# `string` — Common string operations

Source code: Lib/string.py

See also

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

  String of ASCII characters which are considered punctuation characters in the `C` locale: `!"#$%&'()*+,-./:;<=>?@[\]^_`{|}~`.

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()`.

    Changed in version 3.7: 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.

    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.

## 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 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.

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. 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__()`.

Changed in version 3.1: The positional argument specifiers can be omitted for `str.format()`, so `'{} {}'.format(a, b)` is equivalent to `'{0} {1}'.format(a, b)`.

Changed in version 3.4: 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()`.

Some examples:

    "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 f-strings). 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 specification produces the same result as if you had called `str()` on the value. A non-empty format specification typically modifies the result.

The general form of a standard format specifier is:

    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" | "%"

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, excluding `complex`. It becomes the default for numbers 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 `'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.

Changed in version 3.11: Added the `'z'` option (see also 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.

Changed in version 3.1: 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.

Changed in version 3.6: Added the `'_'` option (see also PEP 515).

width is a decimal integer defining the minimum total field width, including any prefixes, separators, and other formatting characters. 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, excluding `complex`. This is equivalent to a fill character of `'0'` with an alignment type of `'='`.

Changed in version 3.10: Preceding the width field by `'0'` no longer affects the default alignment for strings.

The precision is a decimal integer indicating how many digits should be displayed after the decimal point for presentation types `'f'` and `'F'`, or before and after the decimal point for presentation types `'g'` or `'G'`. For string presentation 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 presentation types.

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. In case `'#'` is specified, the prefix `'0x'` will be upper-cased to `'0X'` as well.
> `'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:

> Type | Meaning
> --- | ---
> `'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.
> `'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 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 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.

### 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}'`.

The new format syntax also supports new and different options, shown in the following examples.

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`.

    While other exceptions may still occur, this method is called “safe” because it always tries to return a usable string instead of raising an exception. In another sense, `safe_substitute()` may be anything other than safe, since it will silently ignore malformed templates containing dangling delimiters, unmatched braces, or placeholders that are not valid Python identifiers.

  is_valid()

    Returns false if the template has invalid placeholders that will cause `substitute()` to raise `ValueError`.

    Added in version 3.11.

  get_identifiers()

    Returns a list of the valid identifiers in the template, in the order they first appear, ignoring any invalid identifiers.

    Added in version 3.11.

  `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.

  Note

  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.

  Changed in version 3.7: 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.

  Added in version 3.7.

- 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.

  Added in version 3.2.

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.

The methods on this class will raise `ValueError` if the pattern matches the template without one of these named groups matching.

## 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.