The Python interpreter has a number of functions built into it that are always available. They are listed here in alphabetical order.
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 ni
,
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.
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.
__call__
.
chr(97)
returns the string 'a'
. This is the
inverse of ord()
. The argument must be in the range [0..255],
inclusive.
x
< y
, zero if x == y
and strictly positive if
x > y
.
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).
int
, long
and float
.
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
.
Without arguments, return the list of names in the current local
symbol table. With a module, class or class instance object as
argument (or anything else that has a __dict__
attribute),
returns the list of names in that object's attribute dictionary.
The resulting list is sorted. For example:
>>> import sys >>> dir() ['sys'] >>> dir(sys) ['argv', 'exit', 'modules', 'path', 'stderr', 'stdin', 'stdout'] >>>
(a / b, a % b)
.
For floating point numbers the result is the same as
(math.floor(a / b), a % b)
.
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()
.
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.
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
.
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.
getattr(x, 'foobar')
is equivalent to
x.foobar
.
getattr(object, name)
and
seeing whether it raises an exception or not.)
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.
eval(raw_input(prompt))
. Like
raw_input()
, the prompt argument is optional, and GNU
readline is used when configured. The difference
is that a long input expression may be broken over multiple lines using
the backslash convention.
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.
TypeError
exception is raised.
TypeError
exception is raised.
sequence[:]
.
For instance, list('abc')
returns
returns ['a', 'b', 'c']
and list( (1, 2, 3) )
returns
[1, 2, 3]
.
string.atol(x)
.
Otherwise, the argument may be a plain or
long integer or a floating point number, and a long interger with
the same value is returned. Conversion of floating
point numbers to integers is defined by the C semantics;
see the description of int()
.
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.
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.
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.
ord('a')
returns the integer 97
. This is the inverse of
chr()
.
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.
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 an
exception 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) [] >>>
EOFError
is raised. Example:
>>> s = raw_input('--> ') --> Monty Python's Flying Circus >>> s "Monty Python's Flying Circus" >>>
If the interpreter was built to use the GNU readline library, then
raw_input()
will use it to provide elaborate
line editing and history features.
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.
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.
eval()
.
round(0.5)
is 1.0
and round(-0.5)
is -1.0
).
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
.
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]
.
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('abc')
returns
returns ('a', 'b', 'c')
and tuple([1, 2, 3])
returns
(1, 2, 3)
.
types
defines names for all
built-in types.
For instance:
>>> import types >>> if type(x) == types.StringType: print "It's a string"
__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.
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
).