"string" --- 常見的字串操作
***************************

**原始碼：**Lib/string.py

======================================================================

也參考:

  Text Sequence Type --- str

  String Methods


字串常數
========

此模組中定義的常數為：

string.ascii_letters

   下文描述的 "ascii_lowercase" 和 "ascii_uppercase" 常數的串接，該值
   不依賴於區域設定。

string.ascii_lowercase

   小寫字母 "'abcdefghijklmnopqrstuvwxyz'"。該值與地區設定無關且不會改
   變。

string.ascii_uppercase

   大寫字母  "’ABCDEFGHIJKLMNOPQRSTUVWXYZ’"。 此值與地區設定無關且不會
   改變。

string.digits

   字串 "’0123456789’"。

string.hexdigits

   字串 "’0123456789abcdefABCDEF’"。

string.octdigits

   字串 "'01234567'"。

string.punctuation

   在 "C" 語言中被視為標點符號的 ASCII 字元的字串：
   "!”#$%&’()*+,-./:;<=>?@[\]^_`{|}~"。

string.printable

   ASCII 字元的字串是可被列印輸出的。 這是 "digits"、"ascii_letters"、
   "punctuation" 和 "whitespace" 的組合。

string.whitespace

   包含所有 ASCII 字元的字串都視為空白字元 (whitespace)。 包含空格
   (space)、製表符號 (tab)、換行符號 (linefeed)、return、換頁符號
   (formfeed) 和垂直製表符號 (vertical tab) 這些字元。


自訂字串格式
============

透過 **PEP 3101** 中描述的 "format()" 方法，內建字串類別提供了進行複雜
變數替換和數值格式化的能力。"string" 模組中的 "Formatter" 類別模組可讓
你使用與內建 "format()" 方法相同的實作來建立和自訂你自己的字串格式化行
為。

class string.Formatter

   "Formatter" 類別有以下的公開方法：

   format(format_string, /, *args, **kwargs)

      主要的 API 方法。它接收一個格式字串及一組任意的位置引數與關鍵字
      引數，是呼叫 "vformat()" 的包裝器 (wrapper)。

      在 3.7 版的變更: 現在格式字串引數是僅限位置參數。

   vformat(format_string, args, kwargs)

      This function does the actual work of formatting.  It is exposed
      as a separate function for cases where you want to pass in a
      predefined dictionary of arguments, rather than unpacking and
      repacking the dictionary as individual arguments using the
      "*args" and "**kwargs" syntax.  "vformat()" does the work of
      breaking up the format string into character data and
      replacement fields.  It calls the various methods described
      below.

   In addition, the "Formatter" defines a number of methods that are
   intended to be replaced by subclasses:

   parse(format_string)

      將 format_string 放入迴圈，並回傳一個可疊代物件，其元素為
      (*literal_text*, *field_name*, *format_spec*, *conversion*)。這
      會被 "vformat()" 用於將字串裁切為字面文本或替換欄位。

      The values in the tuple conceptually represent a span of literal
      text followed by a single replacement field.  If there is no
      literal text (which can happen if two replacement fields occur
      consecutively), then *literal_text* will be a zero-length
      string.  If there is no replacement field, then the values of
      *field_name*, *format_spec* and *conversion* will be "None".

   get_field(field_name, args, kwargs)

      Given *field_name* as returned by "parse()" (see above), convert
      it to an object to be formatted.  Returns a tuple (obj,
      used_key).  The default version takes strings of the form
      defined in **PEP 3101**, such as "0[name]" or "label.title".
      *args* and *kwargs* are as passed in to "vformat()".  The return
      value *used_key* has the same meaning as the *key* parameter to
      "get_value()".

   get_value(key, args, kwargs)

      Retrieve a given field value.  The *key* argument will be either
      an integer or a string.  If it is an integer, it represents the
      index of the positional argument in *args*; if it is a string,
      then it represents a named argument in *kwargs*.

      The *args* parameter is set to the list of positional arguments
      to "vformat()", and the *kwargs* parameter is set to the
      dictionary of keyword arguments.

      For compound field names, these functions are only called for
      the first component of the field name; subsequent components are
      handled through normal attribute and indexing operations.

      So for example, the field expression '0.name' would cause
      "get_value()" to be called with a *key* argument of 0.  The
      "name" attribute will be looked up after "get_value()" returns
      by calling the built-in "getattr()" function.

      If the index or keyword refers to an item that does not exist,
      then an "IndexError" or "KeyError" should be raised.

   check_unused_args(used_args, args, kwargs)

      Implement checking for unused arguments if desired.  The
      arguments to this function is the set of all argument keys that
      were actually referred to in the format string (integers for
      positional arguments, and strings for named arguments), and a
      reference to the *args* and *kwargs* that was passed to vformat.
      The set of unused args can be calculated from these parameters.
      "check_unused_args()" is assumed to raise an exception if the
      check fails.

   format_field(value, format_spec)

      "format_field()" simply calls the global "format()" built-in.
      The method is provided so that subclasses can override it.

   convert_field(value, conversion)

      Converts the value (returned by "get_field()") given a
      conversion type (as in the tuple returned by the "parse()"
      method).  The default version understands 's' (str), 'r' (repr)
      and 'a' (ascii) conversion types.


格式化文字語法
==============

The "str.format()" method and the "Formatter" class share the same
syntax for format strings (although in the case of "Formatter",
subclasses can define their own format string syntax).  The syntax is
related to that of formatted string literals, but it is less
sophisticated and, in particular, does not support arbitrary
expressions.

Format strings contain "replacement fields" surrounded by curly braces
"{}". Anything that is not contained in braces is considered literal
text, which is copied unchanged to the output.  If you need to include
a brace character in the literal text, it can be escaped by doubling:
"{{" and "}}".

The grammar for a replacement field is as follows:

   replacement_field ::= "{" [field_name] ["!" conversion] [":" format_spec] "}"
   field_name        ::= arg_name ("." attribute_name | "[" element_index "]")*
   arg_name          ::= [identifier | digit+]
   attribute_name    ::= identifier
   element_index     ::= digit+ | index_string
   index_string      ::= <any source character except "]"> +
   conversion        ::= "r" | "s" | "a"
   format_spec       ::= format-spec:format_spec

In less formal terms, the replacement field can start with a
*field_name* that specifies the object whose value is to be formatted
and inserted into the output instead of the replacement field. The
*field_name* is optionally followed by a  *conversion* field, which is
preceded by an exclamation point "'!'", and a *format_spec*, which is
preceded by a colon "':'".  These specify a non-default format for the
replacement value.

另請參閱 格式規格 (Format Specification) 迷你語言 部份。

The *field_name* itself begins with an *arg_name* that is either a
number or a keyword.  If it's a number, it refers to a positional
argument, and if it's a keyword, it refers to a named keyword
argument. An *arg_name* is treated as a number if a call to
"str.isdecimal()" on the string would return true. If the numerical
arg_names in a format string are 0, 1, 2, ... in sequence, they can
all be omitted (not just some) and the numbers 0, 1, 2, ... will be
automatically inserted in that order. Because *arg_name* is not quote-
delimited, it is not possible to specify arbitrary dictionary keys
(e.g., the strings "'10'" or "':-]'") within a format string. The
*arg_name* can be followed by any number of index or attribute
expressions. An expression of the form "'.name'" selects the named
attribute using "getattr()", while an expression of the form
"'[index]'" does an index lookup using "__getitem__()".

在 3.1 版的變更: The positional argument specifiers can be omitted for
"str.format()", so "'{} {}'.format(a, b)" is equivalent to "'{0}
{1}'.format(a, b)".

在 3.4 版的變更: The positional argument specifiers can be omitted for
"Formatter".

Some simple format string examples:

   "First, thou shalt count to {0}"  # References first positional argument
   "Bring me a {}"                   # Implicitly references the first positional argument
   "From {} to {}"                   # Same as "From {0} to {1}"
   "My quest is {name}"              # References keyword argument 'name'
   "Weight in tons {0.weight}"       # 'weight' attribute of first positional arg
   "Units destroyed: {players[0]}"   # First element of keyword argument 'players'.

The *conversion* field causes a type coercion before formatting.
Normally, the job of formatting a value is done by the "__format__()"
method of the value itself.  However, in some cases it is desirable to
force a type to be formatted as a string, overriding its own
definition of formatting.  By converting the value to a string before
calling "__format__()", the normal formatting logic is bypassed.

目前支援三種轉換旗標："'!s'" 會對該值呼叫 "str()"，"'!r'" 會對該值呼叫
"repr()"，而 "'!a'" 則會對該值呼叫 "ascii()"。

一些範例：

   "Harold's a clever {0!s}"        # Calls str() on the argument first
   "Bring out the holy {name!r}"    # Calls repr() on the argument first
   "More {!a}"                      # Calls ascii() on the argument first

*format_spec* 欄位描述了值的呈現規格，例如欄位寬度、對齊、填充
(padding)、小數精度等細節資訊。每種值類型都可以定義自己的「格式化迷你
語言 (formatting mini-language)」或對 *format_spec* 的解釋。

大多數內建型別都支援常見的格式化迷你語言，下一節將會詳細說明。

*format_spec* 欄位還可以在其內部包含巢狀的替換欄位。這些巢狀的替換欄位
可能包含欄位名稱、轉換旗標、格式規格描述，但是不允許再更深層的巢狀結構
。format_spec 内部的替換欄位會在 *format_spec* 字串被直譯前被替換。這
讓數值的格式能夠被動態地指定。

範例請見 格式範例。


格式規格 (Format Specification) 迷你語言
----------------------------------------

「格式規格」在格式字串 (format string) 中包含的替換欄位中使用，以定義
各個值如何被呈現（請參考 格式化文字語法 和 f-string（f 字串））。它們
也能夠直接傳遞給內建的 "format()" 函式。每個可格式化型別 (formattable
type) 可以定義格式規格如何被直譯。

大部分內建型別為了格式規格實作了下列選項，不過有些選項只被數值型別支援
。

一般來說，輸入空格式規格會產生和對值呼叫 "str()" 函式相同的結果，非空
的格式規格才會修改結果。

*標準格式說明符號 (standard format specifier)* 的一般型式如下：

   format_spec     ::= [[fill]align][sign]["z"]["#"]["0"][width][grouping_option]["." precision][type]
   fill            ::= <any character>
   align           ::= "<" | ">" | "=" | "^"
   sign            ::= "+" | "-" | " "
   width           ::= digit+
   grouping_option ::= "_" | ","
   precision       ::= digit+
   type            ::= "b" | "c" | "d" | "e" | "E" | "f" | "F" | "g" | "G" | "n" | "o" | "s" | "x" | "X" | "%"

如果給定了一個有效的 *align* 值，則可以在它之前加一個 *fill* 字元，且
該字元可為任意字元，若不加的話預設為空格。使用格式字串或
"str.format()" 時是無法在其中使用大括號（""{"" 或 ""}""）作為 *fill*
字元的，但仍可透過巢狀替換欄位的方式插入大括號。此限制不影響
"format()" 函式。

The meaning of the various alignment options is as follows:

+-----------+------------------------------------------------------------+
| 選項      | 含義                                                       |
|===========|============================================================|
| "'<'"     | 強制欄位在可用空間內靠左對齊（這是大多數物件的預設值）。   |
+-----------+------------------------------------------------------------+
| "'>'"     | 強制欄位在可用空間內靠右對齊（這是數字的預設值）。         |
+-----------+------------------------------------------------------------+
| "'='"     | Forces the padding to be placed after the sign (if any)    |
|           | but before the digits.  This is used for printing fields   |
|           | in the form '+000000120'. This alignment option is only    |
|           | valid for numeric types.  It becomes the default for       |
|           | numbers when '0' immediately precedes the field width.     |
+-----------+------------------------------------------------------------+
| "'^'"     | 強制欄位在可用空間內置中。                                 |
+-----------+------------------------------------------------------------+

請注意，除非有定義了最小欄位寬度，否則欄位寬度將始終與填充它的資料大小
相同，故在該情況下的對齊選項是沒有意義的。

*sign* 選項只適用於數字型別，並可為以下之一：

+-----------+------------------------------------------------------------+
| 選項      | 含義                                                       |
|===========|============================================================|
| "'+'"     | 表示正數與負數均需使用符號。                               |
+-----------+------------------------------------------------------------+
| "'-'"     | indicates that a sign should be used only for negative     |
|           | numbers (this is the default behavior).                    |
+-----------+------------------------------------------------------------+
| space     | 表示正數應使用前導空格，負數應使用減號。                   |
+-----------+------------------------------------------------------------+

The "'z'" option coerces negative zero floating-point values to
positive zero after rounding to the format precision.  This option is
only valid for floating-point presentation types.

在 3.11 版的變更: 新增 "'z'" 選項（請見 **PEP 682**）。

The "'#'" option causes the "alternate form" to be used for the
conversion.  The alternate form is defined differently for different
types.  This option is only valid for integer, float and complex
types. For integers, when binary, octal, or hexadecimal output is
used, this option adds the respective prefix "'0b'", "'0o'", "'0x'",
or "'0X'" to the output value. For float and complex the alternate
form causes the result of the conversion to always contain a decimal-
point character, even if no digits follow it. Normally, a decimal-
point character appears in the result of these conversions only if a
digit follows it. In addition, for "'g'" and "'G'" conversions,
trailing zeros are not removed from the result.

The "','" option signals the use of a comma for a thousands separator.
For a locale aware separator, use the "'n'" integer presentation type
instead.

在 3.1 版的變更: 新增 "','" 選項（請見 **PEP 378**）。

"'_'" 選項表示對於浮點表示型別和整數表示型別 "'d'" 使用底線作為千位分
隔符號。對於整數表示型別 "'b'"，"'o'"，"'x'" 和 "'X'"，每 4 位數字會插
入底線。對於其他表示型別，指定此選項會出錯。

在 3.6 版的變更: 新增 "'_'" 選項（請見 **PEP 515**）。

*width* 是一個十進位整數，定義了最小總欄位寬度，包括任何前綴、分隔符號
和其他格式字元。如果未指定，則欄位寬度將由內容決定。

當未給予明確的對齊指示，在 *width* 欄位前面填入零 ("'0'") 字元將會為數
值型別啟用有符號察覺的零填充 (sign-aware zero-padding)。這相當於使用
"'0'" 為 *fill* 字元且對齊類型為 "'='"。

在 3.10 版的變更: 在 *width* 欄位前面加上 "'0'" 不再影響字串的預設對齊
方式。

*precision* 是一個十進位整數，指定表示類型 "'f'" 和 "'F'" 的小數點後應
顯示多少位，或表示類型 "'g'" 或 "'G'" 的小數點前後應顯示多少位。對於字
串表示類型，該欄位指定最大欄位大小 - 換言之，將使用欄位中的多少字元。
整數表示類型不允許使用 *precision*。

最終，型別決定了資料將會如何呈現

可用的字串表示型別有：

   +-----------+------------------------------------------------------------+
   | 型別      | 含義                                                       |
   |===========|============================================================|
   | "'s'"     | String format. This is the default type for strings and    |
   |           | may be omitted.                                            |
   +-----------+------------------------------------------------------------+
   | None      | The same as "'s'".                                         |
   +-----------+------------------------------------------------------------+

The available integer presentation types are:

   +-----------+------------------------------------------------------------+
   | 型別      | 含義                                                       |
   |===========|============================================================|
   | "'b'"     | Binary format. Outputs the number in base 2.               |
   +-----------+------------------------------------------------------------+
   | "'c'"     | Character. Converts the integer to the corresponding       |
   |           | unicode character before printing.                         |
   +-----------+------------------------------------------------------------+
   | "'d'"     | Decimal Integer. Outputs the number in base 10.            |
   +-----------+------------------------------------------------------------+
   | "'o'"     | Octal format. Outputs the number in base 8.                |
   +-----------+------------------------------------------------------------+
   | "'x'"     | 十六進位格式。輸出以 16 為基數的數字，9 以上的數字使用小寫 |
   |           | 字母。                                                     |
   +-----------+------------------------------------------------------------+
   | "'X'"     | 十六進位格式。輸出以 16 為基數的數字，9 以上的數字使用大寫 |
   |           | 字母。如 果指定了 "'#'"，則前綴 "'0x'" 也會被轉成大寫的    |
   |           | "'0X'"。                                                   |
   +-----------+------------------------------------------------------------+
   | "'n'"     | Number. This is the same as "'d'", except that it uses the |
   |           | current locale setting to insert the appropriate number    |
   |           | separator characters.                                      |
   +-----------+------------------------------------------------------------+
   | None      | The same as "'d'".                                         |
   +-----------+------------------------------------------------------------+

In addition to the above presentation types, integers can be formatted
with the floating-point presentation types listed below (except "'n'"
and "None"). When doing so, "float()" is used to convert the integer
to a floating-point number before formatting.

The available presentation types for "float" and "Decimal" values are:

   +-----------+------------------------------------------------------------+
   | 型別      | 含義                                                       |
   |===========|============================================================|
   | "'e'"     | Scientific notation. For a given precision "p", formats    |
   |           | the number in scientific notation with the letter 'e'      |
   |           | separating the coefficient from the exponent. The          |
   |           | coefficient has one digit before and "p" digits after the  |
   |           | decimal point, for a total of "p + 1" significant digits.  |
   |           | With no precision given, uses a precision of "6" digits    |
   |           | after the decimal point for "float", and shows all         |
   |           | coefficient digits for "Decimal". If no digits follow the  |
   |           | decimal point, the decimal point is also removed unless    |
   |           | the "#" option is used.                                    |
   +-----------+------------------------------------------------------------+
   | "'E'"     | Scientific notation. Same as "'e'" except it uses an upper |
   |           | case 'E' as the separator character.                       |
   +-----------+------------------------------------------------------------+
   | "'f'"     | Fixed-point notation. For a given precision "p", formats   |
   |           | the number as a decimal number with exactly "p" digits     |
   |           | following the decimal point. With no precision given, uses |
   |           | a precision of "6" digits after the decimal point for      |
   |           | "float", and uses a precision large enough to show all     |
   |           | coefficient digits for "Decimal". If no digits follow the  |
   |           | decimal point, the decimal point is also removed unless    |
   |           | the "#" option is used.                                    |
   +-----------+------------------------------------------------------------+
   | "'F'"     | Fixed-point notation. Same as "'f'", but converts "nan" to |
   |           | "NAN" and "inf" to "INF".                                  |
   +-----------+------------------------------------------------------------+
   | "'g'"     | General format.  For a given precision "p >= 1", this      |
   |           | rounds the number to "p" significant digits and then       |
   |           | formats the result in either fixed-point format or in      |
   |           | scientific notation, depending on its magnitude. A         |
   |           | precision of "0" is treated as equivalent to a precision   |
   |           | of "1".  The precise rules are as follows: suppose that    |
   |           | the result formatted with presentation type "'e'" and      |
   |           | precision "p-1" would have exponent "exp".  Then, if "m <= |
   |           | exp < p", where "m" is -4 for floats and -6 for            |
   |           | "Decimals", the number is formatted with presentation type |
   |           | "'f'" and precision "p-1-exp".  Otherwise, the number is   |
   |           | formatted with presentation type "'e'" and precision       |
   |           | "p-1". In both cases insignificant trailing zeros are      |
   |           | removed from the significand, and the decimal point is     |
   |           | also removed if there are no remaining digits following    |
   |           | it, unless the "'#'" option is used.  With no precision    |
   |           | given, uses a precision of "6" significant digits for      |
   |           | "float". For "Decimal", the coefficient of the result is   |
   |           | formed from the coefficient digits of the value;           |
   |           | scientific notation is used for values smaller than "1e-6" |
   |           | in absolute value and values where the place value of the  |
   |           | least significant digit is larger than 1, and fixed-point  |
   |           | notation is used otherwise.  Positive and negative         |
   |           | infinity, positive and negative zero, and nans, are        |
   |           | formatted as "inf", "-inf", "0", "-0" and "nan"            |
   |           | respectively, regardless of the precision.                 |
   +-----------+------------------------------------------------------------+
   | "'G'"     | General format. Same as "'g'" except switches to "'E'" if  |
   |           | the number gets too large. The representations of infinity |
   |           | and NaN are uppercased, too.                               |
   +-----------+------------------------------------------------------------+
   | "'n'"     | Number. This is the same as "'g'", except that it uses the |
   |           | current locale setting to insert the appropriate number    |
   |           | separator characters.                                      |
   +-----------+------------------------------------------------------------+
   | "'%'"     | Percentage. Multiplies the number by 100 and displays in   |
   |           | fixed ("'f'") format, followed by a percent sign.          |
   +-----------+------------------------------------------------------------+
   | None      | For "float" this is the same as "'g'", except that when    |
   |           | fixed-point notation is used to format the result, it      |
   |           | always includes at least one digit past the decimal point. |
   |           | The precision used is as large as needed to represent the  |
   |           | given value faithfully.  For "Decimal", this is the same   |
   |           | as either "'g'" or "'G'" depending on the value of         |
   |           | "context.capitals" for the current decimal context.  The   |
   |           | overall effect is to match the output of "str()" as        |
   |           | altered by the other format modifiers.                     |
   +-----------+------------------------------------------------------------+


格式範例
--------

本節包含 "str.format()" 語法以及與舊式 "%" 格式的比較。

此語法在大多情況下與舊式的 "%" 格式類似，只是增加了 "{}" 和 ":" 來取代
"%"。例如，"'%03.2f'" 可以改寫為 "'{:03.2f}'"。

新的語法還支援新的選項，將在以下的範例中說明。

按位置存取引數：

   >>> '{0}, {1}, {2}'.format('a', 'b', 'c')
   'a, b, c'
   >>> '{}, {}, {}'.format('a', 'b', 'c')  # 3.1+ only
   'a, b, c'
   >>> '{2}, {1}, {0}'.format('a', 'b', 'c')
   'c, b, a'
   >>> '{2}, {1}, {0}'.format(*'abc')      # unpacking argument sequence
   'c, b, a'
   >>> '{0}{1}{0}'.format('abra', 'cad')   # arguments' indices can be repeated
   'abracadabra'

按名稱存取引數：

   >>> 'Coordinates: {latitude}, {longitude}'.format(latitude='37.24N', longitude='-115.81W')
   'Coordinates: 37.24N, -115.81W'
   >>> coord = {'latitude': '37.24N', 'longitude': '-115.81W'}
   >>> 'Coordinates: {latitude}, {longitude}'.format(**coord)
   'Coordinates: 37.24N, -115.81W'

存取引數的屬性：

   >>> c = 3-5j
   >>> ('The complex number {0} is formed from the real part {0.real} '
   ...  'and the imaginary part {0.imag}.').format(c)
   'The complex number (3-5j) is formed from the real part 3.0 and the imaginary part -5.0.'
   >>> class Point:
   ...     def __init__(self, x, y):
   ...         self.x, self.y = x, y
   ...     def __str__(self):
   ...         return 'Point({self.x}, {self.y})'.format(self=self)
   ...
   >>> str(Point(4, 2))
   'Point(4, 2)'

存取引數的內容：

   >>> coord = (3, 5)
   >>> 'X: {0[0]};  Y: {0[1]}'.format(coord)
   'X: 3;  Y: 5'

替換 "%s" 和 "%r"：

   >>> "repr() shows quotes: {!r}; str() doesn't: {!s}".format('test1', 'test2')
   "repr() shows quotes: 'test1'; str() doesn't: test2"

對齊文字以及指定寬度：

   >>> '{:<30}'.format('left aligned')
   'left aligned                  '
   >>> '{:>30}'.format('right aligned')
   '                 right aligned'
   >>> '{:^30}'.format('centered')
   '           centered           '
   >>> '{:*^30}'.format('centered')  # use '*' as a fill char
   '***********centered***********'

替換 "%+f"、"%-f" 和 "% f" 以及指定正負號：

   >>> '{:+f}; {:+f}'.format(3.14, -3.14)  # show it always
   '+3.140000; -3.140000'
   >>> '{: f}; {: f}'.format(3.14, -3.14)  # show a space for positive numbers
   ' 3.140000; -3.140000'
   >>> '{:-f}; {:-f}'.format(3.14, -3.14)  # show only the minus -- same as '{:f}; {:f}'
   '3.140000; -3.140000'

替換 "%x" 和 "%o" 並將其值轉換為不同的進位制：

   >>> # format also supports binary numbers
   >>> "int: {0:d};  hex: {0:x};  oct: {0:o};  bin: {0:b}".format(42)
   'int: 42;  hex: 2a;  oct: 52;  bin: 101010'
   >>> # with 0x, 0o, or 0b as prefix:
   >>> "int: {0:d};  hex: {0:#x};  oct: {0:#o};  bin: {0:#b}".format(42)
   'int: 42;  hex: 0x2a;  oct: 0o52;  bin: 0b101010'

使用逗號作為千位分隔符：

   >>> '{:,}'.format(1234567890)
   '1,234,567,890'

表示為百分比：

   >>> points = 19
   >>> total = 22
   >>> 'Correct answers: {:.2%}'.format(points/total)
   'Correct answers: 86.36%'

作為特定型別格式：

   >>> import datetime
   >>> d = datetime.datetime(2010, 7, 4, 12, 15, 58)
   >>> '{:%Y-%m-%d %H:%M:%S}'.format(d)
   '2010-07-04 12:15:58'

巢狀引數及更多複雜範例：

   >>> for align, text in zip('<^>', ['left', 'center', 'right']):
   ...     '{0:{fill}{align}16}'.format(text, fill=align, align=align)
   ...
   'left<<<<<<<<<<<<'
   '^^^^^center^^^^^'
   '>>>>>>>>>>>right'
   >>>
   >>> octets = [192, 168, 0, 1]
   >>> '{:02X}{:02X}{:02X}{:02X}'.format(*octets)
   'C0A80001'
   >>> int(_, 16)
   3232235521
   >>>
   >>> width = 5
   >>> for num in range(5,12): 
   ...     for base in 'dXob':
   ...         print('{0:{width}{base}}'.format(num, base=base, width=width), end=' ')
   ...     print()
   ...
       5     5     5   101
       6     6     6   110
       7     7     7   111
       8     8    10  1000
       9     9    11  1001
      10     A    12  1010
      11     B    13  1011


模板字串
========

模板字串提供如 **PEP 292** 所述更簡單的字串替換。模板字串的主要用例是
國際化 (i18n)，因為在這種情況下，更簡單的語法和功能使得它比其他 Python
內建字串格式化工具更容易翻譯。基於模板字串建構的 i18n 函式庫範例，請參
閱 flufl.i18n 套件。

Template strings support "$"-based substitutions, using the following
rules:

* "$$" is an escape; it is replaced with a single "$".

* "$identifier" names a substitution placeholder matching a mapping
  key of ""identifier"".  By default, ""identifier"" is restricted to
  any case-insensitive ASCII alphanumeric string (including
  underscores) that starts with an underscore or ASCII letter.  The
  first non-identifier character after the "$" character terminates
  this placeholder specification.

* "${identifier}" is equivalent to "$identifier".  It is required when
  valid identifier characters follow the placeholder but are not part
  of the placeholder, such as ""${noun}ification"".

Any other appearance of "$" in the string will result in a
"ValueError" being raised.

The "string" module provides a "Template" class that implements these
rules.  The methods of "Template" are:

class string.Template(template)

   The constructor takes a single argument which is the template
   string.

   substitute(mapping={}, /, **kwds)

      進行模板替換，並回傳一個新的字串。 *mapping* 是任何有金鑰並符合
      模板裡佔位符號的字典型物件。或者如果關鍵字就是佔位符號時，你也可
      以改提供關鍵字引數。當 *mapping* 跟 *kwds* 同時給定並存在重複時
      ，*kwds* 的佔位符號會被優先使用。

   safe_substitute(mapping={}, /, **kwds)

      類似於 "substitute()"，但如果 *mapping* 與 *kwds* 中缺少佔位符號
      的話，原始的佔位符號會完整地出現在結果字串裡，而不會引發
      "KeyError" 例外。此外，與 "substitute()" 不同的是，任何包含 "$"
      的字句會直接回傳 "$" 而非引發 "ValueError"。

      雖然仍可能發生其他例外，但這個方法被認為是「安全」的，因為它總是
      試圖回傳一個有用的字串而不是引發例外。從另一個角度來看，
      "safe_substitute()" 可能並非完全安全，因為它會默默忽略格式錯誤的
      模板，這些模板包含了多餘的左右定界符、不匹配的括號，或者不是有效
      的 Python 識別字的佔位符號。

   is_valid()

      如果模板有將導致 "substitute()" 引發 "ValueError" 的無效佔位符號
      ，就會回傳 false。

      在 3.11 版被加入.

   get_identifiers()

      回傳模板中有效識別字的串列，按照它們首次出現的順序，並忽略任何無
      效的識別字。

      在 3.11 版被加入.

   "Template" 實例也提供一個公開的資料屬性：

   template

      這是傳遞給建構函式 *template* 引數的物件。一般來說，你不應該改變
      它，但並沒有強制設定成唯讀。

以下是如何使用 Template 的一個範例：

   >>> from string import Template
   >>> s = Template('$who likes $what')
   >>> s.substitute(who='tim', what='kung pao')
   'tim likes kung pao'
   >>> d = dict(who='tim')
   >>> Template('Give $who $100').substitute(d)
   Traceback (most recent call last):
   ...
   ValueError: Invalid placeholder in string: line 1, col 11
   >>> Template('$who likes $what').substitute(d)
   Traceback (most recent call last):
   ...
   KeyError: 'what'
   >>> Template('$who likes $what').safe_substitute(d)
   'tim likes $what'

進階用法：你可以衍生 "Template" 類別來自定義佔位符號語法、左右定界符字
元，或者用於剖析模板字串的正規表示式。你可以透過覆寫這些類別屬性來達成
：

* *delimiter* -- 這是描述引入左右定界符的文字字串。預設值是 "$"。請注
  意這*不是*正規表示式，因為實作會在需要時對這個字串呼叫 "re.escape()"
  。也請注意你不能在建立類別後修改左右定界符。（意即在子類別的命名空間
  中必須設置不同的左右定界符）

* *idpattern* -- This is the regular expression describing the pattern
  for non-braced placeholders.  The default value is the regular
  expression "(?a:[_a-z][_a-z0-9]*)".  If this is given and
  *braceidpattern* is "None" this pattern will also apply to braced
  placeholders.

  備註:

    Since default *flags* is "re.IGNORECASE", pattern "[a-z]" can
    match with some non-ASCII characters. That's why we use the local
    "a" flag here.

  在 3.7 版的變更: *braceidpattern* can be used to define separate
  patterns used inside and outside the braces.

* *braceidpattern* -- This is like *idpattern* but describes the
  pattern for braced placeholders.  Defaults to "None" which means to
  fall back to *idpattern* (i.e. the same pattern is used both inside
  and outside braces). If given, this allows you to define different
  patterns for braced and unbraced placeholders.

  在 3.7 版被加入.

* *flags* -- The regular expression flags that will be applied when
  compiling the regular expression used for recognizing substitutions.
  The default value is "re.IGNORECASE".  Note that "re.VERBOSE" will
  always be added to the flags, so custom *idpattern*s must follow
  conventions for verbose regular expressions.

  在 3.2 版被加入.

Alternatively, you can provide the entire regular expression pattern
by overriding the class attribute *pattern*.  If you do this, the
value must be a regular expression object with four named capturing
groups.  The capturing groups correspond to the rules given above,
along with the invalid placeholder rule:

* *escaped* -- 此群組與跳脫序列匹配，例如在預設模式下為 "$$"。

* *named* -- 此群組與不帶大括號的佔位符號名稱匹配；它不應包含擷取群組
  中的左右定界符號。

* *braced* -- 此群組與大括號括起來的佔位符號名稱匹配；它不應在擷取群組
  中包含左右定界符或大括號。

* *invalid* -- 此群組與任何其他左右定界符模式（通常是單一左右定界符）
  匹配，且它應該出現在正規表示式的最後。

當此模式有匹配於模板但這些命名組中卻有任一個不匹配，此類別的方法將引發
"ValueError"。


輔助函式
========

string.capwords(s, sep=None)

   使用 "str.split()" 將引數分割為字詞，使用 "str.capitalize()" 將每個
   單字大寫，並使用 "str.join()" 將大寫字詞連接起來。如果可選的第二引
   數 *sep* 不存在或為 "None"，則連續的空白字元將替換為單一空格，並且
   刪除前導和尾隨空白；在其他情況下則使用 *sep* 來分割和連接單字。
