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

Raymond Hettinger raymond.hettinger at gmail.com
Wed Mar 24 18:39:54 CET 2010

On Mar 24, 2010, at 9:22 AM, Mark Dickinson wrote:

The obvious way to do this nan interning for floats would be to put the interning code into PyFloatFromDouble. I'm not sure whether this would be worth the cost in terms of added code (and possibly reduced performance, since the nan check would be done every time a float was returned), but I'd be willing to review a patch.

-1

Propagating support for NaNs has already consumed an enormous amount of your time. The code for the math module doubled in complexity as a result of adding Nan/Inf support. At each stage, some new weirdness emerges (such as complex(Nan, Nan), etc). And each change introduces the risk of new bugs in code that had been stable for a long time.

IIRC, the original purpose of a NaN was to serve as a placeholder value in a long series of floating point ops so that the programmer would not have to introduce edge case tests at each stage of a calculation. Yet, I look at the code for the decimal module and the C code for the math module and see that the opposite result occurred, the code is littered with is_special(x) tests and handlers.

In terms of practicality, NaNs work great as a way to mark missing values and to propagate through subsequent calculations.
IMO, their least useful feature is the property of being not equal to themselves -- that causes more problems than it solves because it impairs a programmer's ability to reason about their programs.

Raymond

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