One argument form of math.perm()

[rhettinger]

BPO	37178
Nosy	@tim-one, @rhettinger, @mdickinson, @serhiy-storchaka
PRs	bpo-37178: Allow a one argument form of math.perm() #13905 [3.8] bpo-37178: Allow a one argument form of math.perm() (GH-13905) #13919

^{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 2019-06-08.23:55:34.895>
created_at = <Date 2019-06-06.16:30:28.309>
labels = ['3.8', 'library', '3.9']
title = 'One argument form of math.perm()'
updated_at = <Date 2019-06-08.23:55:34.895>
user = 'https://github.com/rhettinger'

bugs.python.org fields:

activity = <Date 2019-06-08.23:55:34.895>
actor = 'rhettinger'
assignee = 'none'
closed = True
closed_date = <Date 2019-06-08.23:55:34.895>
closer = 'rhettinger'
components = ['Library (Lib)']
creation = <Date 2019-06-06.16:30:28.309>
creator = 'rhettinger'
dependencies = []
files = []
hgrepos = []
issue_num = 37178
keywords = ['patch']
message_count = 7.0
messages = ['344833', '344835', '344837', '344840', '345020', '345053', '345054']
nosy_count = 4.0
nosy_names = ['tim.peters', 'rhettinger', 'mark.dickinson', 'serhiy.storchaka']
pr_nums = ['13905', '13919']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue37178'
versions = ['Python 3.8', 'Python 3.9']

[rhettinger]

The perm() function should have a one argument form and change its signature to perm(n, k=None). This matches what itertools:

itertools.permutations(iterable, r=None)
   Return successive r length permutations of elements
   in the iterable.

If r is not specified or is None, then r defaults to
the length of the iterable and all possible full-length
permutations are generated.

[serhiy-storchaka]

perm(n, n) = factorial(n)

[tim-one]

I agree: perm(n) should return factorial(n).

[serhiy-storchaka]

I never seen a one argument form of P(n, k) in mathematics. itertools.permutations() corresponds two functions: math.perm() and math .factorial(). Unless you a going to add a new function in the itertools module equal to a one argument form of itertools.permutations() there will be no full symmetry.

[rhettinger]

It's possible the word permutations means something different to you than it does to me. The itertools.permutations() function is only one function, one that includes the default option to generate all permutations. It does the same as permute(n, r) and permute(n) in Maple; and the same as Permutations[list, n] and Permutations[list] in Mathematica. For the math module, perm(n) is just a common special case that means all possible arrangements.

[rhettinger]

New changeset e119b3d by Raymond Hettinger in branch 'master':
bpo-37178: Allow a one argument form of math.perm() (GH-13905)
e119b3d

[rhettinger]

New changeset feaceaa by Raymond Hettinger (Miss Islington (bot)) in branch '3.8':
bpo-37178: Allow a one argument form of math.perm() (GH-13905) (GH-13919)
feaceaa