[Python-Dev] Partial function application 'from the right' (original) (raw)

Xavier Morel catch-all at masklinn.net
Fri Feb 6 18:24:38 CET 2009

On 5 Feb 2009, at 23:54 , Steven D'Aprano wrote:

Raymond Hettinger wrote:

The arguments for and against the patch could be brought against partial() itself, so I don't understand the -1's at all.

Quite so, but that doesn't justify adding more capabilities to partial(). I concur with Collin. Lever arguments are a road to bloat. One person's "bloat" is another person's rich and complete toolbox. > "In for a penny, in for a pound" is not a language design principle. Neither are "slippery slope" arguments. One of the real problems with partial() and its variants is that they provide almost no advantage over an equivalent lambda. How about speed? I don't have a recent version of Python here to test, but my recollection is that partial is significantly faster than lambda. And even if it currently isn't, there could be (is?) more opportunity to optimize partial. I guess that the voting on this is going to be fall along functional lines: those who like functional languages will vote +1 on partial and co, and those who don't will vote -1. While I don't dislike partial(), I'd rather see one good use-case for partialright() to be removed: built-ins that don't accept keywords. From Ben North's post starting this thread: "I find 'functools.partial' useful, but occasionally I'm unable to use it because it lacks a 'from the right' version. E.g., to create a function which splits a string on commas, you can't say # Won't work when called: splitcomma = partial(str.split, sep = ',') and to create a 'log to base 10' function, you can't say # Won't work when called: log10 = partial(math.log, base = 10.0) because str.split and math.log don't take keyword arguments." Wouldn't a flip function (reverse the order of the function's
arguments) be more flexible and general than a partial_right one?

Your first case would become

we're not ignoring any argument, so we have to provide maxsplit

split_comma = partial(flip(str.split), None, ',')

and the second one would yield

log_10 = partial(flip(math.log), 10.0)

and if we only want to fill-in the rightmost argument, we can flip the
result to get the original order back:

split_5 = flip(partial(flip(str.split), 5))

While better kwargs support would be even better, there probably
always will be kwarg-less functions/methods, so the ability to
partially apply from the right stays interesting.

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