must be int or float.")
TypeError: argument must be int or float.


I think we should not support strange inheritance hierarchies that
break the expected return values of parent class methods.

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[issue26975] Decimal.from_float works incorrectly for non-binary floats

[Mark Dickinson]

Mark Dickinson added the comment:

True. Though having a subclass of float that overrides as_integer_ratio seems a 
fairly unlikely use-case. We could add a check for subclasses of float that the 
denominator is a power of 2 (using the usual trick: a positive integer n is a 
power of 2 if and only if `n & (n-1)` is zero) and raise a suitable error 
otherwise.

I doubt it's worth trying to support arbitrary return values from 
as_integer_ratio. Note that by overriding as_integer_ratio, you're breaking its 
"contract": the docs say

> Return a pair of integers, whose ratio is exactly equal to the original
> float and with a positive denominator.

and you've lost that "exactly equal". I think it's reasonably to get wrong 
results or an exception in that case.

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[issue26975] Decimal.from_float works incorrectly for non-binary floats

[Serhiy Storchaka]

New submission from Serhiy Storchaka:

Decimal.from_float() works correctly only if the denominator of 
as_integer_ratio() of argument is the power of two. Example:

>>> from decimal import Decimal
>>> class DecimalFloat(float):
... def as_integer_ratio(self):
... return Decimal(str(self)).as_integer_ratio()
... def __abs__(self):
... return self
... 
>>> DecimalFloat(1.2).as_integer_ratio()
(6, 5)
>>> Decimal.from_float(DecimalFloat(1.2))
Decimal('1.50')

--
components: Extension Modules, Library (Lib)
messages: 265116
nosy: facundobatista, mark.dickinson, rhettinger, serhiy.storchaka, skrah
priority: normal
severity: normal
status: open
title: Decimal.from_float works incorrectly for non-binary floats
type: behavior
versions: Python 2.7, Python 3.5, Python 3.6

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