"textwrap" --- 文本包装与填充
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**源代码:** Lib/textwrap.py

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"textwrap" 模块提供了一些快捷函数，以及可以完成所有工作的类
"TextWrapper"。 如果你只是要包装或填充一两个文本字符串，快捷函数应该就
够用了；否则的话，你应该使用 "TextWrapper" 的实例来提高效率。

textwrap.wrap(text, width=70, **kwargs)

   包装 *text* (字符串) 中的单独段落以使每行长度最多为 *width* 个字符
   。 返回由输出行组成的列表，行尾不带换行符。

   可选的关键字参数对应于 "TextWrapper" 的实例属性，具体文档见下。
   *width* 默认为 "70"。

   请参阅 "TextWrapper.wrap()" 方法了解有关 "wrap()" 行为的详细信息。

textwrap.fill(text, width=70, **kwargs)

   包装 *text* 中的单独段落，并返回一个包含被包装段落的单独字符串。
   "fill()" 是以下语句的快捷方式

      "\n".join(wrap(text, ...))

   特别要说明的是，"fill()" 接受与 "wrap()" 完全相同的关键字参数。

textwrap.shorten(text, width, **kwargs)

   折叠并截短给定的 *text* 以符合给定的 *width*。

   首先将折叠 *text* 中的空格（所有连续空格替换为单个空格）。 如果结果
   能适合 *width* 则将其返回。 否则将丢弃足够数量的末尾单词以使得剩余
   单词加 "placeholder" 能适合 "width":

      >>> textwrap.shorten("Hello  world!", width=12)
      'Hello world!'
      >>> textwrap.shorten("Hello  world!", width=11)
      'Hello [...]'
      >>> textwrap.shorten("Hello world", width=10, placeholder="...")
      'Hello...'

   可选的关键字参数对应于 "TextWrapper" 的实际属性，具体见下文。 请注
   意文本在被传入 "TextWrapper" 的 "fill()" 函数之前会被折叠，因此改变
   "tabsize", "expand_tabs", "drop_whitespace" 和 "replace_whitespace"
   的值将没有任何效果。

   3.4 新版功能.

textwrap.dedent(text)

   Remove any common leading whitespace from every line in *text*.

   This can be used to make triple-quoted strings line up with the
   left edge of the display, while still presenting them in the source
   code in indented form.

   Note that tabs and spaces are both treated as whitespace, but they
   are not equal: the lines ""  hello"" and ""\thello"" are considered
   to have no common leading whitespace.

   Lines containing only whitespace are ignored in the input and
   normalized to a single newline character in the output.

   例如:

      def test():
          # end first line with \ to avoid the empty line!
          s = '''\
          hello
            world
          '''
          print(repr(s))          # prints '    hello\n      world\n    '
          print(repr(dedent(s)))  # prints 'hello\n  world\n'

textwrap.indent(text, prefix, predicate=None)

   Add *prefix* to the beginning of selected lines in *text*.

   Lines are separated by calling "text.splitlines(True)".

   By default, *prefix* is added to all lines that do not consist
   solely of whitespace (including any line endings).

   例如:

      >>> s = 'hello\n\n \nworld'
      >>> indent(s, '  ')
      '  hello\n\n \n  world'

   The optional *predicate* argument can be used to control which
   lines are indented. For example, it is easy to add *prefix* to even
   empty and whitespace-only lines:

      >>> print(indent(s, '+ ', lambda line: True))
      + hello
      +
      +
      + world

   3.3 新版功能.

"wrap()", "fill()" and "shorten()" work by creating a "TextWrapper"
instance and calling a single method on it.  That instance is not
reused, so for applications that process many text strings using
"wrap()" and/or "fill()", it may be more efficient to create your own
"TextWrapper" object.

Text is preferably wrapped on whitespaces and right after the hyphens
in hyphenated words; only then will long words be broken if necessary,
unless "TextWrapper.break_long_words" is set to false.

class textwrap.TextWrapper(**kwargs)

   The "TextWrapper" constructor accepts a number of optional keyword
   arguments.  Each keyword argument corresponds to an instance
   attribute, so for example

      wrapper = TextWrapper(initial_indent="* ")

   is the same as

      wrapper = TextWrapper()
      wrapper.initial_indent = "* "

   You can re-use the same "TextWrapper" object many times, and you
   can change any of its options through direct assignment to instance
   attributes between uses.

   The "TextWrapper" instance attributes (and keyword arguments to the
   constructor) are as follows:

   width

      (default: "70") The maximum length of wrapped lines.  As long as
      there are no individual words in the input text longer than
      "width", "TextWrapper" guarantees that no output line will be
      longer than "width" characters.

   expand_tabs

      (default: "True") If true, then all tab characters in *text*
      will be expanded to spaces using the "expandtabs()" method of
      *text*.

   tabsize

      (default: "8") If "expand_tabs" is true, then all tab characters
      in *text* will be expanded to zero or more spaces, depending on
      the current column and the given tab size.

      3.3 新版功能.

   replace_whitespace

      (default: "True") If true, after tab expansion but before
      wrapping, the "wrap()" method will replace each whitespace
      character with a single space.  The whitespace characters
      replaced are as follows: tab, newline, vertical tab, formfeed,
      and carriage return ("'\t\n\v\f\r'").

      注解: If "expand_tabs" is false and "replace_whitespace" is
        true, each tab character will be replaced by a single space,
        which is *not* the same as tab expansion.

      注解: If "replace_whitespace" is false, newlines may appear in
        the middle of a line and cause strange output. For this
        reason, text should be split into paragraphs (using
        "str.splitlines()" or similar) which are wrapped separately.

   drop_whitespace

      (default: "True") If true, whitespace at the beginning and
      ending of every line (after wrapping but before indenting) is
      dropped. Whitespace at the beginning of the paragraph, however,
      is not dropped if non-whitespace follows it.  If whitespace
      being dropped takes up an entire line, the whole line is
      dropped.

   initial_indent

      (default: "''") String that will be prepended to the first line
      of wrapped output.  Counts towards the length of the first line.
      The empty string is not indented.

   subsequent_indent

      (default: "''") String that will be prepended to all lines of
      wrapped output except the first.  Counts towards the length of
      each line except the first.

   fix_sentence_endings

      (default: "False") If true, "TextWrapper" attempts to detect
      sentence endings and ensure that sentences are always separated
      by exactly two spaces.  This is generally desired for text in a
      monospaced font. However, the sentence detection algorithm is
      imperfect: it assumes that a sentence ending consists of a
      lowercase letter followed by one of "'.'", "'!'", or "'?'",
      possibly followed by one of "'"'" or ""'"", followed by a space.
      One problem with this is algorithm is that it is unable to
      detect the difference between "Dr." in

         [...] Dr. Frankenstein's monster [...]

      and "Spot." in

         [...] See Spot. See Spot run [...]

      "fix_sentence_endings" is false by default.

      Since the sentence detection algorithm relies on
      "string.lowercase" for the definition of "lowercase letter," and
      a convention of using two spaces after a period to separate
      sentences on the same line, it is specific to English-language
      texts.

   break_long_words

      (default: "True") If true, then words longer than "width" will
      be broken in order to ensure that no lines are longer than
      "width".  If it is false, long words will not be broken, and
      some lines may be longer than "width".  (Long words will be put
      on a line by themselves, in order to minimize the amount by
      which "width" is exceeded.)

   break_on_hyphens

      (default: "True") If true, wrapping will occur preferably on
      whitespaces and right after hyphens in compound words, as it is
      customary in English. If false, only whitespaces will be
      considered as potentially good places for line breaks, but you
      need to set "break_long_words" to false if you want truly
      insecable words.  Default behaviour in previous versions was to
      always allow breaking hyphenated words.

   max_lines

      (default: "None") If not "None", then the output will contain at
      most *max_lines* lines, with *placeholder* appearing at the end
      of the output.

      3.4 新版功能.

   placeholder

      (default: "' [...]'") String that will appear at the end of the
      output text if it has been truncated.

      3.4 新版功能.

   "TextWrapper" also provides some public methods, analogous to the
   module-level convenience functions:

   wrap(text)

      Wraps the single paragraph in *text* (a string) so every line is
      at most "width" characters long.  All wrapping options are taken
      from instance attributes of the "TextWrapper" instance.  Returns
      a list of output lines, without final newlines.  If the wrapped
      output has no content, the returned list is empty.

   fill(text)

      Wraps the single paragraph in *text*, and returns a single
      string containing the wrapped paragraph.
