[Python-Dev] Decimal <-> float comparisons in py3k. (original) (raw)

Nick Coghlan ncoghlan at gmail.com
Thu Mar 18 22:48:33 CET 2010

Mark Dickinson wrote:

Could everyone live with making float<->Decimal comparisons raise an exception in 2.7?

I could, with the caveat that if this causes problems for real world code, then changing it to produce the correct answer (as per your patch) should be applied as a bug fix in both 2.7 and 3.2.

Note that even in Py3k there are some fairly serious weirdnesses kicking around due to the intransitive nature of numeric equality though:

from decimal import Decimal as dec set((1, 1.0, dec("1.0"))) {1} set((1.0, dec("1.0"))) {1.0, Decimal('1.0')}

d = {} from decimal import Decimal as dec d[1] = d[1.0] = d[dec("1.0")] = 42 d {1: 42} d[1.0] = d[dec("1.0")] = 42 d {1: 42} del d[1] d[1.0] = d[dec("1.0")] = 42 d {1.0: 42, Decimal('1.0'): 42}

When there is a clear, correct way (based on Decimal.from_float) to make numeric comparison behave in accordance with the rules of mathematics, do we really want to preserve strange, unintuitive behaviour like the above?

Cheers, Nick.

-- Nick Coghlan | ncoghlan at gmail.com | Brisbane, Australia

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