2.3 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.

__import__ (name[, globals[, locals[, fromlist]]])

This function is invoked by the import statement. It mainly exists so that you can replace it with another function that has a compatible interface, in order to change the semantics of the import statement. For examples of why and how you would do this, see the standard library modules ihooks and rexec. See also the built-in module imp, which defines some useful operations out of which you can build your own __import__() function.     

For example, the statement `import spam' results in the following call: __import__('spam', globals(), locals(), []); the statement from spam.ham import eggs results in __import__('spam.ham', globals(), locals(), ['eggs']). Note that even though locals() and ['eggs'] are passed in as arguments, the __import__() function does not set the local variable named eggs; this is done by subsequent code that is generated for the import statement. (In fact, the standard implementation does not use its locals argument at all, and uses its globals only to determine the package context of the import statement.)

When the name variable is of the form package.module, normally, the top-level package (the name up till the first dot) is returned, not the module named by name. However, when a non-empty fromlist argument is given, the module named by name 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 package spam must be placed in the importing namespace, but when using "from spam.ham import eggs", the spam.ham subpackage must be used to find the eggs variable.

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.

apply (function, args[, keywords])

The function argument must be a callable object (a user-defined or built-in function or method, or a class object) and the args argument must be a tuple. The function is called with args as argument list; the number of arguments is the the length of the tuple. (This is different from just calling func(args), since in that case there is always exactly one argument.) If the optional keywords argument is present, it must be a dictionary whose keys are strings. It specifies keyword arguments to be added to the end of the the argument list.

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, e.g., chr(97) returns the string 'a'. This is the inverse of ord(). The argument must be in the range [0..255], inclusive.

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 if x == y and strictly positive if x > y.

coerce (x, y)

Return a tuple consisting of the two numeric arguments converted to a common type, using the same rules as used by arithmetic operations.

compile (string, filename, kind)

Compile the string into a code object. Code objects can be executed by an exec statement or evaluated by a call to eval(). The filename argument should give the file from which the code was read; pass e.g. '<string>' if it wasn't read from a file. The kind argument specifies what kind of code must be compiled; it can be 'exec' if string 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 than None will printed).

complex (real[, imag])

Create a complex number with the value real + imag*j. 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() and float().

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 to del x.foobar.

dir ()

Without arguments, return the list of names in the current local symbol table. With an argument, attempts to return a list of valid attribute for that object. This information is gleaned from the object's __dict__, __methods__ and __members__ attributes, if defined. The list is not necessarily complete; e.g., for classes, attributes defined in base classes are not included, and for class instances, methods are not included. The resulting list is sorted alphabetically. For example:

>>> import sys
>>> dir()
['sys']
>>> dir(sys)
['argv', 'exit', 'modules', 'path', 'stderr', 'stdin', 'stdout']
>>>

divmod (a, b)

Take two 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 the same as (math.floor(a / b), a % b).

eval (expression[, globals[, locals]])

The arguments are a string and two optional dictionaries. 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 name space. 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 eval 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 (e.g. created by compile()). In this case pass a code object instead of a string. The code object must have been compiled passing 'eval' to the kind argument.

Hints: dynamic execution of statements is supported by the exec statement. Execution of statements from a file is supported by the execfile() function. The globals() and locals() functions returns the current global and local dictionary, respectively, which may be useful to pass around for use by eval() or execfile().

execfile (file[, globals[, locals]])

This function is similar to the exec statement, but parses a file instead of a string. It is different from the import statement in that it does not use the module administration -- it reads the file unconditionally and does not create a new module.^2.7

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 the globals and locals dictionaries as global and local name space. 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 is None.

filter (function, list)

Construct a list from those elements of list for which function returns true. If list 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, i.e. all elements of list that are false (zero or empty) are removed.

float (x)

Convert a string or a number to floating point. If the argument is a string, it must contain a possibly singed decimal or floating point number, possibly embedded in whitespace; this behaves identical to string.atof(x). 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.

getattr (object, name)

The arguments are an object and a string. The string must be the name of one of the object's attributes. The result is the value of that attribute. For example, getattr(x, 'foobar') is equivalent to x.foobar.

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 1 if the string is the name of one of the object's attributes, 0 if not. (This is implemented by calling getattr(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, e.g. 1 and 1.0).

hex (x)

Convert an integer number (of any size) to a hexadecimal string. The result is a valid Python expression. Note: this always yields an unsigned literal, e.g. on a 32-bit machine, hex(-1) yields '0xffffffff'. When evaluated on a machine with the same word size, this literal is evaluated as -1; at a different word size, it may turn up as a large positive number or raise an OverflowError exception.

id (object)

Return the `identity' of an object. This is an integer which is guaranteed to be unique and constant for this object during its lifetime. (Two objects whose lifetimes are disjunct may have the same id() value.) (Implementation note: this is the address of the object.)

input ([prompt])

Almost equivalent to eval(raw_input(prompt)). Like raw_input(), the prompt argument is optional, and the readline module is used when loaded. The difference is that a long input expression may be broken over multiple lines using the backslash convention.

intern (string)

Enter string in the table of ``interned'' strings and return the interned string - which is string itself or a copy. Interning strings is useful to gain a little performance on dictionary lookup - if the keys in a dictionary are interned, and the lookup key is interned, the key comparisons (after hashing) can be done by a pointer compare instead of a string compare. Normally, the names used in Python programs are automatically interned, and the dictionaries used to hold module, class or instance attributes have interned keys. Interned strings are immortal (i.e. never get garbage collected).

int (x)

Convert a string or number to a plain integer. If the argument is a string, it must contain a possibly singed decimal number representable as a Python integer, possibly embedded in whitespace; this behaves identical to string.atoi(x). Otherwise, the argument may be a plain or long integer or a floating point number. Conversion of floating point numbers to integers is defined by the C semantics; normally the conversion truncates towards zero.^2.8

isinstance (object, class)

Return true if the object argument is an instance of the class argument, or of a (direct or indirect) subclass thereof. Also return true if class is a type object and object is an object of that type. If object is not a class instance or a object of the given type, the function always returns false. If class is neither a class object nor a type object, a TypeError exception is raised.

issubclass (class1, class2)

Return true if class1 is a subclass (direct or indirect) of class2. A class is considered a subclass of itself. If either argument is not a class object, a TypeError exception is raised.

len (s)

Return the length (the number of items) of an object. The argument may be a sequence (string, tuple or list) or a mapping (dictionary).

list (sequence)

Return a list whose items are the same and in the same order as sequence's items. If sequence is already a list, a copy is made and returned, similar to sequence[:]. For instance, list('abc') returns returns ['a', 'b', 'c'] and list( (1, 2, 3) ) returns [1, 2, 3].

locals ()

Return a dictionary representing the current local symbol table. Inside a function, modifying this dictionary does not always have the desired effect.

long (x)

Convert a string or number to a long integer. If the argument is a string, it must contain a possibly singed decimal number of arbitrary size, possibly embedded in whitespace; this behaves identical to string.atol(x). 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 is defined by the C semantics; see the description of int().

map (function, list, ...)

Apply function to every item of list and return a list of the results. If additional list arguments are passed, function must take that many arguments and is applied to the items of all lists in parallel; if a list is shorter than another it is assumed to be extended with None items. If function is None, the identity function is assumed; if there are multiple list arguments, map() returns a list consisting of tuples containing the corresponding items from all lists (i.e. a kind of transpose operation). The list arguments may be any kind of sequence; the result is always a list.

max (s)

Return the largest item of a non-empty sequence (string, tuple or list).

min (s)

Return the smallest item of a non-empty sequence (string, tuple or list).

oct (x)

Convert an integer number (of any size) to an octal string. The result is a valid Python expression. Note: this always yields an unsigned literal, e.g. on a 32-bit machine, oct(-1) yields '037777777777'. When evaluated on a machine with the same word size, this literal is evaluated as -1; at a different word size, it may turn up as a large positive number or raise an OverflowError exception.

open (filename[, mode[, bufsize]])

Return a new file object (described earlier under Built-in Types). The first two arguments are the same as for stdio's fopen(): filename is the file name to be opened, mode indicates how the file is to be opened: 'r' for reading, 'w' for writing (truncating an existing file), and 'a' opens it for appending (which on some Unix systems means that all writes append to the end of the file, regardless of the current seek position). Modes 'r+', 'w+' and 'a+' open the file for updating, provided the underlying stdio library understands this. On systems that differentiate between binary and text files, 'b' appended to the mode opens the file in binary mode. If the file cannot be opened, IOError is raised. If mode is omitted, it defaults to 'r'. The optional bufsize 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 negative bufsize means to use the system default, which is usually line buffered for for tty devices and fully buffered for other files. ^2.9

ord (c)

Return the ASCII value of a string of one character. E.g., ord('a') returns the integer 97. This is the inverse of chr().

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 arguments must have numeric types. With mixed operand types, the rules for binary arithmetic operators apply. The effective operand type is also the type of the result; if the result is not expressible in this type, the function raises an exception; e.g., pow(2, -1) or pow(2, 35000) is not allowed.

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 to 1. If the start argument is omitted, it defaults to 0. The full form returns a list of plain integers [start, start + step, start + 2 * step, ...]. If step is positive, the last element is the largest start + i * step less than stop; if step is negative, the last element is the largest start + i * step greater than stop. step must not be zero (or else ValueError 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, then raw_input() will use it to provide elaborate line editing and history features.

reduce (function, list[, initializer])

Apply the binary function to the items of list so as to reduce the list to a single value. E.g., reduce(lambda x, y: x*y, list, 1) returns the product of the elements of list. The optional initializer can be thought of as being prepended to list so as to allow reduction of an empty list. The list arguments may be any kind of sequence.

reload (module)

Re-parse and re-initialize an already 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 (i.e. the same as the module argument).

There are a number of caveats:

If a module is syntactically correct but its initialization fails, the first import statement for it does not bind its name locally, but does store a (partially initialized) module object in sys.modules. To reload the module you must first import it again (this will bind the name to the partially initialized module object) before you can reload() 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 a try statement it can test for the table's presence and skip its initialization if desired.

It is legal though generally not very useful to reload built-in or dynamically loaded modules, except for sys, __main__ and __builtin__. In certain 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 using from ... import ..., calling reload() for the other module does not redefine the objects imported from it -- one way around this is to re-execute the from statement, another is to use import 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().

round (x, n)

Return the floating point value x rounded to n digits after the decimal point. If n 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 n; if two multiples are equally close, rounding is done away from 0 (so e.g. round(0.5) is 1.0 and round(-0.5) is -1.0).

setattr (object, name, value)

This is the counterpart of getattr(). The arguments are an object, a string and an arbitrary value. The string must be the name of one of the object's attributes. The function assigns the value to the attribute, provided the object allows it. For example, setattr(x, 'foobar', 123) is equivalent to x.foobar = 123.

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 to None. Slice objects have read-only data attributes start, stop and step 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, e.g. for "a[start:stop:step]" or "a[start:stop, i]".

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 that str(object) does not always attempt to return a string that is acceptable to eval(); its goal is to return a printable string.

tuple (sequence)

Return a tuple whose items are the same and in the same order as sequence's items. If sequence is already a tuple, it is returned unchanged. For instance, tuple('abc') returns returns ('a', 'b', 'c') and tuple([1, 2, 3]) returns (1, 2, 3).

type (object)

Return the type of an object. The return value is a type object. The standard module types defines names for all built-in types.   For instance:

>>> import types
>>> if isinstance(x, types.StringType): print "It's a string"

vars ([object])

Without arguments, return a dictionary corresponding to the current local symbol table. With a module, class or class instance object as argument (or anything else that has a __dict__ attribute), returns a dictionary corresponding to the object's symbol table. The returned dictionary should not be modified: the effects on the corresponding symbol table are undefined. ^2.10

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 of xrange() over range() is minimal (since xrange() still has to create the values when asked for them) except when a very large range is used on a memory-starved machine (e.g. MS-DOS) or when all of the range's elements are never used (e.g. when the loop is usually terminated with break).