2.3.6 Sequence Types

2.3.6 Sequence Types

There are six sequence types: strings, Unicode strings, lists, tuples, buffers, and xrange objects.

String literals are written in single or double quotes: 'xyzzy', "frobozz". See chapter 2 of the Python Reference Manual for more about string literals. Unicode strings are much like strings, but are specified in the syntax using a preceeding "u" character: u'abc', u"def". Lists are constructed with square brackets, separating items with commas: [a, b, c]. Tuples are constructed by the comma operator (not within square brackets), with or without enclosing parentheses, but an empty tuple must have the enclosing parentheses, such as a, b, c or (). A single item tuple must have a trailing comma, such as (d,).

Buffer objects are not directly supported by Python syntax, but can be created by calling the builtin function buffer(). They don't support concatenation or repetition.

Xrange objects are similar to buffers in that there is no specific syntax to create them, but they are created using the xrange() function. They don't support slicing, concatenation or repetition, and using in, not in, min() or max() on them is inefficient.

Most sequence types support the following operations. The "in" and "not in" operations have the same priorities as the comparison operations. The "+" and "*" operations have the same priority as the corresponding numeric operations.^2.7

This table lists the sequence operations sorted in ascending priority (operations in the same box have the same priority). In the table, s and t are sequences of the same type; n, i and j are integers:

Notes:

(1)

When s is a string or Unicode string object the in and not in operations act like a substring test. In Python versions before 2.3, x had to be a string of length 1. In Python 2.3 and beyond, x may be a string of any length.

(2)

Values of n less than 0 are treated as 0 (which yields an empty sequence of the same type as s). Note also that the copies are shallow; nested structures are not copied. This often haunts new Python programmers; consider:

>>> lists = [[]] * 3
>>> lists
[[], [], []]
>>> lists[0].append(3)
>>> lists
[[3], [3], [3]]

What has happened is that [[]] is a one-element list containing an empty list, so all three elements of [[]] * 3 are (pointers to) this single empty list. Modifying any of the elements of lists modifies this single list. You can create a list of different lists this way:

>>> lists = [[] for i in range(3)]
>>> lists[0].append(3)
>>> lists[1].append(5)
>>> lists[2].append(7)
>>> lists
[[3], [5], [7]]

(3)

If i or j is negative, the index is relative to the end of the string: len(s) + i or len(s) + j is substituted. But note that -0 is still 0.

(4)

The slice of s from i to j is defined as the sequence of items with index k such that i <= k < j. If i or j is greater than len(s), use len(s). If i is omitted, use 0. If j is omitted, use len(s). If i is greater than or equal to j, the slice is empty.

(5)

The slice of s from i to j with step k is defined as the sequence of items with index x = i + n*k such that . In other words, the indices are i, i+k, i+2*k, i+3*k and so on, stopping when j is reached (but never including j). If i or j is greater than len(s), use len(s). If i or j are omitted then they become ``end'' values (which end depends on the sign of k). Note, k cannot be zero.

(6)

If s and t are both strings, some Python implementations such as CPython can usually perform an in-place optimization for assignments of the form s=s+t or s+=t. When applicable, this optimization makes quadratic run-time much less likely. This optimization is both version and implementation dependent. For performance sensitive code, it is preferrable to use the str.join() method which assures consistent linear concatenation performance across versions and implementations. Changed in version 2.4: Formerly, string concatenation never occurred in-place.

Footnotes

... operations.^2.7

They must have since the parser can't tell the type of the operands.

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Release 2.4, documentation updated on 29 November 2004.