3.6.4 Mutable Sequence Types

List objects support additional operations that allow in-place modification of the object. Other mutable sequence types (when added to the language) should also support these operations. Strings and tuples are immutable sequence types: 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 the contents of the iterable t
del s[i:j] same as s[i:j] = []
s[i:j:k] = t the elements of s[i:j:k] are replaced by those of t (1)
del s[i:j:k] removes the elements of s[i:j:k] from the list
s.append(x) same as s[len(s):len(s)] = [x] (2)
s.extend(x) same as s[len(s):len(s)] = x (3)
s.count(x) return number of i's for which s[i] == x
s.index(x[, i[, j]]) return smallest k such that s[k] == x and i <= k < j (4)
s.insert(i, x) same as s[i:i] = [x] (5)
s.pop([i]) same as x = s[i]; del s[i]; return x (6)
s.remove(x) same as del s[s.index(x)] (4)
s.reverse() reverses the items of s in place (7)
s.sort([cmp[, key[, reverse]]]) sort the items of s in place (7), (8), (9), (10)
t must have the same length as the slice it is replacing.

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.

x can be any iterable object.

Raises ValueError when x is not found in s. When a negative index is passed as the second or third parameter to the index() method, the list length is added, as for slice indices. If it is still negative, it is truncated to zero, as for slice indices. Changed in version 2.3: Previously, index() didn't have arguments for specifying start and stop positions.

When a negative index is passed as the first parameter to the insert() method, the list length is added, as for slice indices. If it is still negative, it is truncated to zero, as for slice indices. Changed in version 2.3: Previously, all negative indices were truncated to zero.

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.

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.

The sort() method takes optional arguments for controlling the comparisons.

cmp specifies a custom 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: "cmp=lambda x,y: cmp(x.lower(), y.lower())"

key specifies a function of one argument that is used to extract a comparison key from each list element: "key=str.lower"

reverse is a boolean value. If set to True, then the list elements are sorted as if each comparison were reversed.

In general, the key and reverse conversion processes are much faster than specifying an equivalent cmp function. This is because cmp is called multiple times for each list element while key and reverse touch each element only once.

Changed in version 2.3: Support for None as an equivalent to omitting cmp was added.

Changed in version 2.4: Support for key and reverse was added.

Starting with Python 2.3, the sort() method is guaranteed to be stable. A sort is stable if it guarantees not to change the relative order of elements that compare equal -- this is helpful for sorting in multiple passes (for example, sort by department, then by salary grade).

While a list is being sorted, the effect of attempting to mutate, or even inspect, the list is undefined. The C implementation of Python 2.3 and newer makes the list appear empty for the duration, and raises ValueError if it can detect that the list has been mutated during a sort.

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