"string" --- Common string operations
*************************************

**Source code:** Lib/string.py

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

也參考: Text Sequence Type --- str

  String Methods


String constants
================

The constants defined in this module are:

string.ascii_letters

   The concatenation of the "ascii_lowercase" and "ascii_uppercase"
   constants described below.  This value is not locale-dependent.

string.ascii_lowercase

   The lowercase letters "'abcdefghijklmnopqrstuvwxyz'".  This value
   is not locale-dependent and will not change.

string.ascii_uppercase

   The uppercase letters "'ABCDEFGHIJKLMNOPQRSTUVWXYZ'".  This value
   is not locale-dependent and will not change.

string.digits

   The string "'0123456789'".

string.hexdigits

   The string "'0123456789abcdefABCDEF'".

string.octdigits

   The string "'01234567'".

string.punctuation

   由在 "C" 区域设置中被视为标点符号的 ASCII 字符所组成的字符串:
   "!"#$%&'()*+,-./:;<=>?@[\]^_`{|}~".

string.printable

   String of ASCII characters which are considered printable.  This is
   a combination of "digits", "ascii_letters", "punctuation", and
   "whitespace".

string.whitespace

   A string containing all ASCII characters that are considered
   whitespace. This includes the characters space, tab, linefeed,
   return, formfeed, and vertical tab.


Custom String Formatting
========================

The built-in string class provides the ability to do complex variable
substitutions and value formatting via the "format()" method described
in **PEP 3101**.  The "Formatter" class in the "string" module allows
you to create and customize your own string formatting behaviors using
the same implementation as the built-in "format()" method.

class string.Formatter

   The "Formatter" class has the following public methods:

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

      The primary API method.  It takes a format string and an
      arbitrary set of positional and keyword arguments. It is just a
      wrapper that calls "vformat()".

      3.7 版更變: A format string argument is now positional-only.

   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)

      Loop over the format_string and return an iterable of tuples
      (*literal_text*, *field_name*, *format_spec*, *conversion*).
      This is used by "vformat()" to break the string into either
      literal text, or replacement fields.

      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.

      对于复合字段名称，仅会为字段名称的第一个组件调用这些函数；后续组
      件会通过普通属性和索引操作来进行处理。

      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.


Format String Syntax
====================

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 there are
differences.

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       ::= <described in the next section>

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.

See also the Format Specification Mini-Language section.

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.  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__()".

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

3.4 版更變: 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.

Three conversion flags are currently supported: "'!s'" which calls
"str()" on the value, "'!r'" which calls "repr()" and "'!a'" which
calls "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

The *format_spec* field contains a specification of how the value
should be presented, including such details as field width, alignment,
padding, decimal precision and so on.  Each value type can define its
own "formatting mini-language" or interpretation of the *format_spec*.

Most built-in types support a common formatting mini-language, which
is described in the next section.

A *format_spec* field can also include nested replacement fields
within it. These nested replacement fields may contain a field name,
conversion flag and format specification, but deeper nesting is not
allowed.  The replacement fields within the format_spec are
substituted before the *format_spec* string is interpreted. This
allows the formatting of a value to be dynamically specified.

See the Format examples section for some examples.


Format Specification Mini-Language
----------------------------------

"Format specifications" are used within replacement fields contained
within a format string to define how individual values are presented
(see Format String Syntax and Formatted string literals). They can
also be passed directly to the built-in "format()" function.  Each
formattable type may define how the format specification is to be
interpreted.

Most built-in types implement the following options for format
specifications, although some of the formatting options are only
supported by the numeric types.

A general convention is that an empty format string ("""") produces
the same result as if you had called "str()" on the value. A non-empty
format string typically modifies the result.

The general form of a *standard format specifier* is:

   format_spec     ::= [[fill]align][sign][#][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" | "%"

If a valid *align* value is specified, it can be preceded by a *fill*
character that can be any character and defaults to a space if
omitted. It is not possible to use a literal curly brace (""{"" or
""}"") as the *fill* character in a formatted string literal or when
using the "str.format()" method.  However, it is possible to insert a
curly brace with a nested replacement field.  This limitation doesn't
affect the "format()" function.

The meaning of the various alignment options is as follows:

   +-----------+------------------------------------------------------------+
   | Option    | Meaning                                                    |
   |===========|============================================================|
   | "'<'"     | Forces the field to be left-aligned within the available   |
   |           | space (this is the default for most objects).              |
   +-----------+------------------------------------------------------------+
   | "'>'"     | Forces the field to be right-aligned within the available  |
   |           | space (this is the default for numbers).                   |
   +-----------+------------------------------------------------------------+
   | "'='"     | 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 when '0'  |
   |           | immediately precedes the field width.                      |
   +-----------+------------------------------------------------------------+
   | "'^'"     | Forces the field to be centered within the available       |
   |           | space.                                                     |
   +-----------+------------------------------------------------------------+

Note that unless a minimum field width is defined, the field width
will always be the same size as the data to fill it, so that the
alignment option has no meaning in this case.

The *sign* option is only valid for number types, and can be one of
the following:

   +-----------+------------------------------------------------------------+
   | Option    | Meaning                                                    |
   |===========|============================================================|
   | "'+'"     | indicates that a sign should be used for both positive as  |
   |           | well as negative numbers.                                  |
   +-----------+------------------------------------------------------------+
   | "'-'"     | indicates that a sign should be used only for negative     |
   |           | numbers (this is the default behavior).                    |
   +-----------+------------------------------------------------------------+
   | space     | indicates that a leading space should be used on positive  |
   |           | numbers, and a minus sign on negative numbers.             |
   +-----------+------------------------------------------------------------+

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, complex and
Decimal types. For integers, when binary, octal, or hexadecimal output
is used, this option adds the prefix respective "'0b'", "'0o'", or
"'0x'" to the output value. For floats, complex and Decimal 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.

3.1 版更變: Added the "','" option (see also **PEP 378**).

The "'_'" option signals the use of an underscore for a thousands
separator for floating point presentation types and for integer
presentation type "'d'".  For integer presentation types "'b'", "'o'",
"'x'", and "'X'", underscores will be inserted every 4 digits.  For
other presentation types, specifying this option is an error.

3.6 版更變: Added the "'_'" option (see also **PEP 515**).

*width* is a decimal integer defining the minimum field width.  If not
specified, then the field width will be determined by the content.

When no explicit alignment is given, preceding the *width* field by a
zero ("'0'") character enables sign-aware zero-padding for numeric
types.  This is equivalent to a *fill* character of "'0'" with an
*alignment* type of "'='".

The *precision* is a decimal number indicating how many digits should
be displayed after the decimal point for a floating point value
formatted with "'f'" and "'F'", or before and after the decimal point
for a floating point value formatted with "'g'" or "'G'".  For non-
number types the field indicates the maximum field size - in other
words, how many characters will be used from the field content. The
*precision* is not allowed for integer values.

Finally, the *type* determines how the data should be presented.

The available string presentation types are:

   +-----------+------------------------------------------------------------+
   | Type      | Meaning                                                    |
   |===========|============================================================|
   | "'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:

   +-----------+------------------------------------------------------------+
   | Type      | Meaning                                                    |
   |===========|============================================================|
   | "'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'"     | Hex format. Outputs the number in base 16, using lower-    |
   |           | case letters for the digits above 9.                       |
   +-----------+------------------------------------------------------------+
   | "'X'"     | Hex format. Outputs the number in base 16, using upper-    |
   |           | case letters for the digits above 9.                       |
   +-----------+------------------------------------------------------------+
   | "'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 floating point and decimal values
are:

   +-----------+------------------------------------------------------------+
   | Type      | Meaning                                                    |
   |===========|============================================================|
   | "'e'"     | Exponent notation. Prints the number in scientific         |
   |           | notation using the letter 'e' to indicate the exponent.    |
   |           | The default precision is "6".                              |
   +-----------+------------------------------------------------------------+
   | "'E'"     | Exponent notation. Same as "'e'" except it uses an upper   |
   |           | case 'E' as the separator character.                       |
   +-----------+------------------------------------------------------------+
   | "'f'"     | Fixed-point notation. Displays the number as a fixed-point |
   |           | number. The default precision is "6".                      |
   +-----------+------------------------------------------------------------+
   | "'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.  准确的规 |
   |           | 则如下：假设使用表示类型 "'e'" 和精度 "p-1" 进行格式化的结 |
   |           | 果具有指数值 "exp"。 那么如果 "m <= exp < p"，其中 "m" 以  |
   |           | -4 表示浮 点值而以 -6 表示 "Decimal" 值，该数字将使用类型  |
   |           | "'f'" 和精度 "p-1-exp" 进行格式化。 否则的话，该数字将使用 |
   |           | 表示类型 "'e'" 和精度 "p-1" 进行格式化。 在两种情况下，都  |
   |           | 会从有效数字中移除无意义的末尾零 ，如果小数点之后没有余下  |
   |           | 数字则小数点也会被移除，除非使用了 "'#'" 选 项。  Positive |
   |           | and negative infinity, positive and negative zero, and     |
   |           | nans, are formatted as "inf", "-inf", "0", "-0" and "nan"  |
   |           | respectively, regardless of the precision.  A precision of |
   |           | "0" is treated as equivalent to a precision of "1". The    |
   |           | default precision is "6".                                  |
   +-----------+------------------------------------------------------------+
   | "'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      | Similar to "'g'", except that fixed-point notation, when   |
   |           | used, has at least one digit past the decimal point. The   |
   |           | default precision is as high as needed to represent the    |
   |           | particular value. The overall effect is to match the       |
   |           | output of "str()" as altered by the other format           |
   |           | modifiers.                                                 |
   +-----------+------------------------------------------------------------+


Format examples
---------------

This section contains examples of the "str.format()" syntax and
comparison with the old "%"-formatting.

In most of the cases the syntax is similar to the old "%"-formatting,
with the addition of the "{}" and with ":" used instead of "%". For
example, "'%03.2f'" can be translated to "'{:03.2f}'".

新的格式语法还支持新增的不同选项，将在以下示例中说明。

Accessing arguments by position:

   >>> '{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'

Accessing arguments by name:

   >>> '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'

Accessing arguments' attributes:

   >>> 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)'

Accessing arguments' items:

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

Replacing "%s" and "%r":

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

Aligning the text and specifying a width:

   >>> '{:<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***********'

Replacing "%+f", "%-f", and "% f" and specifying a sign:

   >>> '{:+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'

Replacing "%x" and "%o" and converting the value to different bases:

   >>> # 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'

Using the comma as a thousands separator:

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

Expressing a percentage:

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

Using type-specific formatting:

   >>> 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'

Nesting arguments and more complex examples:

   >>> 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


Template strings
================

Template strings provide simpler string substitutions as described in
**PEP 292**.  A primary use case for template strings is for
internationalization (i18n) since in that context, the simpler syntax
and functionality makes it easier to translate than other built-in
string formatting facilities in Python.  As an example of a library
built on template strings for i18n, see the flufl.i18n package.

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)

      Performs the template substitution, returning a new string.
      *mapping* is any dictionary-like object with keys that match the
      placeholders in the template.  Alternatively, you can provide
      keyword arguments, where the keywords are the placeholders.
      When both *mapping* and *kwds* are given and there are
      duplicates, the placeholders from *kwds* take precedence.

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

      Like "substitute()", except that if placeholders are missing
      from *mapping* and *kwds*, instead of raising a "KeyError"
      exception, the original placeholder will appear in the resulting
      string intact.  Also, unlike with "substitute()", any other
      appearances of the "$" will simply return "$" instead of raising
      "ValueError".

      此方法被认为“安全”，因为虽然仍有可能发生其他异常，但它总是尝试返
      回可用的字符串而不是引发一个异常。 从另一方面来说，
      "safe_substitute()" 也可能根本算不上安全，因为它将静默地忽略错误
      格式的模板，例如包含多余的分隔符、不成对的花括号或不是合法
      Python 标识符的占位符等等。

   "Template" instances also provide one public data attribute:

   template

      This is the object passed to the constructor's *template*
      argument.  In general, you shouldn't change it, but read-only
      access is not enforced.

Here is an example of how to use a Template:

   >>> 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'

Advanced usage: you can derive subclasses of "Template" to customize
the placeholder syntax, delimiter character, or the entire regular
expression used to parse template strings.  To do this, you can
override these class attributes:

* *delimiter* -- This is the literal string describing a placeholder
  introducing delimiter.  The default value is "$".  Note that this
  should *not* be a regular expression, as the implementation will
  call "re.escape()" on this string as needed.  Note further that you
  cannot change the delimiter after class creation (i.e. a different
  delimiter must be set in the subclass's class namespace).

* *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.

  3.7 版更變: *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.

  3.7 版新加入.

* *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.

  3.2 版新加入.

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* -- This group matches the escape sequence, e.g. "$$", in
  the default pattern.

* *named* -- This group matches the unbraced placeholder name; it
  should not include the delimiter in capturing group.

* *braced* -- This group matches the brace enclosed placeholder
  name; it should not include either the delimiter or braces in the
  capturing group.

* *invalid* -- This group matches any other delimiter pattern
  (usually a single delimiter), and it should appear last in the
  regular expression.


Helper functions
================

string.capwords(s, sep=None)

   Split the argument into words using "str.split()", capitalize each
   word using "str.capitalize()", and join the capitalized words using
   "str.join()".  If the optional second argument *sep* is absent or
   "None", runs of whitespace characters are replaced by a single
   space and leading and trailing whitespace are removed, otherwise
   *sep* is used to split and join the words.
