[issue21873] Tuple comparisons with NaNs are broken

[Andreas Kloeckner]

Change by Andreas Kloeckner :


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nosy: +inducer

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[issue21873] Tuple comparisons with NaNs are broken

[akira]

akira added the comment:

> (a, b) < (c, d) is more like: if a != c: return a < c ...

except CPython behaves (undocumented?) as:

  b < d if a is c or a == c else a < c

the difference is in the presence of `is` operator (identity
comparison instead of `__eq__`). `nan is nan` therefore `b < d` is
called and raises TypeError for `(nan, A()) < (nan, A())` expression
where `a = c = nan`, `b = A()`, and `d = A()`.

But `(float("nan"), A()) < (float("nan"), A())` is False (no
TypeError) because `a is not c` in this case and `a < c` is called
instead where `a = float('nan')`, `b = A()`, `c = float('nan')`, and
`d = A()`. Plus `(a, b) < (c, d)` evaluation is lazy (undocumented?)
i.e., once `a < c` determines the final result `b < d` is not called.

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[issue21873] Tuple comparisons with NaNs are broken

[Raymond Hettinger]

Raymond Hettinger added the comment:

FWIW, the logic for tuple ordering is a bit weird due to rich comparisons.   
Each pair of elements is first checked for equality (__eq__).  Only if the 
equality comparison returns False does it call the relevant ordering operations 
(such as __lt__).   The docs get it right, "If not equal, the sequences are 
ordered the same as their first differing elements."

In short tuple ordering is different from scalar ordering because it always 
makes equality tests:
 
   a < b  calls   a.__lt__(b)

in contrast:

   (a, b) < (c, d)is more like:   if a != c:  return a < c ...

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[issue21873] Tuple comparisons with NaNs are broken

[akira]

akira added the comment:

It is about equality. `float('nan') != float('nan')` unlike `0 == 0`.

>From msg221603: 

> If not equal, the sequences are ordered the same as their first differing 
> elements.

The result of the expression: `(a, whatever) < (b, whatever)` is defined by 
`a < b` if a and b differs i.e., it is not necessary to compare other elements 
(though Python language reference doesn't forbid further comparisons. It 
doesn't specify explicitly the short-circuit behavior for sequence comparisons 
unlike for `and`, `or` operators that guarantee the lazy (only as much as 
necessary) evaluation).

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[issue21873] Tuple comparisons with NaNs are broken

[Mak Nazečić-Andrlon]

Mak Nazečić-Andrlon added the comment:

It's not about equality.

>>> class A: pass
... 
>>> (float("nan"), A()) < (float("nan"), A())
False

That < comparison should throw a TypeError, since NaN < NaN is False, in the 
same way that 0 < 0 is False here:

>>> (0, A()) < (0, A())
Traceback (most recent call last):
  File "", line 1, in 
TypeError: unorderable types: A() < A()

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[issue21873] Tuple comparisons with NaNs are broken

[akira]

akira added the comment:

> Python containers are allowed to let identity-imply-equality (the reflesive 
> property of equality).

Is it documented somewhere?

> Dicts, lists, tuples, deques, sets, and frozensets all work this way. 

Is it CPython specific behaviour?

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[issue21873] Tuple comparisons with NaNs are broken

[Raymond Hettinger]

Raymond Hettinger added the comment:

Python core containers support the invariant:

assert all(x in c for x in c)

See also:  
http://bertrandmeyer.com/2010/02/06/reflexivity-and-other-pillars-of-civilization/

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assignee:  -> rhettinger

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[issue21873] Tuple comparisons with NaNs are broken

[Mak Nazečić-Andrlon]

Mak Nazečić-Andrlon added the comment:

The bug is that the comparison should throw a TypeError, but does not (for 
incomparable A).

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[issue21873] Tuple comparisons with NaNs are broken

[Raymond Hettinger]

Raymond Hettinger added the comment:

Python containers are allowed to let identity-imply-equality (the reflesive 
property of equality).  Dicts, lists, tuples, deques, sets, and frozensets all 
work this way.  So for your purposes,  you need to use distinct NaN values 
rather than reusing a single instance of a NaN.

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nosy: +rhettinger
resolution:  -> not a bug
status: open -> closed

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[issue21873] Tuple comparisons with NaNs are broken

[akira]

akira added the comment:

btw, pypy3 (986752d005bb) is broken:

   (1, float('nan')) == (1, float('nan'))
  True

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[issue21873] Tuple comparisons with NaNs are broken

[akira]

akira added the comment:

Is the issue that:

  >>> (1, float('nan')) == (1, float('nan'))
  False

but

  >>> nan = float('nan')
  >>> (1, nan) == (1, nan)
  True

?

`nan != nan` therefore it might be expected that `(a, nan) != (a, nan)` [1]:

> The values float('NaN') and Decimal('NaN') are special. The are identical to 
> themselves, x is x but are not equal to themselves, x != x. 

> Tuples and lists are compared lexicographically using comparison of 
> corresponding elements. This means that to compare equal, each element must 
> compare equal and the two sequences must be of the same type and have the 
> same length.
> If not equal, the sequences are ordered the same as their first differing 
> elements.

[1]: https://docs.python.org/3.4/reference/expressions.html#comparisons

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nosy: +akira

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[issue21873] Tuple comparisons with NaNs are broken

[R. David Murray]

Changes by R. David Murray :


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nosy: +mark.dickinson

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[issue21873] Tuple comparisons with NaNs are broken

[Mak Nazečić-Andrlon]

New submission from Mak Nazečić-Andrlon:

While searching for a way to work around the breakage of the Schwartzian 
transform in Python 3 (and the resulting awkwardness if you wish to use heapq 
or bisect, which do not yet have a key argument), I thought of the good old 
IEEE-754 NaN. Unfortunately, that shouldn't work since lexicographical 
comparisons shouldn't stop for something comparing False all the time. 
Nevertheless:

>>> (1, float("nan"), A()) < (1, float("nan"), A())
False
>>> (0, float("nan"), A()) < (1, float("nan"), A())
True

Instead of as in
>>> nan = float("nan")
>>> (1, nan, A()) < (1, nan, A())
Traceback (most recent call last):
  File "", line 1, in 
TypeError: unorderable types: A() < A()

(As a side note, PyPy3 does not have this bug.)

--
components: Interpreter Core
messages: 221600
nosy: Electro
priority: normal
severity: normal
status: open
title: Tuple comparisons with NaNs are broken
versions: Python 3.4

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