New submission from Mark Dickinson <[EMAIL PROTECTED]>: 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 <[EMAIL PROTECTED]> <http://bugs.python.org/issue4087> _______________________________________ _______________________________________________ Python-bugs-list mailing list Unsubscribe: http://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com