Python: shuffling list, but keeping some elements frozen

Question

I\'ve such a problem:

There is a list of elements of class CAnswer (no need to describe the class), and I need to shuffle it, but with one constraint -

Answer 1

Use the fact that a list has fast remove and insert:

• enumerate fixed elements and copy them and their index
• delete fixed elements from list
• shuffle remaining sub-set
• put fixed elements back in

See https://stackoverflow.com/a/25233037/3449962 for a more general solution.

This will use in-place operations with memory overhead that depends on the number of fixed elements in the list. Linear in time. A possible implementation of shuffle_subset:

#!/usr/bin/env python
"""Shuffle elements in a list, except for a sub-set of the elments.

The sub-set are those elements that should retain their position in
the list.  Some example usage:

>>> from collections import namedtuple
>>> class CAnswer(namedtuple("CAnswer","x fixed")):
...             def __bool__(self):
...                     return self.fixed is True
...             __nonzero__ = __bool__  # For Python 2. Called by bool in Py2.
...             def __repr__(self):
...                     return "".format(self.x)
...
>>> val = [3, 2, 0, 1, 5, 9, 4]
>>> fix = [2, 5]
>>> lst = [ CAnswer(v, i in fix) for i, v in enumerate(val)]

>>> print("Start   ", 0, ": ", lst)
Start    0 :  [, , , , , , ]

Using a predicate to filter.

>>> for i in range(4):  # doctest: +NORMALIZE_WHITESPACE
...     shuffle_subset(lst, lambda x : x.fixed)
...     print([lst[i] for i in fix], end=" ")
...
[, ] [, ] [, ] [, ]

>>> for i in range(4):                # doctest: +NORMALIZE_WHITESPACE
...     shuffle_subset(lst)           # predicate = bool()
...     print([lst[i] for i in fix], end=" ")
...
[, ] [, ] [, ] [, ]

"""
from __future__ import print_function
import random


def shuffle_subset(lst, predicate=None):
    """All elements in lst, except a sub-set, are shuffled.

    The predicate defines the sub-set of elements in lst that should
    not be shuffled:

      + The predicate is a callable that returns True for fixed
      elements, predicate(element) --> True or False.

      + If the predicate is None extract those elements where
      bool(element) == True.

    """
    predicate = bool if predicate is None else predicate
    fixed_subset = [(i, e) for i, e in enumerate(lst) if predicate(e)]

    fixed_subset.reverse()      # Delete fixed elements from high index to low.
    for i, _ in fixed_subset:
        del lst[i]

    random.shuffle(lst)

    fixed_subset.reverse()      # Insert fixed elements from low index to high.
    for i, e in fixed_subset:
        lst.insert(i, e)

if __name__ == "__main__":
    import doctest
    doctest.testmod()