字符串 -- 不可改變的序列

如同大多數高級編程語言一樣，變長字符串是 Python 中的基本類型。Python 在“后臺”分配內存以保存字符串（或其它值），程序員不必為此操心。Python 還有一些其它高級語言沒有的字符串處理功能。

在 Python 中，字符串是“不可改變的序列”。盡管不能“按位置”修改字符串（如字節組），但程序可以引用字符串的元素或子序列，就象使用任何序列一樣。Python 使用靈活的“分片”操作來引用子序列，字符片段的格式類似于電子表格中一定范圍的行或列。以下交互式會話說明了字符串和字符片段的的用法：
字符串和分片

>>> s =     "mary had a little lamb">>> s[0]     # index is zero-based    'm'>>> s[3] =     'x'     # changing element in-place failsTraceback (innermost last): File     "<stdin>", line 1,     in     ?TypeError: object doesn't support item assignment>>> s[11:18]     # 'slice' a subsequence    'little '>>> s[:4]     # empty slice-begin assumes zero    'mary'>>> s[4]     # index 4 is not included in slice [:4]    ' '>>> s[5:-5]     # can use "from end" index with negatives    'had a little'>>> s[:5]+s[5:]     # slice-begin & slice-end are complimentary    'mary had a little lamb'

另一個功能強大的字符串操作就是簡單的 in 關鍵字。它提供了兩個直觀有效的構造：
in 關鍵字

>>> s =     "mary had a little lamb">>>     for     c     in     s[11:18]:     print     c,     # print each char in slice...l i t t l e>>>     if    'x'     in     s:     print    'got x'     # test for char occurrence...>>>     if    'y'     in     s:     print    'got y'     # test for char occurrence...got y

在 Python 中，有幾種方法可以構成字符串文字。可以使用單引號或雙引號，只要左引號和右引號匹配，常用的還有其它引號的變化形式。如果字符串包含換行符或嵌入引號，三重引號可以很方便地定義這樣的字符串，如下例所示：
三重引號的使用

>>> s2 =     """Mary had a little lamb... its fleece was white as snow... and everywhere that Mary went... the lamb was sure to go""">>>     print     s2Mary had a little lambits fleece was white as snow    and     everywhere that Mary wentthe lamb was sure to go

使用單引號或三重引號的字符串前面可以加一個字母 "r" 以表示 Python 不應該解釋規則表達式特殊字符。例如：
使用 "r-strings"

>>> s3 =     "this /n and /n that">>>     print     s3this    and    that>>> s4 = r    "this /n and /n that">>>     print     s4this /n     and     /n that