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2.3.6.4 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 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([cmpfunc=None])` | sort the items of s in place | (7), (8), (9), (10)

Notes:

(1)
  t must have the same length as the slice it is replacing.

(2)
  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.

(3)
  Raises an exception when x is not a list object.

(4)
  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.

(5)
  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.

(6)
  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.

(7)
  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.

(8)
  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; for example to sort a list in reverse order it is much faster to call `sort()` followed by `reverse()` than to use `sort()` with a comparison function that reverses the ordering of the elements. Passing `None` as the comparison function is semantically equivalent to calling `sort()` with no comparison function. Changed in version 2.3: Support for `None` as an equivalent to omitting cmpfunc was added.

  As an example of using the cmpfunc argument to the `sort()` method, consider sorting a list of sequences by the second element of that list:

      def mycmp(a, b):
          return cmp(a[1], b[1])

      mylist.sort(mycmp)

  A more time-efficient approach for reasonably-sized data structures can often be used:

      tmplist = [(x[1], x) for x in mylist]
      tmplist.sort()
      mylist = [x for (key, x) in tmplist]

(9)
  Whether the `sort()` method is stable is not defined by the language (a sort is stable if it guarantees not to change the relative order of elements that compare equal). In the C implementation of Python, sorts were stable only by accident through Python 2.2. The C implementation of Python 2.3 introduced a stable `sort()` method, but code that intends to be portable across implementations and versions must not rely on stability.

(10)
  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 makes the list appear empty for the duration, and raises `ValueError` if it can detect that the list has been mutated during a sort.