 |  |  | Python Library Reference |  |  |  |
| | | Python Library Reference | | | |
The Python interpreter has a number of functions built into it thatare always available. They are listed here in alphabetical order.
| name[, globals[, locals[, fromlist[, level]]]]) |
For example, the statement "import spam" results in the following call:__import__('spam',globals(),locals(), [], -1); the statement "from spam.ham import eggs" results in "__import__('spam.ham', globals(), locals(), ['eggs'], -1)". Note that even thoughlocals() and['eggs'] are passed in as arguments, the__import__() function does not set the local variable namedeggs; this is done by subsequent code that is generated for the import statement. (In fact, the standard implementation does not use itslocals argument at all, and uses itsglobals only to determine the package context of theimport statement.)
When thename variable is of the formpackage.module, normally, the top-level package (the name up till the first dot) is returned,not the module named byname. However, when a non-emptyfromlist argument is given, the module named byname is returned. This is done for compatibility with the bytecode generated for the different kinds of import statement; when using "import spam.ham.eggs", the top-level packagespam must be placed in the importing namespace, but when using "from spam.ham import eggs", thespam.ham subpackage must be used to find theeggs variable. As a workaround for this behavior, usegetattr() to extract the desired components. For example, you could define the following helper:
def my_import(name): mod = __import__(name) components = name.split('.') for comp in components[1:]: mod = getattr(mod, comp) return modlevel specifies whether to use absolute or relative imports. The default is-1 which indicates both absolute and relative imports will be attempted.0 means only perform absolute imports. Positive values forlevel indicate the number of parent directories to search relative to the directory of the module calling__import__.Changed in version 2.5:The level parameter was added.Changed in version 2.5:Keyword support for parameters was added.
| x) |
| iterable) |
def all(iterable): for element in iterable: if not element: return False return True
| iterable) |
def any(iterable): for element in iterable: if element: return True return False
| ) |
isinstance(obj, basestring) is equivalent toisinstance(obj, (str, unicode)).New in version 2.3.| [x]) |
New in version 2.2.1.Changed in version 2.3:If no argument is given, this function returnsFalse.
| object) |
| i) |
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 ifi is outside that range.| 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: @classmethod def f(cls, arg1, arg2, ...): ...
The@classmethod form is a function decorator - see the description of function definitions in chapter 7 of thePython Reference Manual for details.
It can be called either on the class (such asC.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, seestaticmethod() in this section.
For more information on class methods, consult the documentation on the standard type hierarchy in chapter 3 of thePython Reference Manual (at the bottom).New in version 2.2.Changed in version 2.4:Function decorator syntax added.
| x, y) |
x <y, zero ifx ==y and strictly positive ifx >y.| string, filename, kind[, flags[, dont_inherit]]) |
'<string>' is commonly used). Thekind argument specifies what kind of code must be compiled; it can be'exec' ifstring 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 else thanNone will be printed).When compiling multi-line statements, two caveats apply: line endings must be represented by a single newline character ('\n'), and the input must be terminated by at least one newline character. If line endings are represented by'\r\n', use the stringreplace() method to change them into'\n'.
The optional argumentsflags anddont_inherit (which are new in Python 2.2) control which future statements (seePEP 236) affect the compilation ofstring. 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 theflags argument is given anddont_inherit is not (or is zero) then the future statements specified by theflags argument are used in addition to those that would be used anyway. Ifdont_inherit is a non-zero integer then theflags 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 or-ed together to specify multiple statements. The bitfield required to specify a given feature can be found as thecompiler_flag attribute on the_Feature instance in the__future__ module.
| [real[, imag]]) |
0j.| object, name) |
delattr(x, 'foobar') is equivalent todelx.foobar.| [arg]) |
If keyword arguments are given, the keywords themselves with their associated values are added as items to the dictionary. If a key is specified both in the positional argument and as a keyword argument, the value associated with the keyword is retained in the dictionary. For example, these all return a dictionary equal to{"one": 2, "two": 3}:
dict({'one': 2, 'two': 3})dict({'one': 2, 'two': 3}.items())dict({'one': 2, 'two': 3}.iteritems())dict(zip(('one', 'two'), (2, 3)))dict([['two', 3], ['one', 2]])dict(one=2, two=3)dict([(['one', 'two'][i-2], i) for i in (2, 3)])New in version 2.2.Changed in version 2.3:Support for building a dictionary from keyword arguments added.
| [object]) |
>>> import struct>>> dir()['__builtins__', '__doc__', '__name__', 'struct']>>> dir(struct)['__doc__', '__name__', 'calcsize', 'error', 'pack', 'unpack']
Note:Becausedir() 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.
| a, b) |
(a //b,a %b). For floating point numbers the result is(q,a %b), whereq is usuallymath.floor(a /b) but may be 1 less than that. In any caseq *b +a %b is very close toa, ifa %b is non-zero it has the same sign asb, and0 <= abs(a %b) < abs(b).Changed in version 2.3:Usingdivmod() with complex numbers is deprecated.
| iterable) |
(0, seq[0]),(1, seq[1]),(2, seq[2]), ....New in version 2.3.| expression[, globals[, locals]]) |
Theexpression argument is parsed and evaluated as a Python expression (technically speaking, a condition list) using theglobals andlocals dictionaries as global and local name space. If theglobals dictionary is present and lacks '__builtins__', the current globals are copied intoglobals beforeexpression is parsed. This means thatexpression normally has full access to the standard__builtin__ module and restricted environments are propagated. If thelocals dictionary is omitted it defaults to theglobals 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')2This function can also be used to execute arbitrary code objects (such as those created bycompile()). In this case pass a code object instead of a string. The code object must have been compiled passing'eval' as thekind argument.
Hints: dynamic execution of statements is supported by theexec 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().
| filename[, globals[, locals]]) |
The 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 theglobals andlocals dictionaries as global and local namespace. If provided,locals can be any mapping object.Changed in version 2.4:formerlylocals was required to be a dictionary. If thelocals dictionary is omitted it defaults to theglobals dictionary. If both dictionaries are omitted, the expression is executed in the environment whereexecfile() is called. The return value isNone.
Warning:The defaultlocals act as described for functionlocals() below: modifications to the defaultlocals dictionary should not be attempted. Pass an explicitlocals dictionary if you need to see effects of the code onlocals after functionexecfile() returns.execfile() cannot be used reliably to modify a function's locals.
| filename[, mode[, bufsize]]) |
When opening a file, it's preferable to useopen() instead of invoking this constructor directly.file is more suited to type testing (for example, writing "isinstance(f, file)").
New in version 2.2.
| function, iterable) |
None, the identity function is assumed, that is, all elements ofiterable that are false are removed.Note thatfilter(function,iterable) is equivalent to[item for item initerable if function(item)] if function is notNone and[item for item initerable if item] if function isNone.
| [x]) |
0.0.Note:When passing in a string, values for NaN and Infinity may be returned, depending on the underlying C library. The specific set of strings accepted which cause these values to be returned depends entirely on the C library and is known to vary.
| [iterable]) |
frozenset([]).New in version 2.4.| object, name[, default]) |
getattr(x, 'foobar') is equivalent tox.foobar. If the named attribute does not exist,default is returned if provided, otherwiseAttributeError is raised.| ) |
| object, name) |
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.)| object) |
| [object]) |
| x) |
| object) |
| [prompt]) |
eval(raw_input(prompt)).Warning:This function is not safe from user errors! It expects a valid Python expression as input; if the input is not syntactically valid, aSyntaxError will be raised. Other exceptions may be raised if there is an error during evaluation. (On the other hand, sometimes this is exactly what you need when writing a quick script for expert use.)If thereadline module was loaded, theninput() will use it to provide elaborate line editing and history features.
Consider using theraw_input() function for general input from users.
| [x[, radix]]) |
0.| object, classinfo) |
| class, classinfo) |
| o[, sentinel]) |
0). If it does not support either of those protocols,TypeError is raised. If the second argument,sentinel, is given, theno must be a callable object. The iterator created in this case will callo with no arguments for each call to itsnext() method; if the value returned is equal tosentinel,StopIteration will be raised, otherwise the value will be returned.New in version 2.2.| s) |
| [iterable]) |
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,[].| ) |
| [x[, radix]]) |
0L.| function, iterable, ...) |
None items. Iffunction 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). Theiterable arguments may be a sequence or any iterable object; the result is always a list.| iterable[, args...][key]) |
The optionalkey argument specifies a one-argument ordering function like that used forlist.sort(). Thekey 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 optionalkey argument.
| iterable[, args...][key]) |
The optionalkey argument specifies a one-argument ordering function like that used forlist.sort(). Thekey 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 optionalkey argument.
| ) |
Changed in version 2.3:This function does not accept any arguments. Formerly, it accepted arguments but ignored them.
| x) |
| filename[, mode[, bufsize]]) |
The first two arguments are the same as forstdio'sfopen():filename is the file name to be opened, andmode is a string indicating how the file is to be opened.
The most commonly-used values ofmode are'r' for reading,'w' for writing (truncating the file if it already exists), and'a' for appending (which onsomeUnix systems means thatall writes append to the end of the file regardless of the current seek position). Ifmode is omitted, it defaults to'r'. When opening a binary file, you should append'b' to themode 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 ofmode.
The optionalbufsize 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. A negativebufsize 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.2.3
Modes'r+','w+' and'a+' open the file for updating (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 standardfopen() valuesmode may be'U' or'rU'. Python is usually built with universal newline support; supplying'U' opens the file as a text file, but lines may be terminated by any of the following: theUnix 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 newline support amode 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'.
Changed in version 2.5:Restriction on first letter of mode string introduced.
| c) |
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.| x, y[, z]) |
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. Ifz is present,x andy must be of integer types, andy 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.)
| [fget[, fset[, fdel[, doc]]]]) |
fget is a function for getting an attribute value, likewisefset is a function for setting, andfdel a function for del'ing, an attribute. 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 given,doc will be the docstring of the property attribute. Otherwise, the property will copyfget's docstring (if it exists). This makes it possible to create read-only properties easily usingproperty() as a decorator:
class Parrot(object): def __init__(self): self._voltage = 100000 @property def voltage(self): """Get the current voltage.""" return self._voltage
turns thevoltage() method into a ``getter'' for a read-only attribute with the same name.
New in version 2.2.Changed in version 2.5:Usefget's docstring if nodoc given.
| [start,] stop[, step]) |
1. If thestart argument is omitted, it defaults to0. The full form returns a list of plain integers[start,start +step,start + 2 *step, ...]. Ifstep is positive, the last element is the largeststart +i *step less thanstop; ifstep is negative, the last element is the smalleststart +i *step greater thanstop.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)[]
| [prompt]) |
>>> s = raw_input('--> ')--> Monty Python's Flying Circus>>> s"Monty Python's Flying Circus"If thereadline module was loaded, thenraw_input() will use it to provide elaborate line editing and history features.
| function, iterable[, initializer]) |
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 theiterable. If the optionalinitializer is present, it is placed before the items of the iterable in the calculation, and serves as a default when the iterable is empty. Ifinitializer is not given anditerable contains only one item, the first item is returned.| module) |
Whenreload(module) is executed:
init function of extension modules is not called a second time.There are a number of other caveats:
If a module is syntactically correct but its initialization fails, the firstimport statement for it does not bind its name locally, but does store a (partially initialized) module object insys.modules. To reload the module you must firstimport it again (this will bind the name to the partially initialized module object) before you canreload() it.
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 atry statement it can test for the table's presence and skip its initialization if desired:
try: cacheexcept NameError: cache = {}It is legal though generally not very useful to reload built-in or dynamically loaded modules, except forsys,__main__ and__builtin__. In many cases, however, 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 usingfrom ...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.
| object) |
| seq) |
0).New in version 2.4.| x[, n]) |
round(0.5) is1.0 andround(-0.5) is-1.0).| [iterable]) |
set([]).New in version 2.4.| object, name, value) |
setattr(x, 'foobar', 123) is equivalent tox.foobar = 123.| [start,] stop[, step]) |
range(start,stop,step). Thestart andstep 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]" or "a[start:stop, i]".| iterable[, cmp[, key[, reverse]]]) |
The optional argumentscmp,key, andreverse have the same meaning as those for thelist.sort() method (described in section 3.6.4).
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=lambdax,y:cmp(x.lower(), y.lower())"
key specifies a function of one argument that is used to extract a comparison key from each list element: "key=str.lower"
reverse is a boolean value. If set toTrue, then the list elements are sorted as if each comparison were reversed.
In general, thekey andreverse conversion processes are much faster than specifying an equivalentcmp function. This is becausecmp is called multiple times for each list element whilekey andreverse touch each element only once.
New in version 2.4.
| function) |
A static method does not receive an implicit first argument. To declare a static method, use this idiom:
class C: @staticmethod def f(arg1, arg2, ...): ...
The@staticmethod form is a function decorator - see the description of function definitions in chapter 7 of thePython Reference Manual for details.
It can be called either on the class (such asC.f()) or on an instance (such asC().f()). The instance is ignored except for its class.
Static methods in Python are similar to those found in Java or C++. For a more advanced concept, seeclassmethod() in this section.
For more information on static methods, consult the documentation on the standard type hierarchy in chapter 3 of thePython Reference Manual (at the bottom).New in version 2.2.Changed in version 2.4:Function decorator syntax added.
| [object]) |
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,''.| iterable[, start]) |
0. Theiterable's items are normally numbers, and are not allowed to be strings. The fast, correct way to concatenate a sequence of strings is by calling''.join(sequence). Note thatsum(range(n),m) is equivalent toreduce(operator.add, range(n),m)New in version 2.3.| type[, object-or-type]) |
isinstance(obj,type) must be true. If the second argument is a type,issubclass(type2,type) must be true.super() only works for new-style classes.A typical use for calling a cooperative superclass method is:
class C(B): def meth(self, arg): super(C, self).meth(arg)
Note thatsuper is implemented as part of the binding process for explicit dotted attribute lookups such as "super(C, self).__getitem__(name)". Accordingly,super is undefined for implicit lookups using statements or operators such as "super(C, self)[name]".New in version 2.2.
| [iterable]) |
tuple('abc') returns('a', 'b', 'c') andtuple([1, 2, 3]) returns(1, 2, 3). If no argument is given, returns a new empty tuple,().| object) |
With three arguments,type functions as a constructor as detailed below.
| name, bases, dict) |
>>> class X(object): ... a = 1 ... >>> X = type('X', (object,), dict(a=1))| i) |
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.New in version 2.0.| [object[, encoding[, errors]]]) |
Ifencoding and/orerrors are given,unicode() will decode the object which can either be an 8-bit string or a character buffer using the codec forencoding. Theencoding parameter is a string giving the name of an encoding; if the encoding is not known,LookupError is raised. Error handling is done according toerrors; this specifies the treatment of characters which are invalid in the input encoding. Iferrors 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, ifobject 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.
New in version 2.0.Changed in version 2.2:Support for__unicode__() added.
| [object]) |
| [start,] stop[, step]) |
Note: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.
| [iterable, ...]) |
None. With a single sequence argument, it returns a list of 1-tuples. With no arguments, it returns an empty list.New in version 2.0.Changed in version 2.4:Formerly,zip() required at least one argument andzip() raised aTypeError instead of returning an empty list.
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