The Python interpreter has a number of functions built into it that are always available. They are listed here in alphabetical order.
For example, the statement "import spam" results in the
the statement "from spam.ham import eggs" results
in "__import__('spam.ham', globals(), locals(), ['eggs'])".
Note that even though
['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
When the name variable is of the form
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
must be placed in the importing namespace, but when using "from
spam.ham import eggs", the
spam.ham subpackage must be used to
As a workaround for this behavior, use getattr() to extract
the desired components. For example, you could define the following
import string def my_import(name): mod = __import__(name) components = string.split(name, '.') for comp in components[1:]: mod = getattr(mod, comp) return mod
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.
chr(97)returns the string
'a'. This is the inverse of ord(). The argument must be in the range [0..255], inclusive; ValueError will be raised if i is outside that range.
x < y, zero if
x == yand strictly positive if
x > y.
'<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
delattr(x, 'foobar')is equivalent to
>>> import sys >>> dir() ['sys'] >>> dir(sys) ['argv', 'exit', 'modules', 'path', 'stderr', 'stdin', 'stdout']
(a / b, a % b). For floating point numbers the result is
(q, a % b), where q is usually
math.floor(a / b)but may be 1 less than that. In any case
q * b + a % bis very close to a, if
a % bis non-zero it has the same sign as b, and
0 <= abs(a % b) < abs(b).
>>> 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
'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().
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 namespace. 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, the identity function is assumed, i.e. all elements of list that are false (zero or empty) are removed.
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.
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.
getattr(x, 'foobar')is equivalent to
x.foobar. If the named attribute does not exist, default is returned if provided, otherwise AttributeError is raised.
getattr(object, name)and seeing whether it raises an exception or not.)
'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.
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, a SyntaxError 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 the readline module was loaded, then input() will use it to provide elaborate line editing and history features.
Consider using the raw_input() function for general input from users.
string.atoi(x[, radix]). The radix parameter gives the base for the conversion and may be any integer in the range [2, 36], or zero. If radix is zero, the proper radix is guessed based on the contents of string; the interpretation is the same as for integer literals. If radix is specified and x is not a string, TypeError is raised. 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.2.9
sequence[:]. For instance,
['a', 'b', 'c']and
list( (1, 2, 3) )returns
[1, 2, 3].
string.atol(x). The radix 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 is defined by the C semantics; see the description of int().
Noneitems. 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.
'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.
stdio's fopen(): filename is the file name to be opened, mode indicates how the file is to be opened:
'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).
'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 (else it is
ignored). If the file cannot be opened, IOError is
If mode is omitted, it defaults to
'r'. When opening a
binary file, you should append
'b' to the mode value
for improved portability. (It's useful even on systems which don't
treat binary and text files differently, where it serves as
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. If omitted, the system
default is used.2.10
ord('a')returns the integer
8224. This is the inverse of chr() for strings and of unichr() for Unicode characters.
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, 35000)is not allowed.
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 * stepless than stop; if step is negative, the last element is the largest
start + i * stepgreater 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) 
>>> 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(lambda x, y: x+y, [1, 2, 3, 4, 5])calculates
((((1+2)+3)+4)+5). If the optional initializer is present, it is placed before the items of the sequence in the calculation, and serves as a default when the sequence is empty.
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 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 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.
setattr(x, 'foobar', 123)is equivalent to
x.foobar = 123.
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)does not always attempt to return a string that is acceptable to eval(); its goal is to return a printable string.
('a', 'b', 'c')and
tuple([1, 2, 3])returns
(1, 2, 3).
>>> import types >>> if type(x) == types.StringType: print "It's a string"
unichr(97)returns the string
u'a'. This is the inverse of ord() for Unicode strings. The argument must be in the range [0..65535], inclusive. ValueError is raised otherwise. New in version 2.0.
None. With a single sequence argument, it returns a list of 1-tuples. New in version 2.0.