2.2.6.4 Mutable Sequence Types

List objects support additional operations that allow in-place modification of the object. These operations would be supported by other mutable sequence types (when added to the language) as well. Strings and tuples are immutable sequence types and such objects cannot be modified once created. The following operations are defined on mutable sequence types (where x is an arbitrary object):

Operation Result Notes

s[i] = x item i of s is replaced by x
s[i:j] = t slice of s from i to j is replaced by t
del s[i:j] same as s[i:j] = []
s.append(x) same as s[len(s):len(s)] = [x] (1)
s.extend(x) same as s[len(s):len(s)] = x (2)
s.count(x) return number of i's for which s[i] == x
s.index(x) return smallest i such that s[i] == x (3)
s.insert(i, x) same as s[i:i] = [x] if i >= 0 (4)
s.pop([i]) same as x = s[i]; del s[i]; return x (5)
s.remove(x) same as del s[s.index(x)] (3)
s.reverse() reverses the items of s in place (6)
s.sort([cmpfunc]) sort the items of s in place (6), (7)

Notes:

(1): The C implementation of Python has historically accepted multiple parameters and implicitly joined them into a tuple; this no longer works in Python 2.0. Use of this misfeature has been deprecated since Python 1.4.
(2): Raises an exception when x is not a list object. The extend() method is experimental and not supported by mutable sequence types other than lists.
(3): Raises ValueError when x is not found in s.
(4): When a negative index is passed as the first parameter to the insert() method, the new element is prepended to the sequence.
(5): The pop() method is only supported by the list and array types. The optional argument i defaults to -1, so that by default the last item is removed and returned.
(6): The sort() and reverse() methods modify the list in place for economy of space when sorting or reversing a large list. To remind you that they operate by side effect, they don't return the sorted or reversed list.
(7): The sort() method takes an optional argument specifying a comparison function of two arguments (list items) which should return a negative, zero or positive number depending on whether the first argument is considered smaller than, equal to, or larger than the second argument. Note that this slows the sorting process down considerably; e.g. to sort a list in reverse order it is much faster to use calls to the methods sort() and reverse() than to use the built-in function sort() with a comparison function that reverses the ordering of the elements.

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