https://github.com/python/cpython/commit/f9a40c386d4d07dab1c15c6618807306a5119818
commit: f9a40c386d4d07dab1c15c6618807306a5119818
branch: main
author: Raymond Hettinger <[email protected]>
committer: rhettinger <[email protected]>
date: 2025-09-04T10:50:29-05:00
summary:
Add random_derangement recipe gh-138377
files:
M Doc/library/random.rst
diff --git a/Doc/library/random.rst b/Doc/library/random.rst
index b1120b3a4d8eb4..e9cebf46d57b01 100644
--- a/Doc/library/random.rst
+++ b/Doc/library/random.rst
@@ -630,7 +630,8 @@ Recipes
-------
These recipes show how to efficiently make random selections
-from the combinatoric iterators in the :mod:`itertools` module:
+from the combinatoric iterators in the :mod:`itertools` module
+or the :pypi:`more-itertools` project:
.. testcode::
import random
@@ -661,6 +662,17 @@ from the combinatoric iterators in the :mod:`itertools`
module:
indices = sorted(random.choices(range(n), k=r))
return tuple(pool[i] for i in indices)
+ def random_derangement(iterable):
+ "Choose a permutation where no element is in its original position."
+ seq = tuple(iterable)
+ if len(seq) < 2:
+ raise ValueError('derangments require at least two values')
+ perm = list(seq)
+ while True:
+ random.shuffle(perm)
+ if all(p != q for p, q in zip(seq, perm)):
+ return tuple(perm)
+
The default :func:`.random` returns multiples of 2⁻⁵³ in the range
*0.0 ≤ x < 1.0*. All such numbers are evenly spaced and are exactly
representable as Python floats. However, many other representable
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