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2. Built-in Functions
The Python interpreter has a number of functions built into it that are always available. They are listed here in alphabetical order.
Built-in Functions |
||||
---|---|---|---|---|
In addition, there are other four built-in functions that are no longer considered essential: apply()
, buffer()
, coerce()
, and intern()
. They are documented in the Non-essential Built-in Functions section.
abs
( x )-
Return the absolute value of a number. The argument may be a plain or long integer or a floating point number. If the argument is a complex number, its magnitude is returned.
all
( iterable )-
Return
True
if all elements of the iterable are true (or if the iterable is empty). Equivalent to:def all(iterable): for element in iterable: if not element: return False return True
New in version 2.5.
any
( iterable )-
Return
True
if any element of the iterable is true. If the iterable is empty, returnFalse
. Equivalent to:def any(iterable): for element in iterable: if element: return True return False
New in version 2.5.
basestring
( )-
This abstract type is the superclass for
str
andunicode
. It cannot be called or instantiated, but it can be used to test whether an object is an instance ofstr
orunicode
.isinstance(obj, basestring)
is equivalent toisinstance(obj, (str, unicode))
.New in version 2.3.
bin
( x )-
Convert an integer number to a binary string. The result is a valid Python expression. If x is not a Python
int
object, it has to define an__index__()
method that returns an integer.New in version 2.6.
- class
bool
( [ x ] ) -
Return a Boolean value, i.e. one of
True
orFalse
. x is converted using the standard truth testing procedure. If x is false or omitted, this returnsFalse
; otherwise it returnsTrue
.bool
is also a class, which is a subclass ofint
. Classbool
cannot be subclassed further. Its only instances areFalse
andTrue
.New in version 2.2.1.
Changed in version 2.3: If no argument is given, this function returns
False
.
- class
bytearray
( [ source [, encoding [, errors ] ] ] ) -
Return a new array of bytes. The
bytearray
class is a mutable sequence of integers in the range 0 <= x < 256. It has most of the usual methods of mutable sequences, described in Mutable Sequence Types, as well as most methods that thestr
type has, see String Methods.The optional source parameter can be used to initialize the array in a few different ways:
If it is unicode, you must also give the encoding (and optionally, errors) parameters;
bytearray()
then converts the unicode to bytes usingunicode.encode()
.If it is an integer, the array will have that size and will be initialized with null bytes.
If it is an object conforming to the buffer interface, a read-only buffer of the object will be used to initialize the bytes array.
If it is an iterable, it must be an iterable of integers in the range
0 <= x < 256
, which are used as the initial contents of the array.
Without an argument, an array of size 0 is created.
New in version 2.6.
callable
( object )-
Return
True
if the object argument appears callable,False
if not. If this returns true, it is still possible that a call fails, but if it is false, calling object will never succeed. Note that classes are callable (calling a class returns a new instance); class instances are callable if they have a__call__()
method.
chr
( i )-
Return a string of one character whose ASCII code is the integer i. For example,
chr(97)
returns the string'a'
. This is the inverse oford()
. The argument must be in the range [0..255], inclusive;ValueError
will be raised if i is outside that range. See alsounichr()
.
classmethod
( function )-
Return a class method for function.
A class method receives the class as implicit first argument, just like an instance method receives the instance. To declare a class method, use this idiom:
class C(object): @classmethod def f(cls, arg1, arg2, ...): ...
The
@classmethod
form is a function decorator – see Function definitions for details.A class method can be called either on the class (such as
C.f()
) or on an instance (such asC().f()
). The instance is ignored except for its class. If a class method is called for a derived class, the derived class object is passed as the implied first argument.Class methods are different than C++ or Java static methods. If you want those, see
staticmethod()
.For more information on class methods, see The standard type hierarchy.
New in version 2.2.
Changed in version 2.4: Function decorator syntax added.
cmp
( x, y )-
Compare the two objects x and y and return an integer according to the outcome. The return value is negative if
x < y
, zero ifx == y
and strictly positive ifx > y
.
compile
( source, filename, mode [, flags [, dont_inherit ] ] )-
Compile the source into a code or AST object. Code objects can be executed by an
exec
statement or evaluated by a call toeval()
. source can either be a Unicode string, a Latin-1 encoded string or an AST object. Refer to theast
module documentation for information on how to work with AST objects.The filename argument should give the file from which the code was read; pass some recognizable value if it wasn’t read from a file (
'<string>'
is commonly used).The mode argument specifies what kind of code must be compiled; it can be
'exec'
if source consists of a sequence of statements,'eval'
if it consists of a single expression, or'single'
if it consists of a single interactive statement (in the latter case, expression statements that evaluate to something other thanNone
will be printed).The optional arguments flags and dont_inherit control which future statements (see PEP 236 ) affect the compilation of source. If neither is present (or both are zero) the code is compiled with those future statements that are in effect in the code that is calling
compile()
. If the flags argument is given and dont_inherit is not (or is zero) then the future statements specified by the flags argument are used in addition to those that would be used anyway. If dont_inherit is a non-zero integer then the flags argument is it – the future statements in effect around the call to compile are ignored.Future statements are specified by bits which can be bitwise ORed together to specify multiple statements. The bitfield required to specify a given feature can be found as the
compiler_flag
attribute on the_Feature
instance in the__future__
module.This function raises
SyntaxError
if the compiled source is invalid, andTypeError
if the source contains null bytes.If you want to parse Python code into its AST representation, see
ast.parse()
.Note
When compiling a string with multi-line code in
'single'
or'eval'
mode, input must be terminated by at least one newline character. This is to facilitate detection of incomplete and complete statements in thecode
module.Warning
It is possible to crash the Python interpreter with a sufficiently large/complex string when compiling to an AST object due to stack depth limitations in Python’s AST compiler.
Changed in version 2.3: The flags and dont_inherit arguments were added.
Changed in version 2.6: Support for compiling AST objects.
Changed in version 2.7: Allowed use of Windows and Mac newlines. Also input in
'exec'
mode does not have to end in a newline anymore.
- class
complex
( [ real [, imag ] ] ) -
Return a complex number with the value real + imag*1j or convert a string or number to a complex number. If the first parameter is a string, it will be interpreted as a complex number and the function must be called without a second parameter. The second parameter can never be a string. Each argument may be any numeric type (including complex). If imag is omitted, it defaults to zero and the function serves as a numeric conversion function like
int()
,long()
andfloat()
. If both arguments are omitted, returns0j
.Note
When converting from a string, the string must not contain whitespace around the central
+
or-
operator. For example,complex('1+2j')
is fine, butcomplex('1 + 2j')
raisesValueError
.The complex type is described in Numeric Types — int, float, long, complex.
delattr
( object, name )-
This is a relative of
setattr()
. The arguments are an object and a string. The string must be the name of one of the object’s attributes. The function deletes the named attribute, provided the object allows it. For example,delattr(x, 'foobar')
is equivalent todel x.foobar
.
- class
dict
( **kwarg ) - class
dict
( mapping, **kwarg ) - class
dict
( iterable, **kwarg ) -
Create a new dictionary. The
dict
object is the dictionary class. Seedict
and Mapping Types — dict for documentation about this class.For other containers see the built-in
list
,set
, andtuple
classes, as well as thecollections
module.
dir
( [ object ] )-
Without arguments, return the list of names in the current local scope. With an argument, attempt to return a list of valid attributes for that object.
If the object has a method named
__dir__()
, this method will be called and must return the list of attributes. This allows objects that implement a custom__getattr__()
or__getattribute__()
function to customize the waydir()
reports their attributes.If the object does not provide
__dir__()
, the function tries its best to gather information from the object’s__dict__
attribute, if defined, and from its type object. The resulting list is not necessarily complete, and may be inaccurate when the object has a custom__getattr__()
.The default
dir()
mechanism behaves differently with different types of objects, as it attempts to produce the most relevant, rather than complete, information:If the object is a module object, the list contains the names of the module’s attributes.
If the object is a type or class object, the list contains the names of its attributes, and recursively of the attributes of its bases.
Otherwise, the list contains the object’s attributes’ names, the names of its class’s attributes, and recursively of the attributes of its class’s base classes.
The resulting list is sorted alphabetically. For example:
>>> import struct >>> dir() # show the names in the module namespace ['__builtins__', '__doc__', '__name__', 'struct'] >>> dir(struct) # show the names in the struct module ['Struct', '__builtins__', '__doc__', '__file__', '__name__', '__package__', '_clearcache', 'calcsize', 'error', 'pack', 'pack_into', 'unpack', 'unpack_from'] >>> class Shape(object): def __dir__(self): return ['area', 'perimeter', 'location'] >>> s = Shape() >>> dir(s) ['area', 'perimeter', 'location']
Note
Because
dir()
is supplied primarily as a convenience for use at an interactive prompt, it tries to supply an interesting set of names more than it tries to supply a rigorously or consistently defined set of names, and its detailed behavior may change across releases. For example, metaclass attributes are not in the result list when the argument is a class.
divmod
( a, b )-
Take two (non complex) numbers as arguments and return a pair of numbers consisting of their quotient and remainder when using long division. With mixed operand types, the rules for binary arithmetic operators apply. For plain and long integers, the result is the same as
(a // b, a % b)
. For floating point numbers the result is(q, a % b)
, where q is usuallymath.floor(a / b)
but may be 1 less than that. In any caseq * b + a % b
is very close to a, ifa % b
is non-zero it has the same sign as b, and0 <= abs(a % b) < abs(b)
.Changed in version 2.3: Using
divmod()
with complex numbers is deprecated.
enumerate
( sequence, start=0 )-
Return an enumerate object. sequence must be a sequence, an iterator, or some other object which supports iteration. The
next()
method of the iterator returned byenumerate()
returns a tuple containing a count (from start which defaults to 0) and the values obtained from iterating over sequence:>>> seasons = ['Spring', 'Summer', 'Fall', 'Winter'] >>> list(enumerate(seasons)) [(0, 'Spring'), (1, 'Summer'), (2, 'Fall'), (3, 'Winter')] >>> list(enumerate(seasons, start=1)) [(1, 'Spring'), (2, 'Summer'), (3, 'Fall'), (4, 'Winter')]
Equivalent to:
def enumerate(sequence, start=0): n = start for elem in sequence: yield n, elem n += 1
New in version 2.3.
Changed in version 2.6: The start parameter was added.
eval
( expression [, globals [, locals ] ] )-
The arguments are a Unicode or Latin-1 encoded string and optional globals and locals. If provided, globals must be a dictionary. If provided, locals can be any mapping object.
Changed in version 2.4: formerly locals was required to be a dictionary.
The expression argument is parsed and evaluated as a Python expression (technically speaking, a condition list) using the globals and locals dictionaries as global and local namespace. If the globals dictionary is present and lacks ‘__builtins__’, the current globals are copied into globals before expression is parsed. This means that expression normally has full access to the standard
__builtin__
module and restricted environments are propagated. If the locals dictionary is omitted it defaults to the globals dictionary. If both dictionaries are omitted, the expression is executed in the environment whereeval()
is called. The return value is the result of the evaluated expression. Syntax errors are reported as exceptions. Example:>>> x = 1 >>> print eval('x+1') 2
This function can also be used to execute arbitrary code objects (such as those created by
compile()
). In this case pass a code object instead of a string. If the code object has been compiled with'exec'
as the mode argument,eval()
’s return value will beNone
.Hints: dynamic execution of statements is supported by the
exec
statement. Execution of statements from a file is supported by theexecfile()
function. Theglobals()
andlocals()
functions returns the current global and local dictionary, respectively, which may be useful to pass around for use byeval()
orexecfile()
.See
ast.literal_eval()
for a function that can safely evaluate strings with expressions containing only literals.
execfile
( filename [, globals [, locals ] ] )-
This function is similar to the
exec
statement, but parses a file instead of a string. It is different from theimport
statement in that it does not use the module administration — it reads the file unconditionally and does not create a new module. 1The arguments are a file name and two optional dictionaries. The file is parsed and evaluated as a sequence of Python statements (similarly to a module) using the globals and locals dictionaries as global and local namespace. If provided, locals can be any mapping object. Remember that at module level, globals and locals are the same dictionary. If two separate objects are passed as globals and locals, the code will be executed as if it were embedded in a class definition.
Changed in version 2.4: formerly locals was required to be a dictionary.
If the locals dictionary is omitted it defaults to the globals dictionary. If both dictionaries are omitted, the expression is executed in the environment where
execfile()
is called. The return value isNone
.Note
The default locals act as described for function
locals()
below: modifications to the default locals dictionary should not be attempted. Pass an explicit locals dictionary if you need to see effects of the code on locals after functionexecfile()
returns.execfile()
cannot be used reliably to modify a function’s locals.
file
( name [, mode [, buffering ] ] )-
Constructor function for the
file
type, described further in section File Objects. The constructor’s arguments are the same as those of theopen()
built-in function described below.When opening a file, it’s preferable to use
open()
instead of invoking this constructor directly.file
is more suited to type testing (for example, writingisinstance(f, file)
).New in version 2.2.
filter
( function, iterable )-
Construct a list from those elements of iterable for which function returns true. iterable may be either a sequence, a container which supports iteration, or an iterator. If iterable is a string or a tuple, the result also has that type; otherwise it is always a list. If function is
None
, the identity function is assumed, that is, all elements of iterable that are false are removed.Note that
filter(function, iterable)
is equivalent to[item for item in iterable if function(item)]
if function is notNone
and[item for item in iterable if item]
if function isNone
.See
itertools.ifilter()
anditertools.ifilterfalse()
for iterator versions of this function, including a variation that filters for elements where the function returns false.
- class
float
( [ x ] ) -
Return a floating point number constructed from a number or string x.
If the argument is a string, it must contain a possibly signed decimal or floating point number, possibly embedded in whitespace. The argument may also be [+|-]nan or [+|-]inf. Otherwise, the argument may be a plain or long integer or a floating point number, and a floating point number with the same value (within Python’s floating point precision) is returned. If no argument is given, returns
0.0
.Note
When passing in a string, values for NaN and Infinity may be returned, depending on the underlying C library. Float accepts the strings nan, inf and -inf for NaN and positive or negative infinity. The case and a leading + are ignored as well as a leading - is ignored for NaN. Float always represents NaN and infinity as nan, inf or -inf.
The float type is described in Numeric Types — int, float, long, complex.
format
( value [, format_spec ] )-
Convert a value to a “formatted” representation, as controlled by format_spec. The interpretation of format_spec will depend on the type of the value argument, however there is a standard formatting syntax that is used by most built-in types: Format Specification Mini-Language.
Note
format(value, format_spec)
merely callsvalue.__format__(format_spec)
.New in version 2.6.
- class
frozenset
( [ iterable ] ) -
Return a new
frozenset
object, optionally with elements taken from iterable.frozenset
is a built-in class. Seefrozenset
and Set Types — set, frozenset for documentation about this class.For other containers see the built-in
set
,list
,tuple
, anddict
classes, as well as thecollections
module.New in version 2.4.
getattr
( object, name [, default ] )-
Return the value of the named attribute of object. name must be a string. If the string is the name of one of the object’s attributes, the result is the value of that attribute. For example,
getattr(x, 'foobar')
is equivalent tox.foobar
. If the named attribute does not exist, default is returned if provided, otherwiseAttributeError
is raised.
globals
( )-
Return a dictionary representing the current global symbol table. This is always the dictionary of the current module (inside a function or method, this is the module where it is defined, not the module from which it is called).
hasattr
( object, name )-
The arguments are an object and a string. The result is
True
if the string is the name of one of the object’s attributes,False
if not. (This is implemented by callinggetattr(object, name)
and seeing whether it raises an exception or not.)
hash
( object )-
Return the hash value of the object (if it has one). Hash values are integers. They are used to quickly compare dictionary keys during a dictionary lookup. Numeric values that compare equal have the same hash value (even if they are of different types, as is the case for 1 and 1.0).
help
( [ object ] )-
Invoke the built-in help system. (This function is intended for interactive use.) If no argument is given, the interactive help system starts on the interpreter console. If the argument is a string, then the string is looked up as the name of a module, function, class, method, keyword, or documentation topic, and a help page is printed on the console. If the argument is any other kind of object, a help page on the object is generated.
This function is added to the built-in namespace by the
site
module.New in version 2.2.
hex
( x )-
Convert an integer number (of any size) to a lowercase hexadecimal string prefixed with “0x”, for example:
>>> hex(255) '0xff' >>> hex(-42) '-0x2a' >>> hex(1L) '0x1L'
If x is not a Python
int
orlong
object, it has to define a __hex__() method that returns a string.See also
int()
for converting a hexadecimal string to an integer using a base of 16.Note
To obtain a hexadecimal string representation for a float, use the
float.hex()
method.Changed in version 2.4: Formerly only returned an unsigned literal.
id
( object )-
Return the “identity” of an object. This is an integer (or long integer) which is guaranteed to be unique and constant for this object during its lifetime. Two objects with non-overlapping lifetimes may have the same
id()
value.CPython implementation detail: This is the address of the object in memory.
input
( [ prompt ] )-
Equivalent to
eval(raw_input(prompt))
.This function does not catch user errors. If the input is not syntactically valid, a
SyntaxError
will be raised. Other exceptions may be raised if there is an error during evaluation.If the
readline
module was loaded, theninput()
will use it to provide elaborate line editing and history features.Consider using the
raw_input()
function for general input from users.
- class
int
( x=0 ) - class
int
( x, base=10 ) -
Return an integer object constructed from a number or string x, or return
0
if no arguments are given. If x is a number, it can be a plain integer, a long integer, or a floating point number. If x is floating point, the conversion truncates towards zero. If the argument is outside the integer range, the function returns a long object instead.If x is not a number or if base is given, then x must be a string or Unicode object representing an integer literal in radix base. Optionally, the literal can be preceded by
+
or-
(with no space in between) and surrounded by whitespace. A base-n literal consists of the digits 0 to n-1, witha
toz
(orA
toZ
) having values 10 to 35. The default base is 10. The allowed values are 0 and 2–36. Base-2, -8, and -16 literals can be optionally prefixed with0b
/0B
,0o
/0O
/0
, or0x
/0X
, as with integer literals in code. Base 0 means to interpret the string exactly as an integer literal, so that the actual base is 2, 8, 10, or 16.The integer type is described in Numeric Types — int, float, long, complex.
isinstance
( object, classinfo )-
Return true if the object argument is an instance of the classinfo argument, or of a (direct, indirect or virtual) subclass thereof. Also return true if classinfo is a type object (new-style class) and object is an object of that type or of a (direct, indirect or virtual) subclass thereof. If object is not a class instance or an object of the given type, the function always returns false. If classinfo is a tuple of class or type objects (or recursively, other such tuples), return true if object is an instance of any of the classes or types. If classinfo is not a class, type, or tuple of classes, types, and such tuples, a
TypeError
exception is raised.Changed in version 2.2: Support for a tuple of type information was added.
issubclass
( class, classinfo )-
Return true if class is a subclass (direct, indirect or virtual) of classinfo. A class is considered a subclass of itself. classinfo may be a tuple of class objects, in which case every entry in classinfo will be checked. In any other case, a
TypeError
exception is raised.Changed in version 2.3: Support for a tuple of type information was added.
iter
( o [, sentinel ] )-
Return an iterator object. The first argument is interpreted very differently depending on the presence of the second argument. Without a second argument, o must be a collection object which supports the iteration protocol (the
__iter__()
method), or it must support the sequence protocol (the__getitem__()
method with integer arguments starting at0
). If it does not support either of those protocols,TypeError
is raised. If the second argument, sentinel, is given, then o must be a callable object. The iterator created in this case will call o with no arguments for each call to itsnext()
method; if the value returned is equal to sentinel,StopIteration
will be raised, otherwise the value will be returned.One useful application of the second form of
iter()
is to read lines of a file until a certain line is reached. The following example reads a file until thereadline()
method returns an empty string:with open('mydata.txt') as fp: for line in iter(fp.readline, ''): process_line(line)
New in version 2.2.
len
( s )-
Return the length (the number of items) of an object. The argument may be a sequence (such as a string, bytes, tuple, list, or range) or a collection (such as a dictionary, set, or frozen set).
- class
list
( [ iterable ] ) -
Return a list whose items are the same and in the same order as iterable’s items. iterable may be either a sequence, a container that supports iteration, or an iterator object. If iterable is already a list, a copy is made and returned, similar to
iterable[:]
. For instance,list('abc')
returns['a', 'b', 'c']
andlist( (1, 2, 3) )
returns[1, 2, 3]
. If no argument is given, returns a new empty list,[]
.list
is a mutable sequence type, as documented in Sequence Types — str, unicode, list, tuple, bytearray, buffer, xrange. For other containers see the built indict
,set
, andtuple
classes, and thecollections
module.
locals
( )-
Update and return a dictionary representing the current local symbol table. Free variables are returned by
locals()
when it is called in function blocks, but not in class blocks.Note
The contents of this dictionary should not be modified; changes may not affect the values of local and free variables used by the interpreter.
- class
long
( x=0 ) - class
long
( x, base=10 ) -
Return a long integer object constructed from a string or number x. If the argument is a string, it must contain a possibly signed number of arbitrary size, possibly embedded in whitespace. The base argument is interpreted in the same way as for
int()
, and may only be given when x is a string. Otherwise, the argument may be a plain or long integer or a floating point number, and a long integer with the same value is returned. Conversion of floating point numbers to integers truncates (towards zero). If no arguments are given, returns0L
.The long type is described in Numeric Types — int, float, long, complex.
map
( function, iterable, ... )-
Apply function to every item of iterable and return a list of the results. If additional iterable arguments are passed, function must take that many arguments and is applied to the items from all iterables in parallel. If one iterable is shorter than another it is assumed to be extended with
None
items. If function isNone
, the identity function is assumed; if there are multiple arguments,map()
returns a list consisting of tuples containing the corresponding items from all iterables (a kind of transpose operation). The iterable arguments may be a sequence or any iterable object; the result is always a list.
max
( iterable [, key ] )max
( arg1, arg2, *args [, key ] )-
Return the largest item in an iterable or the largest of two or more arguments.
If one positional argument is provided, iterable must be a non-empty iterable (such as a non-empty string, tuple or list). The largest item in the iterable is returned. If two or more positional arguments are provided, the largest of the positional arguments is returned.
The optional key argument specifies a one-argument ordering function like that used for
list.sort()
. The key argument, if supplied, must be in keyword form (for example,max(a,b,c,key=func)
).Changed in version 2.5: Added support for the optional key argument.
memoryview
( obj )-
Return a “memory view” object created from the given argument. See memoryview type for more information.
min
( iterable [, key ] )min
( arg1, arg2, *args [, key ] )-
Return the smallest item in an iterable or the smallest of two or more arguments.
If one positional argument is provided, iterable must be a non-empty iterable (such as a non-empty string, tuple or list). The smallest item in the iterable is returned. If two or more positional arguments are provided, the smallest of the positional arguments is returned.
The optional key argument specifies a one-argument ordering function like that used for
list.sort()
. The key argument, if supplied, must be in keyword form (for example,min(a,b,c,key=func)
).Changed in version 2.5: Added support for the optional key argument.
next
( iterator [, default ] )-
Retrieve the next item from the iterator by calling its
next()
method. If default is given, it is returned if the iterator is exhausted, otherwiseStopIteration
is raised.New in version 2.6.
- class
object
-
Return a new featureless object.
object
is a base for all new style classes. It has the methods that are common to all instances of new style classes.New in version 2.2.
Changed in version 2.3: This function does not accept any arguments. Formerly, it accepted arguments but ignored them.
oct
( x )-
Convert an integer number (of any size) to an octal string. The result is a valid Python expression.
Changed in version 2.4: Formerly only returned an unsigned literal.
open
( name [, mode [, buffering ] ] )-
Open a file, returning an object of the
file
type described in section File Objects. If the file cannot be opened,IOError
is raised. When opening a file, it’s preferable to useopen()
instead of invoking thefile
constructor directly.The first two arguments are the same as for
stdio
’sfopen()
: name is the file name to be opened, and mode is a string indicating how the file is to be opened.The most commonly-used values of mode are
'r'
for reading,'w'
for writing (truncating the file if it already exists), and'a'
for appending (which on some Unix systems means that all writes append to the end of the file regardless of the current seek position). If mode is omitted, it defaults to'r'
. The default is to use text mode, which may convert'\n'
characters to a platform-specific representation on writing and back on reading. Thus, when opening a binary file, you should append'b'
to the mode value to open the file in binary mode, which will improve portability. (Appending'b'
is useful even on systems that don’t treat binary and text files differently, where it serves as documentation.) See below for more possible values of mode.The optional buffering argument specifies the file’s desired buffer size: 0 means unbuffered, 1 means line buffered, any other positive value means use a buffer of (approximately) that size (in bytes). A negative buffering means to use the system default, which is usually line buffered for tty devices and fully buffered for other files. If omitted, the system default is used. 2
Modes
'r+'
,'w+'
and'a+'
open the file for updating (reading and writing); note that'w+'
truncates the file. Append'b'
to the mode to open the file in binary mode, on systems that differentiate between binary and text files; on systems that don’t have this distinction, adding the'b'
has no effect.In addition to the standard
fopen()
values mode may be'U'
or'rU'
. Python is usually built with universal newlines support; supplying'U'
opens the file as a text file, but lines may be terminated by any of the following: the Unix end-of-line convention'\n'
, the Macintosh convention'\r'
, or the Windows convention'\r\n'
. All of these external representations are seen as'\n'
by the Python program. If Python is built without universal newlines support a mode with'U'
is the same as normal text mode. Note that file objects so opened also have an attribute callednewlines
which has a value ofNone
(if no newlines have yet been seen),'\n'
,'\r'
,'\r\n'
, or a tuple containing all the newline types seen.Python enforces that the mode, after stripping
'U'
, begins with'r'
,'w'
or'a'
.Python provides many file handling modules including
fileinput
,os
,os.path
,tempfile
, andshutil
.Changed in version 2.5: Restriction on first letter of mode string introduced.
ord
( c )-
Given a string of length one, return an integer representing the Unicode code point of the character when the argument is a unicode object, or the value of the byte when the argument is an 8-bit string. For example,
ord('a')
returns the integer97
,ord(u'\u2020')
returns8224
. This is the inverse ofchr()
for 8-bit strings and ofunichr()
for unicode objects. If a unicode argument is given and Python was built with UCS2 Unicode, then the character’s code point must be in the range [0..65535] inclusive; otherwise the string length is two, and aTypeError
will be raised.
pow
( x, y [, z ] )-
Return x to the power y; if z is present, return x to the power y, modulo z (computed more efficiently than
pow(x, y) % z
). The two-argument formpow(x, y)
is equivalent to using the power operator:x**y
.The arguments must have numeric types. With mixed operand types, the coercion rules for binary arithmetic operators apply. For int and long int operands, the result has the same type as the operands (after coercion) unless the second argument is negative; in that case, all arguments are converted to float and a float result is delivered. For example,
10**2
returns100
, but10**-2
returns0.01
. (This last feature was added in Python 2.2. In Python 2.1 and before, if both arguments were of integer types and the second argument was negative, an exception was raised.) If the second argument is negative, the third argument must be omitted. If z is present, x and y must be of integer types, and y must be non-negative. (This restriction was added in Python 2.2. In Python 2.1 and before, floating 3-argumentpow()
returned platform-dependent results depending on floating-point rounding accidents.)
print
( *objects, sep=' ', end='\n', file=sys.stdout )-
Print objects to the stream file, separated by sep and followed by end. sep, end and file, if present, must be given as keyword arguments.
All non-keyword arguments are converted to strings like
str()
does and written to the stream, separated by sep and followed by end. Both sep and end must be strings; they can also beNone
, which means to use the default values. If no objects are given,print()
will just write end.The file argument must be an object with a
write(string)
method; if it is not present orNone
,sys.stdout
will be used. Output buffering is determined by file. Usefile.flush()
to ensure, for instance, immediate appearance on a screen.Note
This function is not normally available as a built-in since the name
print
is recognized as theprint
statement. To disable the statement and use theprint()
function, use this future statement at the top of your module:from __future__ import print_function
New in version 2.6.
- class
property
( [ fget [, fset [, fdel [, doc ] ] ] ] ) -
Return a property attribute for new-style classes (classes that derive from
object
).fget is a function for getting an attribute value. fset is a function for setting an attribute value. fdel is a function for deleting an attribute value. And doc creates a docstring for the attribute.
A typical use is to define a managed attribute
x
:class C(object): def __init__(self): self._x = None def getx(self): return self._x def setx(self, value): self._x = value def delx(self): del self._x x = property(getx, setx, delx, "I'm the 'x' property.")
If c is an instance of C,
c.x
will invoke the getter,c.x = value
will invoke the setter anddel c.x
the deleter.If given, doc will be the docstring of the property attribute. Otherwise, the property will copy fget’s docstring (if it exists). This makes it possible to create read-only properties easily using
property()
as a decorator:class Parrot(object): def __init__(self): self._voltage = 100000 @property def voltage(self): """Get the current voltage.""" return self._voltage
The
@property
decorator turns thevoltage()
method into a “getter” for a read-only attribute with the same name, and it sets the docstring for voltage to “Get the current voltage.”A property object has
getter
,setter
, anddeleter
methods usable as decorators that create a copy of the property with the corresponding accessor function set to the decorated function. This is best explained with an example:class C(object): def __init__(self): self._x = None @property def x(self): """I'm the 'x' property.""" return self._x @x.setter def x(self, value): self._x = value @x.deleter def x(self): del self._x
This code is exactly equivalent to the first example. Be sure to give the additional functions the same name as the original property (
x
in this case.)The returned property object also has the attributes
fget
,fset
, andfdel
corresponding to the constructor arguments.New in version 2.2.
Changed in version 2.5: Use fget’s docstring if no doc given.
Changed in version 2.6: The
getter
,setter
, anddeleter
attributes were added.
range
( stop )range
( start, stop [, step ] )-
This is a versatile function to create lists containing arithmetic progressions. It is most often used in
for
loops. The arguments must be plain integers. If the step argument is omitted, it defaults to1
. If the start argument is omitted, it defaults to0
. The full form returns a list of plain integers[start, start + step, start + 2 * step, ...]
. If step is positive, the last element is the largeststart + i * step
less than stop; if step is negative, the last element is the smalleststart + i * step
greater than stop. step must not be zero (or elseValueError
is raised). Example:>>> range(10) [0, 1, 2, 3, 4, 5, 6, 7, 8, 9] >>> range(1, 11) [1, 2, 3, 4, 5, 6, 7, 8, 9, 10] >>> range(0, 30, 5) [0, 5, 10, 15, 20, 25] >>> range(0, 10, 3) [0, 3, 6, 9] >>> range(0, -10, -1) [0, -1, -2, -3, -4, -5, -6, -7, -8, -9] >>> range(0) [] >>> range(1, 0) []
raw_input
( [ prompt ] )-
If the prompt argument is present, it is written to standard output without a trailing newline. The function then reads a line from input, converts it to a string (stripping a trailing newline), and returns that. When EOF is read,
EOFError
is raised. Example:>>> s = raw_input('--> ') --> Monty Python's Flying Circus >>> s "Monty Python's Flying Circus"
If the
readline
module was loaded, thenraw_input()
will use it to provide elaborate line editing and history features.
reduce
( function, iterable [, initializer ] )-
Apply function of two arguments cumulatively to the items of iterable, from left to right, so as to reduce the iterable to a single value. For example,
reduce(lambda x, y: x+y, [1, 2, 3, 4, 5])
calculates((((1+2)+3)+4)+5)
. The left argument, x, is the accumulated value and the right argument, y, is the update value from the iterable. If the optional initializer is present, it is placed before the items of the iterable in the calculation, and serves as a default when the iterable is empty. If initializer is not given and iterable contains only one item, the first item is returned. Roughly equivalent to:def reduce(function, iterable, initializer=None): it = iter(iterable) if initializer is None: try: initializer = next(it) except StopIteration: raise TypeError('reduce() of empty sequence with no initial value') accum_value = initializer for x in it: accum_value = function(accum_value, x) return accum_value
reload
( module )-
Reload a previously imported module. The argument must be a module object, so it must have been successfully imported before. This is useful if you have edited the module source file using an external editor and want to try out the new version without leaving the Python interpreter. The return value is the module object (the same as the module argument).
When
reload(module)
is executed:Python modules’ code is recompiled and the module-level code reexecuted, defining a new set of objects which are bound to names in the module’s dictionary. The
init
function of extension modules is not called a second time.As with all other objects in Python the old objects are only reclaimed after their reference counts drop to zero.
The names in the module namespace are updated to point to any new or changed objects.
Other references to the old objects (such as names external to the module) are not rebound to refer to the new objects and must be updated in each namespace where they occur if that is desired.
There are a number of other caveats:
When a module is reloaded, its dictionary (containing the module’s global variables) is retained. Redefinitions of names will override the old definitions, so this is generally not a problem. If the new version of a module does not define a name that was defined by the old version, the old definition remains. This feature can be used to the module’s advantage if it maintains a global table or cache of objects — with a
try
statement it can test for the table’s presence and skip its initialization if desired:try: cache except NameError: cache = {}
It is generally not very useful to reload built-in or dynamically loaded modules. Reloading
sys
,__main__
,builtins
and other key modules is not recommended. In many cases extension modules are not designed to be initialized more than once, and may fail in arbitrary ways when reloaded.If a module imports objects from another module using
from
…import
…, callingreload()
for the other module does not redefine the objects imported from it — one way around this is to re-execute thefrom
statement, another is to useimport
and qualified names (module.*name*) instead.If a module instantiates instances of a class, reloading the module that defines the class does not affect the method definitions of the instances — they continue to use the old class definition. The same is true for derived classes.
repr
( object )-
Return a string containing a printable representation of an object. This is the same value yielded by conversions (reverse quotes). It is sometimes useful to be able to access this operation as an ordinary function. For many types, this function makes an attempt to return a string that would yield an object with the same value when passed to
eval()
, otherwise the representation is a string enclosed in angle brackets that contains the name of the type of the object together with additional information often including the name and address of the object. A class can control what this function returns for its instances by defining a__repr__()
method.
reversed
( seq )-
Return a reverse iterator. seq must be an object which has a
__reversed__()
method or supports the sequence protocol (the__len__()
method and the__getitem__()
method with integer arguments starting at0
).New in version 2.4.
Changed in version 2.6: Added the possibility to write a custom
__reversed__()
method.
round
( number [, ndigits ] )-
Return the floating point value number rounded to ndigits digits after the decimal point. If ndigits is omitted, it defaults to zero. The result is a floating point number. Values are rounded to the closest multiple of 10 to the power minus ndigits; if two multiples are equally close, rounding is done away from 0 (so, for example,
round(0.5)
is1.0
andround(-0.5)
is-1.0
).Note
The behavior of
round()
for floats can be surprising: for example,round(2.675, 2)
gives2.67
instead of the expected2.68
. This is not a bug: it’s a result of the fact that most decimal fractions can’t be represented exactly as a float. See Floating Point Arithmetic: Issues and Limitations for more information.
- class
set
( [ iterable ] ) -
Return a new
set
object, optionally with elements taken from iterable.set
is a built-in class. Seeset
and Set Types — set, frozenset for documentation about this class.For other containers see the built-in
frozenset
,list
,tuple
, anddict
classes, as well as thecollections
module.New in version 2.4.
setattr
( object, name, value )-
This is the counterpart of
getattr()
. The arguments are an object, a string and an arbitrary value. The string may name an existing attribute or a new attribute. The function assigns the value to the attribute, provided the object allows it. For example,setattr(x, 'foobar', 123)
is equivalent tox.foobar = 123
.
- class
slice
( stop ) - class
slice
( start, stop [, step ] ) -
Return a slice object representing the set of indices specified by
range(start, stop, step)
. The start and step arguments default toNone
. Slice objects have read-only data attributesstart
,stop
andstep
which merely return the argument values (or their default). They have no other explicit functionality; however they are used by Numerical Python and other third party extensions. Slice objects are also generated when extended indexing syntax is used. For example:a[start:stop:step]
ora[start:stop, i]
. Seeitertools.islice()
for an alternate version that returns an iterator.
sorted
( iterable [, cmp [, key [, reverse ] ] ] )-
Return a new sorted list from the items in iterable.
The optional arguments cmp, key, and reverse have the same meaning as those for the
list.sort()
method (described in section Mutable Sequence Types).cmp specifies a custom comparison function of two arguments (iterable elements) 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
(compare the elements directly).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 old-style cmp function to a key function.The built-in
sorted()
function 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).For sorting examples and a brief sorting tutorial, see Sorting HOW TO.
New in version 2.4.
staticmethod
( function )-
Return a static method for function.
A static method does not receive an implicit first argument. To declare a static method, use this idiom:
class C(object): @staticmethod def f(arg1, arg2, ...): ...
The
@staticmethod
form is a function decorator – see Function definitions for details.A static method can be called either on the class (such as
C.f()
) or on an instance (such asC().f()
).Static methods in Python are similar to those found in Java or C++. Also see
classmethod()
for a variant that is useful for creating alternate class constructors.For more information on static methods, see The standard type hierarchy.
New in version 2.2.
Changed in version 2.4: Function decorator syntax added.
- class
str
( object='' ) -
Return a string containing a nicely printable representation of an object. For strings, this returns the string itself. The difference with
repr(object)
is thatstr(object)
does not always attempt to return a string that is acceptable toeval()
; its goal is to return a printable string. If no argument is given, returns the empty string,''
.For more information on strings see Sequence Types — str, unicode, list, tuple, bytearray, buffer, xrange which describes sequence functionality (strings are sequences), 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. In addition see the String Services section. See alsounicode()
.
sum
( iterable [, start ] )-
Sums start and the items of an iterable from left to right and returns the total. start defaults to
0
. The iterable’s items are normally numbers, and the start value is not allowed to be a string.For some use cases, there are good alternatives to
sum()
. The preferred, fast way to concatenate a sequence of strings is by calling''.join(sequence)
. To add floating point values with extended precision, seemath.fsum()
. To concatenate a series of iterables, consider usingitertools.chain()
.New in version 2.3.
super
( type [, object-or-type ] )-
Return a proxy object that delegates method calls to a parent or sibling class of type. This is useful for accessing inherited methods that have been overridden in a class. The search order is same as that used by
getattr()
except that the type itself is skipped.The
__mro__
attribute of the type lists the method resolution search order used by bothgetattr()
andsuper()
. The attribute is dynamic and can change whenever the inheritance hierarchy is updated.If the second argument is omitted, the super object returned is unbound. If the second argument is an object,
isinstance(obj, type)
must be true. If the second argument is a type,issubclass(type2, type)
must be true (this is useful for classmethods).Note
super()
only works for new-style classes.There are two typical use cases for super. In a class hierarchy with single inheritance, super can be used to refer to parent classes without naming them explicitly, thus making the code more maintainable. This use closely parallels the use of super in other programming languages.
The second use case is to support cooperative multiple inheritance in a dynamic execution environment. This use case is unique to Python and is not found in statically compiled languages or languages that only support single inheritance. This makes it possible to implement “diamond diagrams” where multiple base classes implement the same method. Good design dictates that this method have the same calling signature in every case (because the order of calls is determined at runtime, because that order adapts to changes in the class hierarchy, and because that order can include sibling classes that are unknown prior to runtime).
For both use cases, a typical superclass call looks like this:
class C(B): def method(self, arg): super(C, self).method(arg)
Note that
super()
is implemented as part of the binding process for explicit dotted attribute lookups such assuper().__getitem__(name)
. It does so by implementing its own__getattribute__()
method for searching classes in a predictable order that supports cooperative multiple inheritance. Accordingly,super()
is undefined for implicit lookups using statements or operators such assuper()[name]
.Also note that
super()
is not limited to use inside methods. The two argument form specifies the arguments exactly and makes the appropriate references.For practical suggestions on how to design cooperative classes using
super()
, see guide to using super() .New in version 2.2.
tuple
( [ iterable ] )-
Return a tuple whose items are the same and in the same order as iterable’s items. iterable may be a sequence, a container that supports iteration, or an iterator object. If iterable is already a tuple, it is returned unchanged. For instance,
tuple('abc')
returns('a', 'b', 'c')
andtuple([1, 2, 3])
returns(1, 2, 3)
. If no argument is given, returns a new empty tuple,()
.tuple
is an immutable sequence type, as documented in Sequence Types — str, unicode, list, tuple, bytearray, buffer, xrange. For other containers see the built indict
,list
, andset
classes, and thecollections
module.
- class
type
( object ) - class
type
( name, bases, dict ) -
With one argument, return the type of an object. The return value is a type object. The
isinstance()
built-in function is recommended for testing the type of an object.With three arguments, return a new type object. This is essentially a dynamic form of the
class
statement. The name string is the class name and becomes the__name__
attribute; the bases tuple itemizes the base classes and becomes the__bases__
attribute; and the dict dictionary is the namespace containing definitions for class body and becomes the__dict__
attribute. For example, the following two statements create identicaltype
objects:>>> class X(object): ... a = 1 ... >>> X = type('X', (object,), dict(a=1))
New in version 2.2.
unichr
( i )-
Return the Unicode string of one character whose Unicode code is the integer i. For example,
unichr(97)
returns the stringu'a'
. This is the inverse oford()
for Unicode strings. The valid range for the argument depends how Python was configured – it may be either UCS2 [0..0xFFFF] or UCS4 [0..0x10FFFF].ValueError
is raised otherwise. For ASCII and 8-bit strings seechr()
.New in version 2.0.
unicode
( object='' )unicode
( object [, encoding [, errors ] ] )-
Return the Unicode string version of object using one of the following modes:
If encoding and/or errors are given,
unicode()
will decode the object which can either be an 8-bit string or a character buffer using the codec for encoding. The encoding parameter is a string giving the name of an encoding; if the encoding is not known,LookupError
is raised. Error handling is done according to errors; this specifies the treatment of characters which are invalid in the input encoding. If errors is'strict'
(the default), aValueError
is raised on errors, while a value of'ignore'
causes errors to be silently ignored, and a value of'replace'
causes the official Unicode replacement character,U+FFFD
, to be used to replace input characters which cannot be decoded. See also thecodecs
module.If no optional parameters are given,
unicode()
will mimic the behaviour ofstr()
except that it returns Unicode strings instead of 8-bit strings. More precisely, if object is a Unicode string or subclass it will return that Unicode string without any additional decoding applied.For objects which provide a
__unicode__()
method, it will call this method without arguments to create a Unicode string. For all other objects, the 8-bit string version or representation is requested and then converted to a Unicode string using the codec for the default encoding in'strict'
mode.For more information on Unicode strings see Sequence Types — str, unicode, list, tuple, bytearray, buffer, xrange which describes sequence functionality (Unicode strings are sequences), 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. In addition see the String Services section. See alsostr()
.New in version 2.0.
Changed in version 2.2: Support for
__unicode__()
added.
vars
( [ object ] )-
Return the
__dict__
attribute for a module, class, instance, or any other object with a__dict__
attribute.Objects such as modules and instances have an updateable
__dict__
attribute; however, other objects may have write restrictions on their__dict__
attributes (for example, new-style classes use a dictproxy to prevent direct dictionary updates).Without an argument,
vars()
acts likelocals()
. Note, the locals dictionary is only useful for reads since updates to the locals dictionary are ignored.
xrange
( stop )xrange
( start, stop [, step ] )-
This function is very similar to
range()
, but returns an xrange object instead of a list. This is an opaque sequence type which yields the same values as the corresponding list, without actually storing them all simultaneously. The advantage ofxrange()
overrange()
is minimal (sincexrange()
still has to create the values when asked for them) except when a very large range is used on a memory-starved machine or when all of the range’s elements are never used (such as when the loop is usually terminated withbreak
). For more information on xrange objects, see XRange Type and Sequence Types — str, unicode, list, tuple, bytearray, buffer, xrange.CPython implementation detail:
xrange()
is intended to be simple and fast. Implementations may impose restrictions to achieve this. The C implementation of Python restricts all arguments to native C longs (“short” Python integers), and also requires that the number of elements fit in a native C long. If a larger range is needed, an alternate version can be crafted using theitertools
module:islice(count(start, step), (stop-start+step-1+2*(step<0))//step)
.
zip
( [ iterable, ... ] )-
This function returns a list of tuples, where the i-th tuple contains the i-th element from each of the argument sequences or iterables. The returned list is truncated in length to the length of the shortest argument sequence. When there are multiple arguments which are all of the same length,
zip()
is similar tomap()
with an initial argument ofNone
. With a single sequence argument, it returns a list of 1-tuples. With no arguments, it returns an empty list.The left-to-right evaluation order of the iterables is guaranteed. This makes possible an idiom for