[Python-ideas] Introduce `start=1` argument to `math.factorial`

[Terry Reedy]

On 9/18/2014 2:15 AM, Nick Coghlan wrote:
> On 18 September 2014 14:13, Steven D'Aprano <steve at pearwood.info> wrote:
>> On Wed, Sep 17, 2014 at 04:02:44PM -0700, Ram Rachum wrote:
>>> I suggest introducing a `start=1` argument to `math.factorial`, so the
>>> result would be (the C-optimized version of) `product(range(start, x+1),
>>> start=1)`. This'll be useful for combinatorical calculations.
>>
>> Then it wouldn't be the factorial function any more.
>>
>> There are lots of functions which could be useful for combinatorical
>> calculations, including !n and n!!, do you think this particular one
>> would be of broad enough interest that it deserves to be in the standard
>> library?
>>
>> Do you know of any other programming languages which offer this "partial
>> factorial" function in their standard library?
>
> It's also worth noting that "pip install mpmath" will provide rising
> and falling factorials
> (http://mpmath.org/doc/current/functions/gamma.html#rising-and-falling-factorials)
> and a whole lot more. There's no need to add such complexity to the
> standard library.
>
> However, now that CPython ships with pip by default, we may want to
> consider providing more explicit pointers to such "If you want more
> advanced functionality than the standard library provides" libraries.

Having used pip install a few times, I have begun to regard 
pip-installable packages as almost being extensions of the stdlib.

I think the main remaining problem is equally easy access to 
documentation as to the code. It would be nice, for instance, if /Doc, 
like /Lib, had a site-packages subdirectory with an index.html updated 
by pip with a link to either a package.html put in the directory or an 
external file, such as one at readthedocs.org.  If there were something 
like this, I would add an item to Idle's help menu.

> Yes, that may be contentious in the near term as folks argue over
> which "stdlib++" modules to recommend, but in some cases there are
> clear "next step beyond the standard library" category winners that
> are worth introducing to newcomers, rather than making them do their
> own research.

Choosing 1 *or more* packages to list should not be more contentions 
than choosing just 1  package to add to the stdlib.

-- 
Terry Jan Reedy