[Python-Dev] Mixing float and Decimal -- thread reboot (original) (raw)

Nick Coghlan ncoghlan at gmail.com
Mon Mar 22 11:51:33 CET 2010

Raymond Hettinger wrote:

On Mar 21, 2010, at 3:35 PM, Guido van Rossum wrote:

It seems to me that Decimals and floats should be considered at the same level (i.e. both implement Real). Agreed, but doesn't help. (Except against the idea that Decimal goes on the "integer" side of Fraction, which is just wrong.) Woohoo! Glad you agree. I was concerned that idea was gathering a following.

Heck no, it was just a random late night thought from me, and even I thought it was a somewhat dubious idea. I don't mind at all that it since has been knocked soundly (and deservedly) on the head :)

Reasoning for emitting a warning by default:

1) Real actual use cases for mixed decimal / float operations are rare. 2) But accidental mixed decimal / float is an easy mistake to make. 3) Implicit coercion hides the error. 4) A warning flag gives you a chance to catch your error. 5) A warning is educational (it makes sure that you understand what your program is doing) 6). A warning is easily silenced either through a) the warnings module, b) setting a context flag in decimal, or c) by making the coercion explicit using Decimal.fromfloat().

I'll add another one to that list:

(7) For backwards compatible changes, it is easy to go from exception -> warning -> no warning (if we later decide to take that second step). Going from exception -> no warning -> warning (if we were to change our minds the other way) is a lot less user-friendly.

(I was going to try to play devil's advocate and argue in favour of float results and/or no warning, but I got nuthin' - Raymond's points made too much sense to me).

A warning is nice in that you can mix decimals and floats at the interpreter prompt with a single warning per session, but the warning can still act as a pointer into the weird and wonderful world of binary vs decimal floating point.

Cheers, Nick.

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

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