New submission from Mark Dickinson <dickinsm@gmail.com>:
The Decimal module breaks transitivity of equality: Decimal(2) == 2 and
2 == float(2), but Decimal(2) != float(2).
The Python set and dict implementations rely on transitivity of equality
for correct operation.
These two facts together give some strange results when playing with
sets and dicts involving Decimals and floats. For example (with Python
2.6):
>>> s = set([Decimal(2), float(2)])
>>> t = set([2])
>>> s | t == t | s
False
>>> len(s | t)
2
>>> len(t | s)
1
Other strange examples, and possible solutions, were discussed recently
on comp.lang.python; see the thread starting at:
http://mail.python.org/pipermail/python-list/2008-September/508859.html
Possible solutions:
(1) Document the problem, making it clear in the language reference that
correct set operation relies on transitivity of equality, and adding a
note to the decimal documentation indicating that mixing floats and
Decimals in a container is asking for trouble.
(2) Fix up Decimal so that equal numeric objects compare equal; this
would also involve fixing the hash operation. To me, this goes against
the philosophy of keeping the Decimal module close to the specification.
(3) Terry Reedy suggested (in the c.l.python thread linked to above) a
simpler version of (2): allow Decimal(i) == float(i) or Decimal(i) ==
Fraction(i) to return True for all integers i. (Decimal('0.5') == 0.5
would still return False.) This fixes transitivity, and has the
advantage of not requiring any changes to the hash functions:
hash(Decimal(i)) == hash(float(i)) is already true for all integers i.
My own preference would be simply to document the problem; it doesn't
seem like something that's going to affect that vast majority of Python
users.
Raymond, Facundo: any thoughts?
----------
messages: 74576
nosy: facundobatista, marketdickinson, rhettinger, tjreedy
priority: normal
severity: normal
status: open
title: equality involving Decimals is not transitive; strange set behaviour results
type: behavior
versions: Python 2.6, Python 2.7, Python 3.0
_______________________________________
Python tracker <report@bugs.python.org>
<http://bugs.python.org/issue4087>
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The Decimal module breaks transitivity of equality: Decimal(2) == 2 and
2 == float(2), but Decimal(2) != float(2).
The Python set and dict implementations rely on transitivity of equality
for correct operation.
These two facts together give some strange results when playing with
sets and dicts involving Decimals and floats. For example (with Python
2.6):
>>> s = set([Decimal(2), float(2)])
>>> t = set([2])
>>> s | t == t | s
False
>>> len(s | t)
2
>>> len(t | s)
1
Other strange examples, and possible solutions, were discussed recently
on comp.lang.python; see the thread starting at:
http://mail.python.org/pipermail/python-list/2008-September/508859.html
Possible solutions:
(1) Document the problem, making it clear in the language reference that
correct set operation relies on transitivity of equality, and adding a
note to the decimal documentation indicating that mixing floats and
Decimals in a container is asking for trouble.
(2) Fix up Decimal so that equal numeric objects compare equal; this
would also involve fixing the hash operation. To me, this goes against
the philosophy of keeping the Decimal module close to the specification.
(3) Terry Reedy suggested (in the c.l.python thread linked to above) a
simpler version of (2): allow Decimal(i) == float(i) or Decimal(i) ==
Fraction(i) to return True for all integers i. (Decimal('0.5') == 0.5
would still return False.) This fixes transitivity, and has the
advantage of not requiring any changes to the hash functions:
hash(Decimal(i)) == hash(float(i)) is already true for all integers i.
My own preference would be simply to document the problem; it doesn't
seem like something that's going to affect that vast majority of Python
users.
Raymond, Facundo: any thoughts?
----------
messages: 74576
nosy: facundobatista, marketdickinson, rhettinger, tjreedy
priority: normal
severity: normal
status: open
title: equality involving Decimals is not transitive; strange set behaviour results
type: behavior
versions: Python 2.6, Python 2.7, Python 3.0
_______________________________________
Python tracker <report@bugs.python.org>
<http://bugs.python.org/issue4087>
_______________________________________
_______________________________________________
Python-bugs-list mailing list
Unsubscribe: http://mail.python.org/mailman/options/python-bugs-list/list-python-bugs%40lists.gossamer-threads.com