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, unicode, list, tuple, buffer, xrange section, and also the string-specific methods described
in the String Methods section. To output formatted strings use
template strings or the % operator described in the
String Formatting Operations section. Also, see the re module for
string functions based on regular expressions.
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.letters
- The concatenation of the strings lowercase and uppercase
described below. The specific value is locale-dependent, and will be updated
when locale.setlocale() is called.
-
string.lowercase
- A string containing all the characters that are considered lowercase letters.
On most systems this is the string 'abcdefghijklmnopqrstuvwxyz'. Do not
change its definition — the effect on the routines upper() and
swapcase() is undefined. The specific value is locale-dependent, and will
be updated when locale.setlocale() is called.
-
string.octdigits
- The string '01234567'.
-
string.punctuation
- String of ASCII characters which are considered punctuation characters in the
C locale.
-
string.printable
- String of characters which are considered printable. This is a combination of
digits, letters, punctuation, and
whitespace.
-
string.uppercase
- A string containing all the characters that are considered uppercase letters.
On most systems this is the string 'ABCDEFGHIJKLMNOPQRSTUVWXYZ'. Do not
change its definition — the effect on the routines lower() and
swapcase() is undefined. The specific value is locale-dependent, and will
be updated when locale.setlocale() is called.
-
string.whitespace
- A string containing all characters that are considered whitespace. On most
systems this includes the characters space, tab, linefeed, return, formfeed, and
vertical tab. Do not change its definition — the effect on the routines
strip() and split() is undefined.
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"
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.
Two conversion flags are currently supported: '!s' which calls str()
on the value, and '!r' which calls repr().
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:
Then the outer replacement field would be evaluated, producing:
Which is substituted into the string, yielding:
(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.
New in version 2.4.
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:
-
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 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 and Unicode objects.
They are not available as string methods.
-
string.capwords(s)
- 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(from, to)
Return a translation table suitable for passing to 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.
Warning
Don’t use strings derived from lowercase and uppercase as
arguments; in some locales, these don’t have the same length. For case
conversions, always use lower() and upper().
Deprecated string functions
The following list of functions are also defined as methods of string and
Unicode objects; see section String Methods for more information on
those. You should consider these functions as deprecated, although they will
not be removed until Python 3.0. The functions defined in this module are:
-
string.atof(s)
Deprecated since version 2.0: Use the float() built-in function.
Convert a string to a floating point number. The string must have the standard
syntax for a floating point literal in Python, optionally preceded by a sign
(+ or -). Note that this behaves identical to the built-in function
float() when passed a string.
Note
When passing in a string, values for NaN and Infinity may be returned, depending
on the underlying C library. The specific set of strings accepted which cause
these values to be returned depends entirely on the C library and is known to
vary.
-
string.atoi(s[, base])
Deprecated since version 2.0: Use the int() built-in function.
Convert string s to an integer in the given base. The string must consist
of one or more digits, optionally preceded by a sign (+ or -). The
base defaults to 10. If it is 0, a default base is chosen depending on the
leading characters of the string (after stripping the sign): 0x or 0X
means 16, 0 means 8, anything else means 10. If base is 16, a leading
0x or 0X is always accepted, though not required. This behaves
identically to the built-in function int() when passed a string. (Also
note: for a more flexible interpretation of numeric literals, use the built-in
function eval().)
-
string.atol(s[, base])
Deprecated since version 2.0: Use the long() built-in function.
Convert string s to a long integer in the given base. The string must
consist of one or more digits, optionally preceded by a sign (+ or -).
The base argument has the same meaning as for atoi(). A trailing l
or L is not allowed, except if the base is 0. Note that when invoked
without base or with base set to 10, this behaves identical to the built-in
function long() when passed a string.
-
string.capitalize(word)
- Return a copy of word with only its first character capitalized.
-
string.expandtabs(s[, tabsize])
- Expand tabs in a string replacing them by one or more spaces, depending on the
current column and the given tab size. The column number is reset to zero after
each newline occurring in the string. This doesn’t understand other non-printing
characters or escape sequences. The tab size defaults to 8.
-
string.find(s, sub[, start[, end]])
- Return the lowest index in s where the substring sub is found such that
sub is wholly contained in s[start:end]. Return -1 on failure.
Defaults for start and end and interpretation of negative values is the same
as for slices.
-
string.rfind(s, sub[, start[, end]])
- Like find() but find the highest index.
-
string.index(s, sub[, start[, end]])
- Like find() but raise ValueError when the substring is not found.
-
string.rindex(s, sub[, start[, end]])
- Like rfind() but raise ValueError when the substring is not found.
-
string.count(s, sub[, start[, end]])
- Return the number of (non-overlapping) occurrences of substring sub in string
s[start:end]. Defaults for start and end and interpretation of negative
values are the same as for slices.
-
string.lower(s)
- Return a copy of s, but with upper case letters converted to lower case.
-
string.split(s[, sep[, maxsplit]])
Return a list of the words of the string s. If the optional second argument
sep is absent or None, the words are separated by arbitrary strings of
whitespace characters (space, tab, newline, return, formfeed). If the second
argument sep is present and not None, it specifies a string to be used as
the word separator. The returned list will then have one more item than the
number of non-overlapping occurrences of the separator in the string. The
optional third argument maxsplit defaults to 0. If it is nonzero, at most
maxsplit number of splits occur, and the remainder of the string is returned
as the final element of the list (thus, the list will have at most
maxsplit+1 elements).
The behavior of split on an empty string depends on the value of sep. If sep
is not specified, or specified as None, the result will be an empty list.
If sep is specified as any string, the result will be a list containing one
element which is an empty string.
-
string.rsplit(s[, sep[, maxsplit]])
Return a list of the words of the string s, scanning s from the end. To all
intents and purposes, the resulting list of words is the same as returned by
split(), except when the optional third argument maxsplit is explicitly
specified and nonzero. When maxsplit is nonzero, at most maxsplit number of
splits – the rightmost ones – occur, and the remainder of the string is
returned as the first element of the list (thus, the list will have at most
maxsplit+1 elements).
New in version 2.4.
-
string.splitfields(s[, sep[, maxsplit]])
- This function behaves identically to split(). (In the past, split()
was only used with one argument, while splitfields() was only used with
two arguments.)
-
string.join(words[, sep])
- Concatenate a list or tuple of words with intervening occurrences of sep.
The default value for sep is a single space character. It is always true that
string.join(string.split(s, sep), sep) equals s.
-
string.joinfields(words[, sep])
- This function behaves identically to join(). (In the past, join()
was only used with one argument, while joinfields() was only used with two
arguments.) Note that there is no joinfields() method on string objects;
use the join() method instead.
-
string.lstrip(s[, chars])
Return a copy of the string with leading characters removed. If chars is
omitted or None, whitespace characters are removed. If given and not
None, chars must be a string; the characters in the string will be
stripped from the beginning of the string this method is called on.
Changed in version 2.2.3: The chars parameter was added. The chars parameter cannot be passed in
earlier 2.2 versions.
-
string.rstrip(s[, chars])
Return a copy of the string with trailing characters removed. If chars is
omitted or None, whitespace characters are removed. If given and not
None, chars must be a string; the characters in the string will be
stripped from the end of the string this method is called on.
Changed in version 2.2.3: The chars parameter was added. The chars parameter cannot be passed in
earlier 2.2 versions.
-
string.strip(s[, chars])
Return a copy of the string with leading and trailing characters removed. If
chars is omitted or None, whitespace characters are removed. If given and
not None, chars must be a string; the characters in the string will be
stripped from the both ends of the string this method is called on.
Changed in version 2.2.3: The chars parameter was added. The chars parameter cannot be passed in
earlier 2.2 versions.
-
string.swapcase(s)
- Return a copy of s, but with lower case letters converted to upper case and
vice versa.
-
string.translate(s, table[, deletechars])
- Delete all characters from s that are in deletechars (if present), and then
translate the characters using table, which must be a 256-character string
giving the translation for each character value, indexed by its ordinal. If
table is None, then only the character deletion step is performed.
-
string.upper(s)
- Return a copy of s, but with lower case letters converted to upper case.
-
string.ljust(s, width)
-
string.rjust(s, width)
-
string.center(s, width)
- These functions respectively left-justify, right-justify and center a string in
a field of given width. They return a string that is at least width
characters wide, created by padding the string s with spaces until the given
width on the right, left or both sides. The string is never truncated.
-
string.zfill(s, width)
- Pad a numeric string on the left with zero digits until the given width is
reached. Strings starting with a sign are handled correctly.
-
string.replace(str, old, new[, maxreplace])
- Return a copy of string str with all occurrences of substring old replaced
by new. If the optional argument maxreplace is given, the first
maxreplace occurrences are replaced.
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