[docs] [issue18928] Remove misleading documentation for random.shuffle

David Benbennick report at bugs.python.org
Thu Sep 5 02:18:34 CEST 2013

New submission from David Benbennick:

Since Python 2.1 [1], when random.shuffle was added, the documentation has said:

"""Note that for even rather small len(x), the total number of permutations of x is larger than the period of most random number generators; this implies that most permutations of a long sequence can never be generated."""

This comment is incorrect and misleading.  In fact, I claim that shuffle can produce "all" permutations for any representable sequence.

To shuffle a sequence of length N requires log(N!) ~ N * log(N/e) bits of randomness [2].  The random module provides a generator with "a period of 2**19937-1", meaning you can get 2**19937 bits of randomness out of it before it starts repeating.

All of which is to say that any representable sequence, say N < 2**50, will need no more than 2**60 bits of randomness to shuffle.  That is well within the period of the random number generator.

Attached is a patch that deletes the comment.

An illustration of this misconception is at [3].

1: http://docs.python.org/release/2.1/lib/module-random.html
2: https://en.wikipedia.org/wiki/Factorial#Rate_of_growth_and_approximations_for_large_n
3: http://stackoverflow.com/questions/3062741/maximal-length-of-list-to-shuffle-with-python-random-shuffle

assignee: docs at python
components: Documentation
files: mywork.patch
keywords: patch
messages: 196971
nosy: dbenbenn, docs at python
priority: normal
severity: normal
status: open
title: Remove misleading documentation for random.shuffle
Added file: http://bugs.python.org/file31593/mywork.patch

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