[ python-Bugs-1163325 ] "special" decimals aren't hashable

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Bugs item #1163325, was opened at 2005-03-14 17:37
Message generated for change (Comment added) made by foom
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Category: Python Library
Group: Python 2.4
Status: Open
Resolution: None
Priority: 5
Submitted By: Marien Zwart (marienz)
Assigned to: Nobody/Anonymous (nobody)
Summary: "special" decimals aren't hashable

Initial Comment:
Python 2.4 (#1, Feb 22 2005, 15:02:34) 
[GCC 3.4.3-20050110 (Gentoo Linux 3.4.3.20050110,
ssp-3.4.3.20050110-0, pie-8.7 on linux2
Type "help", "copyright", "credits" or "license" for
more information.
>>> import decimal
>>> hash(decimal.NaN)
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
  File "/usr/lib/python2.4/decimal.py", line 720, in
__hash__
    i = int(self)
  File "/usr/lib/python2.4/decimal.py", line 1410, in
__int__
    return context._raise_error(InvalidContext)
  File "/usr/lib/python2.4/decimal.py", line 2215, in
_raise_error
    raise error, explanation
decimal.InvalidOperation

This behaviour doesn't match the comment in
decimal.py's __hash__:
# Decimal integers must hash the same as the ints
# Non-integer decimals are normalized and hashed as strings
# Normalization assures that hast(100E-1) == hash(10)

Would it make sense to wrap the int(self) in an except
block and return hash(str(self.normalize())) if this
raises?


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Comment By: James Y Knight (foom)
Date: 2005-03-14 23:15

Message:
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NaN, Inf, and negInf all fail to hash. 

Inf and negInf are equal to themselves, so they don't have any problem 
being used as a dict key. 

As for NaN, I don't see any harm in allowing it to return a hashcode 
anyways, but perhaps you're right. 

However, in that case, it would be nice to have the exception raised be 
somewhat more regular and expected-looking than a InvalidOperation 
from int(). Perhaps it should raise TypeError('Decimal("NaN") is 
unhashable').


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Comment By: Tim Peters (tim_one)
Date: 2005-03-14 19:04

Message:
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Well, I'm not really a fan of Python's tying hashability 
to "usable as a dict key", but given that's what Python 
_does_, ya, hashing a NaN doesn't make sense.

marienz, by "special" decimals did you mean only NaNs, or 
do you have other cases in mind too?

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Comment By: Raymond Hettinger (rhettinger)
Date: 2005-03-14 17:42

Message:
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Since two NaNs are never equal to one another, I would think
that it doesn't make sense to hash them.  Tim, do you have
another thoughts on the subject?

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