[Python-Dev] Symmetry arguments for API expansion (original) (raw)

Steven D'Aprano steve at pearwood.info
Wed Mar 21 00:34:29 EDT 2018

On Wed, Mar 21, 2018 at 12:32:11AM +0000, Chris Barker wrote:

Could float et al. add an index method that would return a ValueError if the value was not an integer?

That would allow us to write things like:

"abcdefgh"[5.0]

which is one of the things index was invented to prevent.

Of course, as pointed out earlier in this thread, an "exact" integer is probably not what you want with a float anyway....

Not always. For example, I might want a function factorial(x) which returns x! when x is an exact integer value, and gamma(x+1) when it is not. That is what the HP-48 series of calculators do.

(This is just an illustration.)

Another example is that pow() functions sometimes swap to an exact algorithm if the power is an int. There's no particular reason why xn and xn.0 ought to be different, but they are:

py> 123**10 792594609605189126649

py> 123**10.0 7.925946096051892e+20

On the other hand, some might argue that by passing 10.0 as the power, I am specifically requesting a float implementation and result. I don't wish to argue in favour of either position, but just to point out that it is sometimes reasonable to want to know whether a float represents an exact integer value or not.

-- Steve

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