 |
 |
 |
Python Reference Manual |  |
 |
 |
|
|
|
Python Reference Manual | |
|
|
Below is a list of the types that are built into Python. Extension modules (written in C, Java, or other languages, depending on the implementation) can define additional types. Future versions of Python may add types to the type hierarchy (e.g., rational numbers, efficiently stored arrays of integers, etc.).
Some of the type descriptions below contain a paragraph listing `special attributes.' These are attributes that provide access to the implementation and are not intended for general use. Their definition may change in the future.
None.
It is used to signify the absence of a value in many situations, e.g.,
it is returned from functions that don't explicitly return anything.
Its truth value is false.
NotImplemented.
Numeric methods and rich comparison methods may return this value if
they do not implement the operation for the operands provided. (The
interpreter will then try the reflected operation, or some other
fallback, depending on the operator.) Its truth value is true.
Ellipsis.
It is used to indicate the presence of the "..." syntax in a
slice. Its truth value is true.
Python distinguishes between integers, floating point numbers, and complex numbers:
There are three types of integers:
"False" or "True" are returned, respectively.
The rules for integer representation are intended to give the most meaningful interpretation of shift and mask operations involving negative integers and the least surprises when switching between the plain and long integer domains. Any operation except left shift, if it yields a result in the plain integer domain without causing overflow, will yield the same result in the long integer domain or when using mixed operands.
z can be retrieved through the read-only attributes
z.real and z.imag.
a[i].
Sequences also support slicing: a[i:j]
selects all items with index k such that i <=
k < j. When used as an expression, a slice is a
sequence of the same type. This implies that the index set is
renumbered so that it starts at 0.
Some sequences also support ``extended slicing'' with a third ``step''
parameter: a[i:j:k] selects all items
of a with index x where x = i +
n*k, n >= 0 and i <=
x < j.
Sequences are distinguished according to their mutability:
The following types are immutable sequences:
(On systems whose native character set is not ASCII, strings may use EBCDIC in their internal representation, provided the functions chr() and ord() implement a mapping between ASCII and EBCDIC, and string comparison preserves the ASCII order. Or perhaps someone can propose a better rule?)
sys.maxunicode, and
depends on how Python is configured at compile time). Surrogate pairs
may be present in the Unicode object, and will be reported as two
separate items. The built-in functions
unichr() and
ord() convert between code units and
nonnegative integers representing the Unicode ordinals as defined in
the Unicode Standard 3.0. Conversion from and to other encodings are
possible through the Unicode method encode and the built-in
function unicode().
There is currently a single intrinsic mutable sequence type:
The extension module array provides an additional example of a mutable sequence type.
a[k] selects the item indexed
by k from the mapping a; this can be used in
expressions and as the target of assignments or del statements.
The built-in function len() returns the number of items
in a mapping.
There is currently a single intrinsic mapping type:
1 and
1.0) then they can be used interchangeably to index the same
dictionary entry.
Dictionaries are mutable; they can be created by the
{...} notation (see section 5.2.5, ``Dictionary
Displays'').
The extension modules dbm gdbm bsddb provide additional examples of mapping types.
Special attributes: func_doc or __doc__ is the
function's documentation string, or None if unavailable;
func_name or __name__ is the function's name;
__module__ is the name of the module the function was defined
in, or None if unavailable;
func_defaults is a tuple containing default argument values for
those arguments that have defaults, or None if no arguments
have a default value; func_code is the code object representing
the compiled function body; func_globals is (a reference to)
the dictionary that holds the function's global variables -- it
defines the global namespace of the module in which the function was
defined; func_dict or __dict__ contains the
namespace supporting arbitrary function attributes;
func_closure is None or a tuple of cells that contain
bindings for the function's free variables.
Of these, func_code, func_defaults, func_doc/__doc__, and func_dict/__dict__ may be writable; the others can never be changed. Additional information about a function's definition can be retrieved from its code object; see the description of internal types below.
None) and any callable object (normally a user-defined
function).
Special read-only attributes: im_self is the class instance
object, im_func is the function object;
im_class is the class of im_self for bound methods
or the class that asked for the method for unbound methods;
__doc__ is the method's documentation (same as
im_func.__doc__); __name__ is the method name (same as
im_func.__name__); __module__ is the name of the
module the method was defined in, or None if unavailable.
Changed in version 2.2:
im_self used to refer to the class that
defined the method.
Methods also support accessing (but not setting) the arbitrary function attributes on the underlying function object.
User-defined method objects may be created when getting an attribute of a class (perhaps via an instance of that class), if that attribute is a user-defined function object, an unbound user-defined method object, or a class method object. When the attribute is a user-defined method object, a new method object is only created if the class from which it is being retrieved is the same as, or a derived class of, the class stored in the original method object; otherwise, the original method object is used as it is.
When a user-defined method object is created by retrieving
a user-defined function object from a class, its im_self
attribute is None and the method object is said to be unbound.
When one is created by retrieving a user-defined function object
from a class via one of its instances, its im_self attribute
is the instance, and the method object is said to be bound.
In either case, the new method's im_class attribute
is the class from which the retrieval takes place, and
its im_func attribute is the original function object.
When a user-defined method object is created by retrieving another method object from a class or instance, the behaviour is the same as for a function object, except that the im_func attribute of the new instance is not the original method object but its im_func attribute.
When a user-defined method object is created by retrieving a class method object from a class or instance, its im_self attribute is the class itself (the same as the im_class attribute), and its im_func attribute is the function object underlying the class method.
When an unbound user-defined method object is called, the underlying function (im_func) is called, with the restriction that the first argument must be an instance of the proper class (im_class) or of a derived class thereof.
When a bound user-defined method object is called, the underlying
function (im_func) is called, inserting the class instance
(im_self) in front of the argument list. For instance, when
C is a class which contains a definition for a function
f(), and x is an instance of C, calling
x.f(1) is equivalent to calling C.f(x, 1).
When a user-defined method object is derived from a class method object,
the ``class instance'' stored in im_self will actually be the
class itself, so that calling either x.f(1) or C.f(1) is
equivalent to calling f(C,1) where f is the underlying
function.
Note that the transformation from function object to (unbound or bound) method object happens each time the attribute is retrieved from the class or instance. In some cases, a fruitful optimization is to assign the attribute to a local variable and call that local variable. Also notice that this transformation only happens for user-defined functions; other callable objects (and all non-callable objects) are retrieved without transformation. It is also important to note that user-defined functions which are attributes of a class instance are not converted to bound methods; this only happens when the function is an attribute of the class.
None if unavailable; __name__
is the function's name; __self__ is set to None (but see
the next item); __module__ is the name of the module the
function was defined in or None if unavailable.
alist.append(), assuming
alist is a list object.
In this case, the special read-only attribute __self__ is set
to the object denoted by list.
x(arguments)
is a shorthand for x.__call__(arguments).
m.x is equivalent to
m.__dict__["x"].
A module object does not contain the code object used to
initialize the module (since it isn't needed once the initialization
is done).
Attribute assignment updates the module's namespace dictionary, e.g., "m.x = 1" is equivalent to "m.__dict__["x"] = 1".
Special read-only attribute: __dict__ is the module's namespace as a dictionary object.
Predefined (writable) attributes: __name__
is the module's name; __doc__ is the
module's documentation string, or
None if unavailable; __file__ is the pathname of the
file from which the module was loaded, if it was loaded from a file.
The __file__ attribute is not present for C modules that are
statically linked into the interpreter; for extension modules loaded
dynamically from a shared library, it is the pathname of the shared
library file.
When a class attribute reference (for class C, say) would yield a user-defined function object or an unbound user-defined method object whose associated class is either C or one of its base classes, it is transformed into an unbound user-defined method object whose im_class attribute is C. When it would yield a class method object, it is transformed into a bound user-defined method object whose im_class and im_self attributes are both C. When it would yield a static method object, it is transformed into the object wrapped by the static method object. See section 3.3.2 for another way in which attributes retrieved from a class may differ from those actually contained in its __dict__.
Class attribute assignments update the class's dictionary, never the dictionary of a base class.
A class object can be called (see above) to yield a class instance (see below).
Special attributes: __name__ is the class name; __module__ is the module name in which the class was defined; __dict__ is the dictionary containing the class's namespace; __bases__ is a tuple (possibly empty or a singleton) containing the base classes, in the order of their occurrence in the base class list; __doc__ is the class's documentation string, or None if undefined.
Attribute assignments and deletions update the instance's dictionary, never a class's dictionary. If the class has a __setattr__() or __delattr__() method, this is called instead of updating the instance dictionary directly.
Class instances can pretend to be numbers, sequences, or mappings if they have methods with certain special names. See section 3.3, ``Special method names.''
Special attributes: __dict__ is the attribute dictionary; __class__ is the instance's class.
sys.stdin,
sys.stdout and
sys.stderr are initialized to file objects
corresponding to the interpreter's standard input, output
and error streams. See the Python Library
Reference for complete documentation of file objects.
Special read-only attributes: co_name gives the function name; co_argcount is the number of positional arguments (including arguments with default values); co_nlocals is the number of local variables used by the function (including arguments); co_varnames is a tuple containing the names of the local variables (starting with the argument names); co_cellvars is a tuple containing the names of local variables that are referenced by nested functions; co_freevars is a tuple containing the names of free variables; co_code is a string representing the sequence of bytecode instructions; co_consts is a tuple containing the literals used by the bytecode; co_names is a tuple containing the names used by the bytecode; co_filename is the filename from which the code was compiled; co_firstlineno is the first line number of the function; co_lnotab is a string encoding the mapping from byte code offsets to line numbers (for details see the source code of the interpreter); co_stacksize is the required stack size (including local variables); co_flags is an integer encoding a number of flags for the interpreter.
The following flag bits are defined for co_flags: bit
0x04 is set if the function uses the "*arguments" syntax
to accept an arbitrary number of positional arguments; bit
0x08 is set if the function uses the "**keywords" syntax
to accept arbitrary keyword arguments; bit 0x20 is set if the
function is a generator.
Future feature declarations ("from __future__ import division")
also use bits in co_flags to indicate whether a code object
was compiled with a particular feature enabled: bit 0x2000 is
set if the function was compiled with future division enabled; bits
0x10 and 0x1000 were used in earlier versions of Python.
Other bits in co_flags are reserved for internal use.
If a code object represents a function,
the first item in
co_consts is the documentation string of the function, or
None if undefined.
Special read-only attributes: f_back is to the previous
stack frame (towards the caller), or None if this is the bottom
stack frame; f_code is the code object being executed in this
frame; f_locals is the dictionary used to look up local
variables; f_globals is used for global variables;
f_builtins is used for built-in (intrinsic) names;
f_restricted is a flag indicating whether the function is
executing in restricted execution mode; f_lasti gives the
precise instruction (this is an index into the bytecode string of
the code object).
Special writable attributes: f_trace, if not None, is a
function called at the start of each source code line (this is used by
the debugger); f_exc_type, f_exc_value,
f_exc_traceback represent the most recent exception caught in
this frame; f_lineno is the current line number of the frame
-- writing to this from within a trace function jumps to the given line
(only for the bottom-most frame). A debugger can implement a Jump
command (aka Set Next Statement) by writing to f_lineno.
try statement.'')
It is accessible as sys.exc_traceback, and also as the third
item of the tuple returned by sys.exc_info(). The latter is
the preferred interface, since it works correctly when the program is
using multiple threads.
When the program contains no suitable handler, the stack trace is written
(nicely formatted) to the standard error stream; if the interpreter is
interactive, it is also made available to the user as
sys.last_traceback.
Special read-only attributes: tb_next is the next level in the
stack trace (towards the frame where the exception occurred), or
None if there is no next level; tb_frame points to the
execution frame of the current level; tb_lineno gives the line
number where the exception occurred; tb_lasti indicates the
precise instruction. The line number and last instruction in the
traceback may differ from the line number of its frame object if the
exception occurred in a try statement with no matching
except clause or with a finally clause.
a[i:j:step], a[i:j,
k:l], or a[..., i:j]. They are also created by the built-in
slice() function.
Special read-only attributes: start is the lower bound;
stop is the upper bound; step is the step value; each is
None if omitted. These attributes can have any type.
Slice objects support one method:
| self, length) |
|
|
|
Python Reference Manual | |
|
|