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string — Common string operations — Python v3.0 documentation

string — Common string operations¶

The 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 Sequence Types — str, bytes, bytearray, list, tuple, range section, and also the string-specific methods described in the String Methods section. To output formatted strings, see the String Formatting section. Also, see the re module for string functions based on regular expressions.

String constants¶

The constants defined in this module are:

The concatenation of the ascii_lowercase and ascii_uppercase constants described below. This value is not locale-dependent.
The lowercase letters 'abcdefghijklmnopqrstuvwxyz'. This value is not locale-dependent and will not change.
The uppercase letters 'ABCDEFGHIJKLMNOPQRSTUVWXYZ'. This value is not locale-dependent and will not change.
The string '0123456789'.
The string '0123456789abcdefABCDEF'.
The string '01234567'.
String of ASCII characters which are considered punctuation characters in the C locale.
String of ASCII characters which are considered printable. This is a combination of digits, ascii_letters, punctuation, and whitespace.
A string containing all ASCII characters that are considered whitespace. This includes the characters space, tab, linefeed, return, formfeed, and vertical tab.

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)¶
format() is the primary API method. It takes a format template string, and an arbitrary set of positional and keyword argument. format() is just a wrapper that calls vformat().
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. 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 Formatter defines a number of methods that are intended to be replaced by subclasses:


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 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.

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 ‘’ 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 throw 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 ‘r’ (repr) and ‘s’ (str) 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.)

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        ::=  (identifier | integer) ("." attribute_name | "[" element_index "]")*
attribute_name    ::=  identifier
element_index     ::=  integer
conversion        ::=  "r" | "s" | "a"
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 getattr(), while an expression of the form '[index]' does an index lookup using __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 __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

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 substituted 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.)

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.) They can also be passed directly to the builtin 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.

The general form of a standard format specifier is:

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.

The '#' option is only valid for integers, and only for binary, octal, or hexadecimal output. If present, it specifies that the output will be prefixed by '0b', '0o', or '0x', respectively.

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 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 ignored for integer values.

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

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'.

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. Infinity and NaN values are formatted as inf, -inf and nan, respectively.
'G' General format. Same as 'g' except switches to 'E' if the number gets to 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 The same as 'g'.

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 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[, **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.
safe_substitute(mapping[, **kws])¶

Like substitute(), except that if placeholders are missing from mapping and kws, 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 substitutions 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.

Template instances also provide one public data attribute:

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 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 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.

Split the argument into words using split(), capitalize each word using capitalize(), and join the capitalized words using join(). Note that this replaces runs of whitespace characters by a single space, and removes leading and trailing whitespace.
string.maketrans(frm, to)¶
Return a translation table suitable for passing to 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.