| |
| :mod:`string` --- Common string operations |
| ========================================== |
| |
| .. module:: string |
| :synopsis: Common string operations. |
| |
| |
| .. index:: module: re |
| |
| The :mod:`string` module contains a number of useful constants and classes, as |
| well as some deprecated legacy functions that are also available as methods on |
| strings. In addition, Python's built-in string classes support the sequence type |
| methods described in the :ref:`typesseq` section, and also the string-specific |
| methods described in the :ref:`string-methods` section. To output formatted |
| strings, see the :ref:`string-formatting` section. Also, see the :mod:`re` |
| module for string functions based on regular expressions. |
| |
| |
| String constants |
| ---------------- |
| |
| The constants defined in this module are: |
| |
| |
| .. data:: ascii_letters |
| |
| The concatenation of the :const:`ascii_lowercase` and :const:`ascii_uppercase` |
| constants described below. This value is not locale-dependent. |
| |
| |
| .. data:: ascii_lowercase |
| |
| The lowercase letters ``'abcdefghijklmnopqrstuvwxyz'``. This value is not |
| locale-dependent and will not change. |
| |
| |
| .. data:: ascii_uppercase |
| |
| The uppercase letters ``'ABCDEFGHIJKLMNOPQRSTUVWXYZ'``. This value is not |
| locale-dependent and will not change. |
| |
| |
| .. data:: digits |
| |
| The string ``'0123456789'``. |
| |
| |
| .. data:: hexdigits |
| |
| The string ``'0123456789abcdefABCDEF'``. |
| |
| |
| .. data:: octdigits |
| |
| The string ``'01234567'``. |
| |
| |
| .. data:: punctuation |
| |
| String of ASCII characters which are considered punctuation characters |
| in the ``C`` locale. |
| |
| |
| .. data:: printable |
| |
| String of ASCII characters which are considered printable. This is a |
| combination of :const:`digits`, :const:`ascii_letters`, :const:`punctuation`, |
| and :const:`whitespace`. |
| |
| |
| .. data:: whitespace |
| |
| A string containing all characters that are considered whitespace. |
| This includes the characters space, tab, linefeed, return, formfeed, and |
| vertical tab. |
| |
| |
| .. _string-formatting: |
| |
| String Formatting |
| ----------------- |
| |
| The built-in string class provides the ability to do complex variable |
| substitutions and value formatting via the :func:`format` method described in |
| :pep:`3101`. The :class:`Formatter` class in the :mod:`string` module allows |
| you to create and customize your own string formatting behaviors using the same |
| implementation as the built-in :meth:`format` method. |
| |
| .. class:: Formatter |
| |
| The :class:`Formatter` class has the following public methods: |
| |
| .. method:: format(format_string, *args, *kwargs) |
| |
| :meth:`format` is the primary API method. It takes a format template |
| string, and an arbitrary set of positional and keyword argument. |
| :meth:`format` is just a wrapper that calls :meth:`vformat`. |
| |
| .. method:: 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 ``**kwds`` |
| syntax. :meth:`vformat` does the work of breaking up the format template |
| string into character data and replacement fields. It calls the various |
| methods described below. |
| |
| In addition, the :class:`Formatter` defines a number of methods that are |
| intended to be replaced by subclasses: |
| |
| .. method:: 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 :meth:`vformat` to break the string in to 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``. |
| |
| .. method:: get_field(field_name, args, kwargs) |
| |
| Given *field_name* as returned by :meth:`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 |
| :meth:`vformat`. The return value *used_key* has the same meaning as the |
| *key* parameter to :meth:`get_value`. |
| |
| .. method:: 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 |
| :meth:`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 |
| :meth:`get_value` to be called with a *key* argument of 0. The ``name`` |
| attribute will be looked up after :meth:`get_value` returns by calling the |
| built-in :func:`getattr` function. |
| |
| If the index or keyword refers to an item that does not exist, then an |
| :exc:`IndexError` or :exc:`KeyError` should be raised. |
| |
| .. method:: 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. :meth:`check_unused_args` is assumed to throw an exception if |
| the check fails. |
| |
| .. method:: format_field(value, format_spec) |
| |
| :meth:`format_field` simply calls the global :func:`format` built-in. The |
| method is provided so that subclasses can override it. |
| |
| .. method:: convert_field(value, conversion) |
| |
| Converts the value (returned by :meth:`get_field`) given a conversion type |
| (as in the tuple returned by the :meth:`parse` method.) The default |
| version understands 'r' (repr) and 's' (str) conversion types. |
| |
| |
| .. _formatstrings: |
| |
| Format String Syntax |
| -------------------- |
| |
| The :meth:`str.format` method and the :class:`Formatter` class share the same |
| syntax for format strings (although in the case of :class:`Formatter`, |
| subclasses can define their own format string syntax.) |
| |
| 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: |
| |
| .. productionlist:: sf |
| replacement_field: "{" `field_name` ["!" `conversion`] [":" `format_spec`] "}" |
| field_name: (`identifier` | `integer`) ("." `attribute_name` | "[" element_index "]")* |
| attribute_name: `identifier` |
| element_index: `integer` |
| conversion: "r" | "s" |
| format_spec: <described in the next section> |
| |
| In less formal terms, the replacement field starts with a *field_name*, which |
| can either be a number (for a positional argument), or an identifier (for |
| keyword arguments). Following this is an optional *conversion* field, which is |
| preceded by an exclamation point ``'!'``, and a *format_spec*, which is preceded |
| by a colon ``':'``. |
| |
| The *field_name* itself begins with 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. This can be followed by any number of index or |
| attribute expressions. An expression of the form ``'.name'`` selects the named |
| attribute using :func:`getattr`, while an expression of the form ``'[index]'`` |
| does an index lookup using :func:`__getitem__`. |
| |
| Some simple format string examples:: |
| |
| "First, thou shalt count to {0}" # References first positional argument |
| "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 :meth:`__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 :meth:`__format__`, the normal formatting logic |
| is bypassed. |
| |
| Three conversion flags are currently supported: ``'!s'`` which calls :func:`str` |
| on the value, ``'!r'`` which calls :func:`repr` and ``'!a'`` which calls |
| :func:`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 |
| |
| 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 it's 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 can contain only a field name; conversion flags |
| and format specifications are 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. |
| |
| For example, suppose you wanted to have a replacement field whose field width is |
| determined by another variable:: |
| |
| "A man with two {0:{1}}".format("noses", 10) |
| |
| This would first evaluate the inner replacement field, making the format string |
| effectively:: |
| |
| "A man with two {0:10}" |
| |
| Then the outer replacement field would be evaluated, producing:: |
| |
| "noses " |
| |
| Which is subsitituted into the string, yielding:: |
| |
| "A man with two noses " |
| |
| (The extra space is because we specified a field width of 10, and because left |
| alignment is the default for strings.) |
| |
| |
| .. _formatspec: |
| |
| Format Specification Mini-Language |
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ |
| |
| "Format specifications" are used within replacement fields contained within a |
| format string to define how individual values are presented (see |
| :ref:`formatstrings`.) They can also be passed directly to the builtin |
| :func:`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 :func:`str` on the value. |
| |
| The general form of a *standard format specifier* is: |
| |
| .. productionlist:: sf |
| format_spec: [[`fill`]`align`][`sign`][0][`width`][.`precision`][`type`] |
| fill: <a character other than '}'> |
| align: "<" | ">" | "=" | "^" |
| sign: "+" | "-" | " " |
| width: `integer` |
| precision: `integer` |
| type: "b" | "c" | "d" | "e" | "E" | "f" | "F" | "g" | "G" | "n" | "o" | "x" | "X" | "%" |
| |
| The *fill* character can be any character other than '}' (which signifies the |
| end of the field). The presence of a fill character is signaled by the *next* |
| character, which must be one of the alignment options. If the second character |
| of *format_spec* is not a valid alignment option, then it is assumed that both |
| the fill character and the alignment option are absent. |
| |
| 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.) | |
| +---------+----------------------------------------------------------+ |
| | ``'>'`` | Forces the field to be right-aligned within the | |
| | | available space. | |
| +---------+----------------------------------------------------------+ |
| | ``'='`` | 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. | |
| +---------+----------------------------------------------------------+ |
| | ``'^'`` | 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. | |
| +---------+----------------------------------------------------------+ |
| |
| *width* is a decimal integer defining the minimum field width. If not |
| specified, then the field width will be determined by the content. |
| |
| If the *width* field is preceded by a zero (``'0'``) character, this enables |
| zero-padding. This is equivalent to an *alignment* type of ``'='`` and a *fill* |
| character of ``'0'``. |
| |
| The *precision* is a decimal number indicating how many digits should be |
| displayed after the decimal point for a floating point value. 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 ignored for |
| integer values. |
| |
| Finally, the *type* determines how the data should be presented. |
| |
| The available integer presentation types are: |
| |
| +---------+----------------------------------------------------------+ |
| | Type | Meaning | |
| +=========+==========================================================+ |
| | ``'b'`` | Binary. 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'`` | |
| +---------+----------------------------------------------------------+ |
| |
| 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. | |
| +---------+----------------------------------------------------------+ |
| | ``'E'`` | Exponent notation. Same as ``'e'`` except it uses an | |
| | | upper case 'E' as the separator character. | |
| +---------+----------------------------------------------------------+ |
| | ``'f'`` | Fixed point. Displays the number as a fixed-point | |
| | | number. | |
| +---------+----------------------------------------------------------+ |
| | ``'F'`` | Fixed point. Same as ``'f'``. | |
| +---------+----------------------------------------------------------+ |
| | ``'g'`` | General format. This prints the number as a fixed-point | |
| | | number, unless the number is too large, in which case | |
| | | it switches to ``'e'`` exponent notation. | |
| +---------+----------------------------------------------------------+ |
| | ``'G'`` | General format. Same as ``'g'`` except switches to | |
| | | ``'E'`` if the number gets to large. | |
| +---------+----------------------------------------------------------+ |
| | ``'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 | the same as ``'g'`` | |
| +---------+----------------------------------------------------------+ |
| |
| |
| .. _template-strings: |
| |
| Template strings |
| ---------------- |
| |
| Templates provide simpler string substitutions as described in :pep:`292`. |
| Instead of the normal ``%``\ -based substitutions, Templates 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"`` must spell a Python |
| identifier. 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 :exc:`ValueError` |
| being raised. |
| |
| The :mod:`string` module provides a :class:`Template` class that implements |
| these rules. The methods of :class:`Template` are: |
| |
| |
| .. class:: Template(template) |
| |
| The constructor takes a single argument which is the template string. |
| |
| |
| .. method:: substitute(mapping[, **kws]) |
| |
| 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 *kws* are given |
| and there are duplicates, the placeholders from *kws* take precedence. |
| |
| |
| .. method:: safe_substitute(mapping[, **kws]) |
| |
| Like :meth:`substitute`, except that if placeholders are missing from |
| *mapping* and *kws*, instead of raising a :exc:`KeyError` exception, the |
| original placeholder will appear in the resulting string intact. Also, |
| unlike with :meth:`substitute`, any other appearances of the ``$`` will |
| simply return ``$`` instead of raising :exc:`ValueError`. |
| |
| While other exceptions may still occur, this method is called "safe" |
| because substitutions always tries to return a usable string instead of |
| raising an exception. In another sense, :meth:`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. |
| |
| :class:`Template` instances also provide one public data attribute: |
| |
| |
| .. attribute:: string.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 10 |
| >>> 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 :class:`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 ``$``. Note that this should *not* be a regular |
| expression, as the implementation will call :meth:`re.escape` on this string as |
| needed. |
| |
| * *idpattern* -- This is the regular expression describing the pattern for |
| non-braced placeholders (the braces will be added automatically as |
| appropriate). The default value is the regular expression |
| ``[_a-z][_a-z0-9]*``. |
| |
| 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. |
| |
| |
| String functions |
| ---------------- |
| |
| The following functions are available to operate on string objects. |
| They are not available as string methods. |
| |
| |
| .. function:: capwords(s) |
| |
| Split the argument into words using :func:`split`, capitalize each word using |
| :func:`capitalize`, and join the capitalized words using :func:`join`. Note |
| that this replaces runs of whitespace characters by a single space, and removes |
| leading and trailing whitespace. |
| |
| |
| .. function:: maketrans(frm, to) |
| |
| Return a translation table suitable for passing to :meth:`bytes.translate`, |
| that will map each character in *from* into the character at the same |
| position in *to*; *from* and *to* must have the same length. |