GNU Info
(python2.1-lib.info)Built-in Functions
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. As a workaround for this behavior, use
`getattr()' to extract the desired components. For example, you
could define the following helper:
import string
def my_import(name):
mod = __import__(name)
components = string.split(name, '.')
for comp in components[1:]:
mod = getattr(mod, comp)
return mod
`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 sequence (if it is not a tuple, the sequence is
first converted to a tuple). The FUNCTION is called with ARGS as
the 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.
`buffer(object[, offset[, size]])'
The OBJECT argument must be an object that supports the buffer
call interface (such as strings, arrays, and buffers). A new
buffer object will be created which references the OBJECT argument.
The buffer object will be a slice from the beginning of OBJECT (or
from the specified OFFSET). The slice will extend to the end of
OBJECT (or will have a length given by the SIZE argument).
`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; `ValueError' will be raised if I is outside that range.
`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 or convert a
string or number to a complex number. 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()'; in this case it also accepts
a string argument which should be a valid complex number.
`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([object])'
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 `(Q, A %{} B)', where Q is usually
`math.floor(A / B)' but may be 1 less than that. In any case `Q *
B + A %{} B' is very close to A, if `A %{} B' is non-zero it has
the same sign as B, and `0 <= abs(A %{} B) < abs(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.(1)
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'.
`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 signed 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.
*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(object, name[, default])'
Return the value of the named attributed 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 to `x.foobar'. If
the named attribute does not exist, DEFAULT is returned if
provided, otherwise `AttributeError' 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 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 (or long
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])'
Equivalent to `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.
`int(x[, radix])'
Convert a string or number to a plain integer. If the argument is
a string, it must contain a possibly signed decimal number
representable as a Python integer, possibly embedded in whitespace;
this behaves identical to `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)
`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).
`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.
*Warning:* The contents of this dictionary should not be modified;
changes may not affect the values of local variables used by the
interpreter.
`long(x[, radix])'
Convert a string or number to a long integer. If the argument is a
string, it must contain a possibly signed number of arbitrary
size, possibly embedded in whitespace; this behaves identical to
`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()'.
`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[, args...])'
With a single argument S, return the largest item of a non-empty
sequence (e.g., a string, tuple or list). With more than one
argument, return the largest of the arguments.
`min(s[, args...])'
With a single argument S, return the smallest item of a non-empty
sequence (e.g., a string, tuple or list). With more than one
argument, return the smallest of the arguments.
`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 (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 raised.
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
documentation.) 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.(3)
`ord(c)'
Return the ASCII value of a string of one character or a Unicode
character. E.g., `ord('a')' returns the integer `97',
`ord(u'\u2020')' returns `8224'. This is the inverse of `chr()'
for strings and of `unichr()' for Unicode characters.
`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, sequence[, initializer])'
Apply FUNCTION of two arguments cumulatively to the items of
SEQUENCE, from left to right, so as to reduce the sequence to a
single value. For example, `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.
`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 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.
`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 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 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 type(x) == types.StringType: print "It's a string"
`unichr(i)'
Return the Unicode string of one character whose Unicode code is
the integer I, e.g., `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. _Added in Python version 2.0_
`unicode(string[, encoding[, errors]])'
Decodes STRING using the codec for ENCODING. Error handling is
done according to ERRORS. The default behavior is to decode UTF-8
in strict mode, meaning that encoding errors raise `ValueError'.
See also the `codecs' module. _Added in Python version 2.0_
`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.(4)
`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').
`zip(seq1, ...)'
This function returns a list of tuples, where each tuple contains
the I-th element from each of the argument sequences. At least one
sequence is required, otherwise a `TypeError' is raised. The
returned list is truncated in length to the length of the shortest
argument sequence. When there are multiple argument sequences
which are all of the same length, `zip()' is similar to `map()'
with an initial argument of `None'. With a single sequence
argument, it returns a list of 1-tuples. _Added in Python version
2.0_
---------- Footnotes ----------
(1) It is used relatively rarely so does not warrant being made into a
statement.
(2) This is ugly -- the language definition should require truncation
towards zero.
(3) Specifying a buffer size currently has no effect on systems that
don't have `setvbuf()'. The interface to specify the buffer size is
not done using a method that calls `setvbuf()', because that may dump
core when called after any I/O has been performed, and there's no
reliable way to determine whether this is the case.
(4) In the current implementation, local variable bindings cannot
normally be affected this way, but variables retrieved from other
scopes (e.g. modules) can be. This may change.
automatically generated by info2www version 1.2.2.9