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5. Builtin Types
The following sections describe the standard types that are built into the interpreter.
Note
Historically (until release 2.2), Python’s builtin types have differed from userdefined types because it was not possible to use the builtin types as the basis for objectoriented inheritance. This limitation no longer exists.
The principal builtin types are numerics, sequences, mappings, files, classes, instances and exceptions.
Some operations are supported by several object types; in particular, practically all objects can be compared, tested for truth value, and converted to a string (with the repr() function or the slightly different str()
function). The latter function is implicitly used when an object is written by the print()
function.
5.1. Truth Value Testing
Any object can be tested for truth value, for use in an if
or while
condition or as operand of the Boolean operations below. The following values are considered false:
None
False
zero of any numeric type, for example,
0
,0L
,0.0
,0j
.any empty sequence, for example,
''
,()
,[]
.any empty mapping, for example,
{}
.instances of userdefined classes, if the class defines a
__nonzero__()
or__len__()
method, when that method returns the integer zero orbool
valueFalse
. 1
All other values are considered true — so objects of many types are always true.
Operations and builtin functions that have a Boolean result always return 0
or False
for false and 1
or True
for true, unless otherwise stated. (Important exception: the Boolean operations or
and and
always return one of their operands.)
5.2. Boolean Operations — and
, or
, not
These are the Boolean operations, ordered by ascending priority:
Operation 
Result 
Notes 


if x is false, then y, else x 
(1) 

if x is false, then x, else y 
(2) 

if x is false, then 
(3) 
Notes:
This is a shortcircuit operator, so it only evaluates the second argument if the first one is false.
This is a shortcircuit operator, so it only evaluates the second argument if the first one is true.
not
has a lower priority than nonBoolean operators, sonot a == b
is interpreted asnot (a == b)
, anda == not b
is a syntax error.
5.3. Comparisons
Comparison operations are supported by all objects. They all have the same priority (which is higher than that of the Boolean operations). Comparisons can be chained arbitrarily; for example, x < y <= z
is equivalent to x < y and y <= z
, except that y is evaluated only once (but in both cases z is not evaluated at all when x < y
is found to be false).
This table summarizes the comparison operations:
Operation 
Meaning 
Notes 


strictly less than 


less than or equal 


strictly greater than 


greater than or equal 


equal 


not equal 
(1) 

object identity 


negated object identity 
Notes:
!=
can also be written<>
, but this is an obsolete usage kept for backwards compatibility only. New code should always use!=
.
Objects of different types, except different numeric types and different string types, never compare equal; such objects are ordered consistently but arbitrarily (so that sorting a heterogeneous array yields a consistent result). Furthermore, some types (for example, file objects) support only a degenerate notion of comparison where any two objects of that type are unequal. Again, such objects are ordered arbitrarily but consistently. The <
, <=
, >
and >=
operators will raise a TypeError
exception when any operand is a complex number.
Nonidentical instances of a class normally compare as nonequal unless the class defines the __eq__()
method or the __cmp__()
method.
Instances of a class cannot be ordered with respect to other instances of the same class, or other types of object, unless the class defines either enough of the rich comparison methods (__lt__()
, __le__()
, __gt__()
, and __ge__()
) or the __cmp__()
method.
CPython implementation detail: Objects of different types except numbers are ordered by their type names; objects of the same types that don’t support proper comparison are ordered by their address.
Two more operations with the same syntactic priority, in
and not in
, are supported only by sequence types (below).
5.4. Numeric Types — int
, float
, long
, complex
There are four distinct numeric types: plain integers, long integers, floating point numbers, and complex numbers. In addition, Booleans are a subtype of plain integers. Plain integers (also just called integers) are implemented using long
in C, which gives them at least 32 bits of precision (sys.maxint
is always set to the maximum plain integer value for the current platform, the minimum value is sys.maxint  1
). Long integers have unlimited precision. Floating point numbers are usually implemented using double
in C; information about the precision and internal representation of floating point numbers for the machine on which your program is running is available in sys.float_info
. Complex numbers have a real and imaginary part, which are each a floating point number. To extract these parts from a complex number z, use z.real
and z.imag
. (The standard library includes additional numeric types, fractions
that hold rationals, and decimal
that hold floatingpoint numbers with userdefinable precision.)
Numbers are created by numeric literals or as the result of builtin functions and operators. Unadorned integer literals (including binary, hex, and octal numbers) yield plain integers unless the value they denote is too large to be represented as a plain integer, in which case they yield a long integer. Integer literals with an 'L'
or 'l'
suffix yield long integers ('L'
is preferred because 1l
looks too much like eleven!). Numeric literals containing a decimal point or an exponent sign yield floating point numbers. Appending 'j'
or 'J'
to a numeric literal yields an imaginary number (a complex number with a zero real part) which you can add to an integer or float to get a complex number with real and imaginary parts.
Python fully supports mixed arithmetic: when a binary arithmetic operator has operands of different numeric types, the operand with the “narrower” type is widened to that of the other, where plain integer is narrower than long integer is narrower than floating point is narrower than complex. Comparisons between numbers of mixed type use the same rule. 2 The constructors int()
, long()
, float()
, and complex()
can be used to produce numbers of a specific type.
All builtin numeric types support the following operations. See The power operator and later sections for the operators’ priorities.
Operation 
Result 
Notes 


sum of x and y 


difference of x and y 


product of x and y 


quotient of x and y 
(1) 

(floored) quotient of x and y 
(4)(5) 

remainder of 
(4) 

x negated 


x unchanged 


absolute value or magnitude of x 
(3) 

x converted to integer 
(2) 

x converted to long integer 
(2) 

x converted to floating point 
(6) 

a complex number with real part re, imaginary part im. im defaults to zero. 


conjugate of the complex number c. (Identity on real numbers) 


the pair 
(3)(4) 

x to the power y 
(3)(7) 

x to the power y 
(7) 
Notes:
For (plain or long) integer division, the result is an integer. The result is always rounded towards minus infinity: 1/2 is 0, (1)/2 is 1, 1/(2) is 1, and (1)/(2) is 0. Note that the result is a long integer if either operand is a long integer, regardless of the numeric value.
Conversion from floats using
int()
orlong()
truncates toward zero like the related function,math.trunc()
. Use the functionmath.floor()
to round downward andmath.ceil()
to round upward.See Builtin Functions for a full description.
Also referred to as integer division. The resultant value is a whole integer, though the result’s type is not necessarily int.
float also accepts the strings “nan” and “inf” with an optional prefix “+” or “” for Not a Number (NaN) and positive or negative infinity.
New in version 2.6.
Python defines
pow(0, 0)
and0 ** 0
to be1
, as is common for programming languages.
All numbers.Real
types (int
, long
, and float
) also include the following operations:
Operation 
Result 

x truncated to 

x rounded to n digits, rounding ties away from zero. If n is omitted, it defaults to 0. 

the greatest integer as a float <= x 

the least integer as a float >= x 
5.4.1. Bitwise Operations on Integer Types
Bitwise operations only make sense for integers. Negative numbers are treated as their 2’s complement value (this assumes a sufficiently large number of bits that no overflow occurs during the operation).
The priorities of the binary bitwise operations are all lower than the numeric operations and higher than the comparisons; the unary operation ~
has the same priority as the other unary numeric operations (+
and 
).
This table lists the bitwise operations sorted in ascending priority:
Operation 
Result 
Notes 


bitwise or of x and y 


bitwise exclusive or of x and y 


bitwise and of x and y 


x shifted left by n bits 
(1)(2) 

x shifted right by n bits 
(1)(3) 

the bits of x inverted 
Notes:
Negative shift counts are illegal and cause a
ValueError
to be raised.A left shift by n bits is equivalent to multiplication by
pow(2, n)
. A long integer is returned if the result exceeds the range of plain integers.A right shift by n bits is equivalent to division by
pow(2, n)
.
5.4.2. Additional Methods on Integer Types
The integer types implement the numbers.Integral
abstract base class. In addition, they provide one more method:
long.
bit_length
( )
Return the number of bits necessary to represent an integer in binary, excluding the sign and leading zeros:
>>> n = 37 >>> bin(n) '0b100101' >>> n.bit_length() 6
More precisely, if
x
is nonzero, thenx.bit_length()
is the unique positive integerk
such that2**(k1) <= abs(x) < 2**k
. Equivalently, whenabs(x)
is small enough to have a correctly rounded logarithm, thenk = 1 + int(log(abs(x), 2))
. Ifx
is zero, thenx.bit_length()
returns0
.Equivalent to:
def bit_length(self): s = bin(self) # binary representation: bin(37) > '0b100101' s = s.lstrip('0b') # remove leading zeros and minus sign return len(s) # len('100101') > 6
New in version 2.7.
5.4.3. Additional Methods on Float
The float type implements the numbers.Real
abstract base class. float also has the following additional methods.
float.
as_integer_ratio
( )
Return a pair of integers whose ratio is exactly equal to the original float and with a positive denominator. Raises
OverflowError
on infinities and aValueError
on NaNs.New in version 2.6.
float.
is_integer
( )
Return
True
if the float instance is finite with integral value, andFalse
otherwise:>>> (2.0).is_integer() True >>> (3.2).is_integer() False
New in version 2.6.
Two methods support conversion to and from hexadecimal strings. Since Python’s floats are stored internally as binary numbers, converting a float to or from a decimal string usually involves a small rounding error. In contrast, hexadecimal strings allow exact representation and specification of floatingpoint numbers. This can be useful when debugging, and in numerical work.
float.
hex
( )
Return a representation of a floatingpoint number as a hexadecimal string. For finite floatingpoint numbers, this representation will always include a leading
0x
and a trailingp
and exponent.New in version 2.6.
float.
fromhex
( s )
Class method to return the float represented by a hexadecimal string s. The string s may have leading and trailing whitespace.
New in version 2.6.
Note that float.hex()
is an instance method, while float.fromhex()
is a class method.
A hexadecimal string takes the form:
[sign] ['0x'] integer ['.' fraction] ['p' exponent]
where the optional sign
may by either +
or 
, integer
and fraction
are strings of hexadecimal digits, and exponent
is a decimal integer with an optional leading sign. Case is not significant, and there must be at least one hexadecimal digit in either the integer or the fraction. This syntax is similar to the syntax specified in section 6.4.4.2 of the C99 standard, and also to the syntax used in Java 1.5 onwards. In particular, the output of float.hex()
is usable as a hexadecimal floatingpoint literal in C or Java code, and hexadecimal strings produced by C’s %a
format character or Java’s Double.toHexString
are accepted by float.fromhex()
.
Note that the exponent is written in decimal rather than hexadecimal, and that it gives the power of 2 by which to multiply the coefficient. For example, the hexadecimal string 0x3.a7p10
represents the floatingpoint number (3 + 10./16 + 7./16**2) * 2.0**10
, or 3740.0
:
>>> float.fromhex('0x3.a7p10')
3740.0
Applying the reverse conversion to 3740.0
gives a different hexadecimal string representing the same number:
>>> float.hex(3740.0)
'0x1.d380000000000p+11'
5.5. Iterator Types
New in version 2.2.
Python supports a concept of iteration over containers. This is implemented using two distinct methods; these are used to allow userdefined classes to support iteration. Sequences, described below in more detail, always support the iteration methods.
One method needs to be defined for container objects to provide iteration support:
container.
__iter__
( )
Return an iterator object. The object is required to support the iterator protocol described below. If a container supports different types of iteration, additional methods can be provided to specifically request iterators for those iteration types. (An example of an object supporting multiple forms of iteration would be a tree structure which supports both breadthfirst and depthfirst traversal.) This method corresponds to the
tp_iter
slot of the type structure for Python objects in the Python/C API.
The iterator objects themselves are required to support the following two methods, which together form the iterator protocol:
iterator.
__iter__
( )
Return the iterator object itself. This is required to allow both containers and iterators to be used with the
for
andin
statements. This method corresponds to thetp_iter
slot of the type structure for Python objects in the Python/C API.
iterator.
next
( )
Return the next item from the container. If there are no further items, raise the
StopIteration
exception. This method corresponds to thetp_iternext
slot of the type structure for Python objects in the Python/C API.
Python defines several iterator objects to support iteration over general and specific sequence types, dictionaries, and other more specialized forms. The specific types are not important beyond their implementation of the iterator protocol.
The intention of the protocol is that once an iterator’s next()
method raises StopIteration
, it will continue to do so on subsequent calls. Implementations that do not obey this property are deemed broken. (This constraint was added in Python 2.3; in Python 2.2, various iterators are broken according to this rule.)
5.5.1. Generator Types
Python’s generators provide a convenient way to implement the iterator protocol. If a container object’s __iter__()
method is implemented as a generator, it will automatically return an iterator object (technically, a generator object) supplying the __iter__()
and next()
methods. More information about generators can be found in the documentation for the yield expression.
5.6. Sequence Types — str
, unicode
, list
, tuple
, bytearray
, buffer
, xrange
There are seven sequence types: strings, Unicode strings, lists, tuples, bytearrays, buffers, and xrange objects.
For other containers see the built in dict
and set
classes, and the collections
module.
String literals are written in single or double quotes: 'xyzzy'
, "frobozz"
. See String literals for more about string literals. Unicode strings are much like strings, but are specified in the syntax using a preceding 'u'
character: u'abc'
, u"def"
. In addition to the functionality described here, there are also stringspecific methods described in the String Methods section. Lists are constructed with square brackets, separating items with commas: [a, b, c]
. Tuples are constructed by the comma operator (not within square brackets), with or without enclosing parentheses, but an empty tuple must have the enclosing parentheses, such as a, b, c
or ()
. A single item tuple must have a trailing comma, such as (d,)
.
Bytearray objects are created with the builtin function bytearray()
.
Buffer objects are not directly supported by Python syntax, but can be created by calling the builtin function buffer()
. They don’t support concatenation or repetition.
Objects of type xrange are similar to buffers in that there is no specific syntax to create them, but they are created using the xrange()
function. They don’t support slicing, concatenation or repetition, and using in
, not in
, min()
or max()
on them is inefficient.
Most sequence types support the following operations. The in
and not in
operations have the same priorities as the comparison operations. The +
and *
operations have the same priority as the corresponding numeric operations. 3 Additional methods are provided for Mutable Sequence Types.
This table lists the sequence operations sorted in ascending priority. In the table, s and t are sequences of the same type; n, i and j are integers:
Operation 
Result 
Notes 



(1) 


(1) 

the concatenation of s and t 
(6) 

equivalent to adding s to itself n times 
(2) 

ith item of s, origin 0 
(3) 

slice of s from i to j 
(3)(4) 

slice of s from i to j with step k 
(3)(5) 

length of s 


smallest item of s 


largest item of s 


index of the first occurrence of x in s 


total number of occurrences of x in s 
Sequence types also support comparisons. In particular, tuples and lists are compared lexicographically by comparing corresponding elements. This means that to compare equal, every element must compare equal and the two sequences must be of the same type and have the same length. (For full details see Comparisons in the language reference.)
Notes:
When s is a string or Unicode string object the
in
andnot in
operations act like a substring test. In Python versions before 2.3, x had to be a string of length 1. In Python 2.3 and beyond, x may be a string of any length.Values of n less than
0
are treated as0
(which yields an empty sequence of the same type as s). Note that items in the sequence s are not copied; they are referenced multiple times. This often haunts new Python programmers; consider:>>> lists = [[]] * 3 >>> lists [[], [], []] >>> lists[0].append(3) >>> lists [[3], [3], [3]]
What has happened is that
[[]]
is a oneelement list containing an empty list, so all three elements of[[]] * 3
are references to this single empty list. Modifying any of the elements oflists
modifies this single list. You can create a list of different lists this way:>>> lists = [[] for i in range(3)] >>> lists[0].append(3) >>> lists[1].append(5) >>> lists[2].append(7) >>> lists [[3], [5], [7]]
Further explanation is available in the FAQ entry How do I create a multidimensional list?.
If i or j is negative, the index is relative to the end of sequence s:
len(s) + i
orlen(s) + j
is substituted. But note that0
is still0
.The slice of s from i to j is defined as the sequence of items with index k such that
i <= k < j
. If i or j is greater thanlen(s)
, uselen(s)
. If i is omitted orNone
, use0
. If j is omitted orNone
, uselen(s)
. If i is greater than or equal to j, the slice is empty.The slice of s from i to j with step k is defined as the sequence of items with index
x = i + n*k
such that0 <= n < (ji)/k
. In other words, the indices arei
,i+k
,i+2*k
,i+3*k
and so on, stopping when j is reached (but never including j). When k is positive, i and j are reduced tolen(s)
if they are greater. When k is negative, i and j are reduced tolen(s)  1
if they are greater. If i or j are omitted orNone
, they become “end” values (which end depends on the sign of k). Note, k cannot be zero. If k isNone
, it is treated like1
.
CPython implementation detail: If s and t are both strings, some Python implementations such as CPython can usually perform an inplace optimization for assignments of the form
s = s + t
ors += t
. When applicable, this optimization makes quadratic runtime much less likely. This optimization is both version and implementation dependent. For performance sensitive code, it is preferable to use thestr.join()
method which assures consistent linear concatenation performance across versions and implementations.Changed in version 2.4: Formerly, string concatenation never occurred inplace.
5.6.1. String Methods
Below are listed the string methods which both 8bit strings and Unicode objects support. Some of them are also available on bytearray
objects.
In addition, Python’s strings support the sequence type methods described in the Sequence Types — str, unicode, list, tuple, bytearray, buffer, xrange 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.
str.
capitalize
( )
Return a copy of the string with its first character capitalized and the rest lowercased.
For 8bit strings, this method is localedependent.
str.
center
( width [, fillchar ] )
Return centered in a string of length width. Padding is done using the specified fillchar (default is a space).
Changed in version 2.4: Support for the fillchar argument.
str.
count
( sub [, start [, end ] ] )
Return the number of nonoverlapping occurrences of substring sub in the range [start, end]. Optional arguments start and end are interpreted as in slice notation.
str.
decode
( [ encoding [, errors ] ] )
Decodes the string using the codec registered for encoding. encoding defaults to the default string encoding. errors may be given to set a different error handling scheme. The default is
'strict'
, meaning that encoding errors raiseUnicodeError
. Other possible values are'ignore'
,'replace'
and any other name registered viacodecs.register_error()
, see section Codec Base Classes.New in version 2.2.
Changed in version 2.3: Support for other error handling schemes added.
Changed in version 2.7: Support for keyword arguments added.
str.
encode
( [ encoding [, errors ] ] )
Return an encoded version of the string. Default encoding is the current default string encoding. errors may be given to set a different error handling scheme. The default for errors is
'strict'
, meaning that encoding errors raise aUnicodeError
. Other possible values are'ignore'
,'replace'
,'xmlcharrefreplace'
,'backslashreplace'
and any other name registered viacodecs.register_error()
, see section Codec Base Classes. For a list of possible encodings, see section Standard Encodings.New in version 2.0.
Changed in version 2.3: Support for
'xmlcharrefreplace'
and'backslashreplace'
and other error handling schemes added.Changed in version 2.7: Support for keyword arguments added.
str.
endswith
( suffix [, start [, end ] ] )
Return
True
if the string ends with the specified suffix, otherwise returnFalse
. suffix can also be a tuple of suffixes to look for. With optional start, test beginning at that position. With optional end, stop comparing at that position.Changed in version 2.5: Accept tuples as suffix.
str.
expandtabs
( [ tabsize ] )
Return a copy of the string where all tab characters are replaced by one or more spaces, depending on the current column and the given tab size. Tab positions occur every tabsize characters (default is 8, giving tab positions at columns 0, 8, 16 and so on). To expand the string, the current column is set to zero and the string is examined character by character. If the character is a tab (
\t
), one or more space characters are inserted in the result until the current column is equal to the next tab position. (The tab character itself is not copied.) If the character is a newline (\n
) or return (\r
), it is copied and the current column is reset to zero. Any other character is copied unchanged and the current column is incremented by one regardless of how the character is represented when printed.>>> '01\t012\t0123\t01234'.expandtabs() '01 012 0123 01234' >>> '01\t012\t0123\t01234'.expandtabs(4) '01 012 0123 01234'
str.
find
( sub [, start [, end ] ] )
Return the lowest index in the string where substring sub is found within the slice
s[start:end]
. Optional arguments start and end are interpreted as in slice notation. Return1
if sub is not found.
str.
format
( *args, **kwargs )
Perform a string formatting operation. The string on which this method is called can contain literal text or replacement fields delimited by braces
{}
. Each replacement field contains either the numeric index of a positional argument, or the name of a keyword argument. Returns a copy of the string where each replacement field is replaced with the string value of the corresponding argument.>>> "The sum of 1 + 2 is {0}".format(1+2) 'The sum of 1 + 2 is 3'
See Format String Syntax for a description of the various formatting options that can be specified in format strings.
This method of string formatting is the new standard in Python 3, and should be preferred to the
%
formatting described in String Formatting Operations in new code.New in version 2.6.
str.
index
( sub [, start [, end ] ] )
Like
find()
, but raiseValueError
when the substring is not found.
str.
isalnum
( )
Return true if all characters in the string are alphanumeric and there is at least one character, false otherwise.
For 8bit strings, this method is localedependent.
str.
isalpha
( )
Return true if all characters in the string are alphabetic and there is at least one character, false otherwise.
For 8bit strings, this method is localedependent.
str.
isdigit
( )
Return true if all characters in the string are digits and there is at least one character, false otherwise.
For 8bit strings, this method is localedependent.
str.
islower
( )
Return true if all cased characters 4 in the string are lowercase and there is at least one cased character, false otherwise.
For 8bit strings, this method is localedependent.
str.
isspace
( )
Return true if there are only whitespace characters in the string and there is at least one character, false otherwise.
For 8bit strings, this method is localedependent.
str.
istitle
( )
Return true if the string is a titlecased string and there is at least one character, for example uppercase characters may only follow uncased characters and lowercase characters only cased ones. Return false otherwise.
For 8bit strings, this method is localedependent.
str.
isupper
( )
Return true if all cased characters 4 in the string are uppercase and there is at least one cased character, false otherwise.
For 8bit strings, this method is localedependent.
str.
join
( iterable )
Return a string which is the concatenation of the strings in iterable. If there is any Unicode object in iterable, return a Unicode instead. A
TypeError
will be raised if there are any nonstring or non Unicode object values in iterable. The separator between elements is the string providing this method.
str.
ljust
( width [, fillchar ] )
Return the string left justified in a string of length width. Padding is done using the specified fillchar (default is a space). The original string is returned if width is less than or equal to
len(s)
.Changed in version 2.4: Support for the fillchar argument.
str.
lower
( )
Return a copy of the string with all the cased characters 4 converted to lowercase.
For 8bit strings, this method is localedependent.
str.
lstrip
( [ chars ] )
Return a copy of the string with leading characters removed. The chars argument is a string specifying the set of characters to be removed. If omitted or
None
, the chars argument defaults to removing whitespace. The chars argument is not a prefix; rather, all combinations of its values are stripped:>>> ' spacious '.lstrip() 'spacious ' >>> 'www.example.com'.lstrip('cmowz.') 'example.com'
Changed in version 2.2.2: Support for the chars argument.
str.
partition
( sep )
Split the string at the first occurrence of sep, and return a 3tuple containing the part before the separator, the separator itself, and the part after the separator. If the separator is not found, return a 3tuple containing the string itself, followed by two empty strings.
New in version 2.5.
str.
replace
( old, new [, count ] )
Return a copy of the string with all occurrences of substring old replaced by new. If the optional argument count is given, only the first count occurrences are replaced.
str.
rfind
( sub [, start [, end ] ] )
Return the highest index in the string where substring sub is found, such that sub is contained within
s[start:end]
. Optional arguments start and end are interpreted as in slice notation. Return1
on failure.
str.
rindex
( sub [, start [, end ] ] )
Like
rfind()
but raisesValueError
when the substring sub is not found.
str.
rjust
( width [, fillchar ] )
Return the string right justified in a string of length width. Padding is done using the specified fillchar (default is a space). The original string is returned if width is less than or equal to
len(s)
.Changed in version 2.4: Support for the fillchar argument.
str.
rpartition
( sep )
Split the string at the last occurrence of sep, and return a 3tuple containing the part before the separator, the separator itself, and the part after the separator. If the separator is not found, return a 3tuple containing two empty strings, followed by the string itself.
New in version 2.5.
str.
rsplit
( [ sep [, maxsplit ] ] )
Return a list of the words in the string, using sep as the delimiter string. If maxsplit is given, at most maxsplit splits are done, the rightmost ones. If sep is not specified or
None
, any whitespace string is a separator. Except for splitting from the right,rsplit()
behaves likesplit()
which is described in detail below.New in version 2.4.
str.
rstrip
( [ chars ] )
Return a copy of the string with trailing characters removed. The chars argument is a string specifying the set of characters to be removed. If omitted or
None
, the chars argument defaults to removing whitespace. The chars argument is not a suffix; rather, all combinations of its values are stripped:>>> ' spacious '.rstrip() ' spacious' >>> 'mississippi'.rstrip('ipz') 'mississ'
Changed in version 2.2.2: Support for the chars argument.
str.
split
( [ sep [, maxsplit ] ] )
Return a list of the words in the string, using sep as the delimiter string. If maxsplit is given, at most maxsplit splits are done (thus, the list will have at most
maxsplit+1
elements). If maxsplit is not specified or1
, then there is no limit on the number of splits (all possible splits are made).If sep is given, consecutive delimiters are not grouped together and are deemed to delimit empty strings (for example,
'1,,2'.split(',')
returns['1', '', '2']
). The sep argument may consist of multiple characters (for example,'1<>2<>3'.split('<>')
returns['1', '2', '3']
). Splitting an empty string with a specified separator returns['']
.If sep is not specified or is
None
, a different splitting algorithm is applied: runs of consecutive whitespace are regarded as a single separator, and the result will contain no empty strings at the start or end if the string has leading or trailing whitespace. Consequently, splitting an empty string or a string consisting of just whitespace with aNone
separator returns[]
.For example,
' 1 2 3 '.split()
returns['1', '2', '3']
, and' 1 2 3 '.split(None, 1)
returns['1', '2 3 ']
.
str.
splitlines
( [ keepends ] )
Return a list of the lines in the string, breaking at line boundaries. This method uses the universal newlines approach to splitting lines. Line breaks are not included in the resulting list unless keepends is given and true.
Python recognizes
"\r"
,"\n"
, and"\r\n"
as line boundaries for 8bit strings.For example:
>>> 'ab c\n\nde fg\rkl\r\n'.splitlines() ['ab c', '', 'de fg', 'kl'] >>> 'ab c\n\nde fg\rkl\r\n'.splitlines(True) ['ab c\n', '\n', 'de fg\r', 'kl\r\n']
Unlike
split()
when a delimiter string sep is given, this method returns an empty list for the empty string, and a terminal line break does not result in an extra line:>>> "".splitlines() [] >>> "One line\n".splitlines() ['One line']
For comparison,
split('\n')
gives:>>> ''.split('\n') [''] >>> 'Two lines\n'.split('\n') ['Two lines', '']
unicode.
splitlines
( [ keepends ] )
Return a list of the lines in the string, like
str.splitlines()
. However, the Unicode method splits on the following line boundaries, which are a superset of the universal newlines recognized for 8bit strings.Representation
Description
\n
Line Feed
\r
Carriage Return
\r\n
Carriage Return + Line Feed
\v
or\x0b
Line Tabulation
\f
or\x0c
Form Feed
\x1c
File Separator
\x1d
Group Separator
\x1e
Record Separator
\x85
Next Line (C1 Control Code)
\u2028
Line Separator
\u2029
Paragraph Separator
Changed in version 2.7:
\v
and\f
added to list of line boundaries.
str.
startswith
( prefix [, start [, end ] ] )
Return
True
if string starts with the prefix, otherwise returnFalse
. prefix can also be a tuple of prefixes to look for. With optional start, test string beginning at that position. With optional end, stop comparing string at that position.Changed in version 2.5: Accept tuples as prefix.
str.
strip
( [ chars ] )
Return a copy of the string with the leading and trailing characters removed. The chars argument is a string specifying the set of characters to be removed. If omitted or
None
, the chars argument defaults to removing whitespace. The chars argument is not a prefix or suffix; rather, all combinations of its values are stripped:>>> ' spacious '.strip() 'spacious' >>> 'www.example.com'.strip('cmowz.') 'example'
Changed in version 2.2.2: Support for the chars argument.
str.
swapcase
( )
Return a copy of the string with uppercase characters converted to lowercase and vice versa.
For 8bit strings, this method is localedependent.
str.
title
( )
Return a titlecased version of the string where words start with an uppercase character and the remaining characters are lowercase.
The algorithm uses a simple languageindependent definition of a word as groups of consecutive letters. The definition works in many contexts but it means that apostrophes in contractions and possessives form word boundaries, which may not be the desired result:
>>> "they're bill's friends from the UK".title() "They'Re Bill'S Friends From The Uk"
A workaround for apostrophes can be constructed using regular expressions:
>>> import re >>> def titlecase(s): ... return re.sub(r"[AZaz]+('[AZaz]+)?", ... lambda mo: mo.group(0)[0].upper() + ... mo.group(0)[1:].lower(), ... s) ... >>> titlecase("they're bill's friends.") "They're Bill's Friends."
For 8bit strings, this method is localedependent.
str.
translate
( table [, deletechars ] )
Return a copy of the string where all characters occurring in the optional argument deletechars are removed, and the remaining characters have been mapped through the given translation table, which must be a string of length 256.
You can use the
maketrans()
helper function in thestring
module to create a translation table. For string objects, set the table argument toNone
for translations that only delete characters:>>> 'read this short text'.translate(None, 'aeiou') 'rd ths shrt txt'
New in version 2.6: Support for a
None
table argument.For Unicode objects, the
translate()
method does not accept the optional deletechars argument. Instead, it returns a copy of the s where all characters have been mapped through the given translation table which must be a mapping of Unicode ordinals to Unicode ordinals, Unicode strings orNone
. Unmapped characters are left untouched. Characters mapped toNone
are deleted. Note, a more flexible approach is to create a custom character mapping codec using thecodecs
module (seeencodings.cp1251
for an example).
str.
upper
( )
Return a copy of the string with all the cased characters 4 converted to uppercase. Note that
s.upper().isupper()
might beFalse
ifs
contains uncased characters or if the Unicode category of the resulting character(s) is not “Lu” (Letter, uppercase), but e.g. “Lt” (Letter, titlecase).For 8bit strings, this method is localedependent.
str.
zfill
( width )
Return the numeric string left filled with zeros in a string of length width. A sign prefix is handled correctly. The original string is returned if width is less than or equal to
len(s)
.New in version 2.2.2.
The following methods are present only on unicode objects:
5.6.2. String Formatting Operations
String and Unicode objects have one unique builtin operation: the %
operator (modulo). This is also known as the string formatting or interpolation operator. Given format % values
(where format is a string or Unicode object), %
conversion specifications in format are replaced with zero or more elements of values. The effect is similar to the using sprintf()
in the C language. If format is a Unicode object, or if any of the objects being converted using the %s
conversion are Unicode objects, the result will also be a Unicode object.
If format requires a single argument, values may be a single nontuple object. 5 Otherwise, values must be a tuple with exactly the number of items specified by the format string, or a single mapping object (for example, a dictionary).
A conversion specifier contains two or more characters and has the following components, which must occur in this order:
The
'%'
character, which marks the start of the specifier.Mapping key (optional), consisting of a parenthesised sequence of characters (for example,
(somename)
).Conversion flags (optional), which affect the result of some conversion types.
Minimum field width (optional). If specified as an
'*'
(asterisk), the actual width is read from the next element of the tuple in values, and the object to convert comes after the minimum field width and optional precision.Precision (optional), given as a
'.'
(dot) followed by the precision. If specified as'*'
(an asterisk), the actual width is read from the next element of the tuple in values, and the value to convert comes after the precision.Length modifier (optional).
Conversion type.
When the right argument is a dictionary (or other mapping type), then the formats in the string must include a parenthesised mapping key into that dictionary inserted immediately after the '%'
character. The mapping key selects the value to be formatted from the mapping. For example:
>>> print '%(language)s has %(number)03d quote types.' % \
... {"language": "Python", "number": 2}
Python has 002 quote types.
In this case no *
specifiers may occur in a format (since they require a sequential parameter list).
The conversion flag characters are:
Flag 
Meaning 


The value conversion will use the “alternate form” (where defined below). 

The conversion will be zero padded for numeric values. 

The converted value is left adjusted (overrides the 

(a space) A blank should be left before a positive number (or empty string) produced by a signed conversion. 

A sign character ( 
A length modifier (h
, l
, or L
) may be present, but is ignored as it is not necessary for Python – so e.g. %ld
is identical to %d
.
The conversion types are:
Conversion 
Meaning 
Notes 


Signed integer decimal. 


Signed integer decimal. 


Signed octal value. 
(1) 

Obsolete type – it is identical to 
(7) 

Signed hexadecimal (lowercase). 
(2) 

Signed hexadecimal (uppercase). 
(2) 

Floating point exponential format (lowercase). 
(3) 

Floating point exponential format (uppercase). 
(3) 

Floating point decimal format. 
(3) 

Floating point decimal format. 
(3) 

Floating point format. Uses lowercase exponential format if exponent is less than 4 or not less than precision, decimal format otherwise. 
(4) 

Floating point format. Uses uppercase exponential format if exponent is less than 4 or not less than precision, decimal format otherwise. 
(4) 

Single character (accepts integer or single character string). 


String (converts any Python object using repr()). 
(5) 

String (converts any Python object using 
(6) 

No argument is converted, results in a 
Notes:
The alternate form causes a leading zero (
'0'
) to be inserted between lefthand padding and the formatting of the number if the leading character of the result is not already a zero.The alternate form causes a leading
'0x'
or'0X'
(depending on whether the'x'
or'X'
format was used) to be inserted before the first digit.The alternate form causes the result to always contain a decimal point, even if no digits follow it.
The precision determines the number of digits after the decimal point and defaults to 6.
The alternate form causes the result to always contain a decimal point, and trailing zeroes are not removed as they would otherwise be.
The precision determines the number of significant digits before and after the decimal point and defaults to 6.
The
%r
conversion was added in Python 2.0.The precision determines the maximal number of characters used.
If the object or format provided is a
unicode
string, the resulting string will also beunicode
.The precision determines the maximal number of characters used.
See PEP 237 .
Since Python strings have an explicit length, %s
conversions do not assume that '\0'
is the end of the string.
Changed in version 2.7: %f
conversions for numbers whose absolute value is over 1e50 are no longer replaced by %g
conversions.
Additional string operations are defined in standard modules string
and re
.
5.6.3. XRange Type
The xrange
type is an immutable sequence which is commonly used for looping. The advantage of the xrange
type is that an xrange
object will always take the same amount of memory, no matter the size of the range it represents. There are no consistent performance advantages.
XRange objects have very little behavior: they only support indexing, iteration, and the len()
function.
5.6.4. Mutable Sequence Types
List and bytearray
objects support additional operations that allow inplace modification of the object. Other mutable sequence types (when added to the language) should also support these operations. Strings and tuples are immutable sequence types: such objects cannot be modified once created. The following operations are defined on mutable sequence types (where x is an arbitrary object):
Operation 
Result 
Notes 


item i of s is replaced by x 


slice of s from i to j is replaced by the contents of the iterable t 


same as 


the elements of 
(1) 

removes the elements of 


same as 
(2) 

for the most part the same as 
(3) 

updates s with its contents repeated n times 
(11) 

return number of i’s for which 


return smallest k such that 
(4) 

same as 
(5) 

same as 
(6) 

same as 
(4) 

reverses the items of s in place 
(7) 

sort the items of s in place 
(7)(8)(9)(10) 
Notes:
t must have the same length as the slice it is replacing.
The C implementation of Python has historically accepted multiple parameters and implicitly joined them into a tuple; this no longer works in Python 2.0. Use of this misfeature has been deprecated since Python 1.4.
t can be any iterable object.
Raises
ValueError
when x is not found in s. When a negative index is passed as the second or third parameter to theindex()
method, the list length is added, as for slice indices. If it is still negative, it is truncated to zero, as for slice indices.Changed in version 2.3: Previously,
index()
didn’t have arguments for specifying start and stop positions.When a negative index is passed as the first parameter to the
insert()
method, the list length is added, as for slice indices. If it is still negative, it is truncated to zero, as for slice indices.Changed in version 2.3: Previously, all negative indices were truncated to zero.
The
pop()
method’s optional argument i defaults to1
, so that by default the last item is removed and returned.The
sort()
andreverse()
methods modify the list in place for economy of space when sorting or reversing a large list. To remind you that they operate by side effect, they don’t return the sorted or reversed list.The
sort()
method takes optional arguments for controlling the comparisons.cmp specifies a custom comparison function of two arguments (list items) which should return a negative, zero or positive number depending on whether the first argument is considered smaller than, equal to, or larger than the second argument:
cmp=lambda x,y: cmp(x.lower(), y.lower())
. The default value isNone
.key specifies a function of one argument that is used to extract a comparison key from each list element:
key=str.lower
. The default value isNone
.reverse is a boolean value. If set to
True
, then the list elements are sorted as if each comparison were reversed.In general, the key and reverse conversion processes are much faster than specifying an equivalent cmp function. This is because cmp is called multiple times for each list element while key and reverse touch each element only once. Use
functools.cmp_to_key()
to convert an oldstyle cmp function to a key function.Changed in version 2.3: Support for
None
as an equivalent to omitting cmp was added.Changed in version 2.4: Support for key and reverse was added.
Starting with Python 2.3, the
sort()
method is guaranteed to be stable. A sort is stable if it guarantees not to change the relative order of elements that compare equal — this is helpful for sorting in multiple passes (for example, sort by department, then by salary grade).
CPython implementation detail: While a list is being sorted, the effect of attempting to mutate, or even inspect, the list is undefined. The C implementation of Python 2.3 and newer makes the list appear empty for the duration, and raises
ValueError
if it can detect that the list has been mutated during a sort. The value n is an integer, or an object implementing
__index__()
. Zero and negative values of n clear the sequence. Items in the sequence are not copied; they are referenced multiple times, as explained fors * n
under Sequence Types — str, unicode, list, tuple, bytearray, buffer, xrange.
5.7. Set Types — set
, frozenset
A set object is an unordered collection of distinct hashable objects. Common uses include membership testing, removing duplicates from a sequence, and computing mathematical operations such as intersection, union, difference, and symmetric difference. (For other containers see the built in dict
, list
, and tuple
classes, and the collections
module.)
New in version 2.4.
Like other collections, sets support x in set
, len(set)
, and for x in set
. Being an unordered collection, sets do not record element position or order of insertion. Accordingly, sets do not support indexing, slicing, or other sequencelike behavior.
There are currently two builtin set types, set
and frozenset
. The set
type is mutable — the contents can be changed using methods like add()
and remove()
. Since it is mutable, it has no hash value and cannot be used as either a dictionary key or as an element of another set. The frozenset
type is immutable and hashable — its contents cannot be altered after it is created; it can therefore be used as a dictionary key or as an element of another set.
As of Python 2.7, nonempty sets (not frozensets) can be created by placing a commaseparated list of elements within braces, for example: {'jack', 'sjoerd'}
, in addition to the set
constructor.
The constructors for both classes work the same:
 class
set
( [ iterable ] )  class
frozenset
( [ iterable ] ) 
Return a new set or frozenset object whose elements are taken from iterable. The elements of a set must be hashable. To represent sets of sets, the inner sets must be
frozenset
objects. If iterable is not specified, a new empty set is returned.Instances of
set
andfrozenset
provide the following operations:len(s)

Return the number of elements in set s (cardinality of s).
x in s

Test x for membership in s.
x not in s

Test x for nonmembership in s.
isdisjoint
( other )
Return
True
if the set has no elements in common with other. Sets are disjoint if and only if their intersection is the empty set.New in version 2.6.
set < other

Test whether the set is a proper subset of other, that is,
set <= other and set != other
.
set > other

Test whether the set is a proper superset of other, that is,
set >= other and set != other
.
union
( *others )set  other  ...

Return a new set with elements from the set and all others.
Changed in version 2.6: Accepts multiple input iterables.
intersection
( *others )set & other & ...

Return a new set with elements common to the set and all others.
Changed in version 2.6: Accepts multiple input iterables.
difference
( *others )set  other  ...

Return a new set with elements in the set that are not in the others.
Changed in version 2.6: Accepts multiple input iterables.
symmetric_difference
( other )set ^ other

Return a new set with elements in either the set or other but not both.
Note, the nonoperator versions of
union()
,intersection()
,difference()
, andsymmetric_difference()
,issubset()
, andissuperset()
methods will accept any iterable as an argument. In contrast, their operator based counterparts require their arguments to be sets. This precludes errorprone constructions likeset('abc') & 'cbs'
in favor of the more readableset('abc').intersection('cbs')
.Both
set
andfrozenset
support set to set comparisons. Two sets are equal if and only if every element of each set is contained in the other (each is a subset of the other). A set is less than another set if and only if the first set is a proper subset of the second set (is a subset, but is not equal). A set is greater than another set if and only if the first set is a proper superset of the second set (is a superset, but is not equal).Instances of
set
are compared to instances offrozenset
based on their members. For example,set('abc') == frozenset('abc')
returnsTrue
and so doesset('abc') in set([frozenset('abc')])
.The subset and equality comparisons do not generalize to a total ordering function. For example, any two nonempty disjoint sets are not equal and are not subsets of each other, so all of the following return
False
:a<b
,a==b
, ora>b
. Accordingly, sets do not implement the__cmp__()
method.Since sets only define partial ordering (subset relationships), the output of the
list.sort()
method is undefined for lists of sets.Set elements, like dictionary keys, must be hashable.
Binary operations that mix
set
instances withfrozenset
return the type of the first operand. For example:frozenset('ab')  set('bc')
returns an instance offrozenset
.The following table lists operations available for
set
that do not apply to immutable instances offrozenset
:update
( *others )set = other  ...

Update the set, adding elements from all others.
Changed in version 2.6: Accepts multiple input iterables.
intersection_update
( *others )set &= other & ...

Update the set, keeping only elements found in it and all others.
Changed in version 2.6: Accepts multiple input iterables.
difference_update
( *others )set = other  ...

Update the set, removing elements found in others.
Changed in version 2.6: Accepts multiple input iterables.
symmetric_difference_update
( other )set ^= other

Update the set, keeping only elements found in either set, but not in both.
remove
( elem )
Remove element elem from the set. Raises
KeyError
if elem is not contained in the set.
pop
( )
Remove and return an arbitrary element from the set. Raises
KeyError
if the set is empty.
Note, the nonoperator versions of the
update()
,intersection_update()
,difference_update()
, andsymmetric_differenc