# `string` --- 常見的字串操作

原始碼： Lib/string/__init__.py

也參考

Text Sequence Type --- str

字串方法

## 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 字元的字串被 Python 是為可被列印輸出的。這是 `digits`、`ascii_letters`、`punctuation` 和 `whitespace` 的組合。

  備註

  By design, `string.printable.isprintable()` returns `False`. In particular, `string.printable` is not printable in the POSIX sense (see LC_CTYPE).

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`. The value of field_name is unmodified and auto-numbering of non-numbered positional fields is done by `vformat()`.

  get_field(field_name, args, kwargs)

    Given field_name, convert it to an object to be formatted. Auto-numbering of field_name returned from `parse()` is done by `vformat()` before calling this method. 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 and template string literals, but it is less sophisticated and, in particular, does not support arbitrary expressions in interpolations.

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 內部的替換欄位會在 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、f-string（f 字串） 和 t-string（t 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:             [options][width_and_precision][type]
    options:                 [[fill]align][sign]["z"]["#"]["0"]
    fill:                    <any character>
    align:                   "<" | ">" | "=" | "^"
    sign:                    "+" | "-" | " "
    width_and_precision:     [width_with_grouping][precision_with_grouping]
    width_with_grouping:     [width][grouping]
    precision_with_grouping: "." [precision][grouping] | "." grouping
    width:                   digit+
    precision:               digit+
    grouping:                "," | "_"
    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, excluding `complex`. 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。
`'-'` | 表示正數應使用前導空格，負數應使用減號。
空格 | 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.

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 `complex` 以外的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。

The grouping option after width and precision fields specifies a digit group separator for the integral and fractional parts of a number respectively. It can be one of the following:

選項 | 含義
--- | ---
`','` | Inserts a comma every 3 digits for integer presentation type `'d'` and floating-point presentation types, excluding `'n'`. For other presentation types, this option is not supported.
`'_'` | 在整數表示型別 `'d'` 和浮點數表示型別中，每 3 位數插入一個底線，這不包括 `'n'`。對於整數表示型別 `'b'`，`'o'`，`'x'` 和 `'X'`，每 4 位數字會插入底線。對於其他表示型別來說此選項並不支援。

For a locale-aware separator, use the `'n'` float presentation type or integer presentation type instead.

在 3.1 版的變更: 新增 `','` 選項（請見 PEP 378）。

在 3.6 版的變更: 新增 `'_'` 選項（請見 PEP 515）。

在 3.14 版的變更: Support the grouping option for the fractional part.

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 | 與 `'s'` 相同。

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 digit group separators. Note that the default locale is not the system locale. Depending on your use case, you may wish to set `LC_NUMERIC` with `locale.setlocale()` before using `'n'`.
> None | 與 `'d'` 相同。

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 `p=0`, the decimal point is omitted unless the `#` option is used.

> For `float`, the exponent always contains at least two digits, and is zero if the value is zero.

> `'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 `p=0`, the decimal point is omitted 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 digit group separators for the integral part of a number. Note that the default locale is not the system locale. Depending on your use case, you may wish to set `LC_NUMERIC` with `locale.setlocale()` before using `'n'`.
> `'%'` | Percentage. Multiplies the number by 100 and displays in fixed (`'f'`) format, followed by a percent sign.

> None

> For `float` this is like the `'g'` type, except that when fixed-point notation is used to format the result, it always includes at least one digit past the decimal point, and switches to the scientific notation when `exp >= p - 1`. When the precision is not specified, the latter will be 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.

The result should be correctly rounded to a given precision `p` of digits after the decimal point. The rounding mode for `float` matches that of the `round()` builtin. For `Decimal`, the rounding mode of the current context will be used.

The available presentation types for `complex` are the same as those for `float` (`'%'` is not allowed). Both the real and imaginary components of a complex number are formatted as floating-point numbers, according to the specified presentation type. They are separated by the mandatory sign of the imaginary part, the latter being terminated by a `j` suffix. If the presentation type is missing, the result will match the output of `str()` (complex numbers with a non-zero real part are also surrounded by parentheses), possibly altered by 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')      # 解包引數序列
    'c, b, a'
    >>> '{0}{1}{0}'.format('abra', 'cad')   # 引數索引可以重複
    '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)  # 總會顯示
    '+3.140000; -3.140000'
    >>> '{: f}; {: f}'.format(3.14, -3.14)  # 若為正數則顯示空格
    ' 3.140000; -3.140000'
    >>> '{:-f}; {:-f}'.format(3.14, -3.14)  # 只顯示負號 -- 與 '{:f}; {:f}' 相同
    '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'

使用逗號或底線作為數字分組分隔符號：

    >>> '{:,}'.format(1234567890)
    '1,234,567,890'
    >>> '{:_}'.format(1234567890)
    '1_234_567_890'
    >>> '{:_b}'.format(1234567890)
    '100_1001_1001_0110_0000_0010_1101_0010'
    >>> '{:_x}'.format(1234567890)
    '4996_02d2'
    >>> '{:_}'.format(123456789.123456789)
    '123_456_789.12345679'
    >>> '{:.,}'.format(123456789.123456789)
    '123456789.123,456,79'
    >>> '{:,._}'.format(123456789.123456789)
    '123,456,789.123_456_79'

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 as dt
    >>> d = dt.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 ($-strings)

備註

The feature described here was introduced in Python 2.4; a simple templating method based upon regular expressions. It predates `str.format()`, formatted string literals, and template string literals.

It is unrelated to template string literals (t-strings), which were introduced in Python 3.14. These evaluate to `string.templatelib.Template` objects, found in the `string.templatelib` module.

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` 類別來自訂佔位符號語法、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 idpatterns 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。