Add math.perm()

[serhiy-storchaka]

BPO	37128
Nosy	@malemburg, @rhettinger, @mdickinson, @serhiy-storchaka
PRs	bpo-37128: Add math.perm(). #13731

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2019-06-02.08:53:34.710>
created_at = <Date 2019-06-01.20:47:29.810>
labels = ['3.8', 'type-feature', 'library']
title = 'Add math.perm()'
updated_at = <Date 2019-06-02.08:53:41.312>
user = 'https://github.com/serhiy-storchaka'

bugs.python.org fields:

activity = <Date 2019-06-02.08:53:41.312>
actor = 'serhiy.storchaka'
assignee = 'none'
closed = True
closed_date = <Date 2019-06-02.08:53:34.710>
closer = 'serhiy.storchaka'
components = ['Library (Lib)']
creation = <Date 2019-06-01.20:47:29.810>
creator = 'serhiy.storchaka'
dependencies = []
files = []
hgrepos = []
issue_num = 37128
keywords = ['patch']
message_count = 4.0
messages = ['344226', '344227', '344228', '344259']
nosy_count = 5.0
nosy_names = ['lemburg', 'rhettinger', 'mark.dickinson', 'stutzbach', 'serhiy.storchaka']
pr_nums = ['13731']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue37128'
versions = ['Python 3.8']

[serhiy-storchaka]

The function which returns the number of ways to choose k items from n items without repetition and without order was added in bpo-35431. This functions is always goes in pair with other function, which returns the number of ways to choose k items from n items without repetition and with order. These functions are always learned together in curses of combinatorics. Often C(n,k) is determined via P(n,k) (and both are determined via factorial).

P(n, k) = n! / (n-k)!
C(n, k) = P(n, k) / k!

The proposed PR adds meth.perm(). It shares most of the code with math.comb().

[rhettinger]

+0 from me. It is inevitable that this will be requested now that we have comb(). The need for this is much less though.

[mdickinson]

+0 from me. It is inevitable that this will be requested now that we have comb().

Exactly my thoughts, too. I don't think I'll have any use for math.perm; the main reason to add it will be to satisfy all those who got taught combinations and permutations at the same time, see them as a unit, and then get surprised when they find math.comb without math.perm.

The good part is that now that we have math.comb, I think everything about the API of math.perm is already clear (including what to call it).

[serhiy-storchaka]

New changeset 5ae299a by Serhiy Storchaka in branch 'master':
bpo-37128: Add math.perm(). (GH-13731)
5ae299a