7.1. string — Common string operations & Jython v2.5.2 documentation
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7.1. 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.
7.1.1. 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 ''.
string.hexdigits
The string 'abcdefABCDEF'.
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'.
The specific value is locale-
dependent, and will be updated when locale.setlocale() is
string.octdigits
The string ''.
string.punctuation
String of ASCII characters which are considered punctuation
characters in the C locale.
string.printable
String of characters which are considered printable.
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'.
The specific value is locale-
dependent, and will be updated when locale.setlocale() is
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.
7.1.2. String Formatting
Starting in Python 2.6, the built-in str and unicode classes provide
the ability to do complex variable substitutions and value formatting
via the str.format() method described in PEP 3101.
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 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
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 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
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().
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 compone 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.
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.
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-
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()
The default version understands ‘r’ (repr) and ‘s’
(str) conversion types.
7.1.3. 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.
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
conversion flags and format specifications are not allowed.
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.)
7.1.3.1. 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]
::= &a character other than ‘}’&
::= “&” | “&” | “=” | “^”
::= “+” | “-” | ” ”
::= integer
::= integer
::= “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:
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
Forces the padding to be placed after the sign (if any)
but before the digits.
This is used for printing fields
in the form &#120’. This alignment option is only
valid for numeric types.
Forces the field to be centered within the available
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:
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).
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.
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 not allowed for integer values.
Finally, the type determines how the data should be presented.
The available integer presentation types are:
Binary format. Outputs the number in base 2.
Character. Converts the integer to the corresponding
unicode character before printing.
Decimal Integer. Outputs the number in base 10.
Octal format. Outputs the number in base 8.
Hex format. Outputs the number in base 16, using lower-
case letters for the digits above 9.
Hex format. Outputs the number in base 16, using upper-
case letters for the digits above 9.
Number. This is the same as 'd', except that it uses
the current locale setting to insert the appropriate
number separator characters.
The same as 'd'.
The available presentation types for floating point and decimal values
Exponent notation. Prints the number in scientific
notation using the letter ‘e’ to indicate the exponent.
Exponent notation. Same as 'e' except it uses an upper
case ‘E’ as the separator character.
Fixed point. Displays the number as a fixed-point number.
Fixed point. Same as 'f'.
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.
General format. Same as 'g' except switches to 'E'
if the number gets to large. The representations of
infinity and NaN are uppercased, too.
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.
The same as 'g'.
7.1.4. 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:
$$ 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 class string.Template(template)
The constructor takes a single argument which is the template
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
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
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 unbra it
should not include the delimiter in capturing group.
braced – This group matches the brace enclo
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.
7.1.5. 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
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.
Note: Don’t use strings derived from lowercase and uppercase as
in some locales, these don’t have the same length.
For case conversions, always use str.lower() and
str.upper().
7.1.6. Deprecated string functions
The following list of functions are also defined as methods of string
and U see section String Methods for more information
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
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
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
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
(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].
-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
string.rindex(s, sub[, start[, end]])
Like rfind() but raise ValueError when the substring is not
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,
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.rsplit(s[, sep[, maxsplit]])
Return a list of the words of the string s, scanning s from the
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.
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
sep. The default value for sep is a single space character.
It is always true that string.join(string.split(s, sep), sep)
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() meth use the join()
method instead.
string.lstrip(s[, chars])
Return a copy of the string with leading characters removed.
chars is omitted or None, whitespace characters are removed.
If given and not None, chars 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.
chars parameter cannot be passed in earlier 2.2 versions.
string.rstrip(s[, chars])
Return a copy of the string with trailing characters removed.
chars is omitted or None, whitespace characters are removed.
If given and not None, chars 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.
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
If chars is omitted or None, whitespace characters
are removed.
If given and not None, chars
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.
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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