3.2 The standard type hierarchy
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
- This type has a single value. There is a single object with this value. This object
is accessed through the built-in name
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
- This type has a single value. There is a single object with this value. This object
is accessed through the built-in name
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
- This type has a single value. There is a single object with this value. This object
is accessed through the built-in name
Ellipsis. It is used to indicate
the presence of the "..." syntax in a slice. Its truth
value is true.
- Numbers
- These are created by numeric literals and returned as results by arithmetic
operators and arithmetic built-in functions. Numeric objects are immutable; once
created their value never changes. Python numbers are of course strongly related to
mathematical numbers, but subject to the limitations of numerical representation in
computers.
Python distinguishes between integers, floating point numbers, and complex numbers:
- Integers
- These represent elements from the mathematical set of whole numbers.
There are three types of integers:
- Plain integers
- These represent numbers in the range -2147483648 through 2147483647. (The
range may be larger on machines with a larger natural word size, but not
smaller.) When the result of an operation would fall outside this range, the
result is normally returned as a long integer (in some cases, the exception OverflowError is raised instead). For the purpose of
shift and mask operations, integers are assumed to have a binary, 2's
complement notation using 32 or more bits, and hiding no bits from the user
(i.e., all 4294967296 different bit patterns correspond to different values).
- Long integers
- These represent numbers in an unlimited range, subject to available
(virtual) memory only. For the purpose of shift and mask operations, a binary
representation is assumed, and negative numbers are represented in a variant
of 2's complement which gives the illusion of an infinite string of sign bits
extending to the left.
- Booleans
- These represent the truth values False and True. The two objects
representing the values False and True are the only Boolean objects. The
Boolean type is a subtype of plain integers, and Boolean values behave like
the values 0 and 1, respectively, in almost all contexts, the exception being
that when converted to a string, the strings
"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.
- Floating point numbers
- These represent machine-level double precision floating point numbers. You are
at the mercy of the underlying machine architecture (and C or Java implementation)
for the accepted range and handling of overflow. Python does not support
single-precision floating point numbers; the savings in processor and memory usage
that are usually the reason for using these is dwarfed by the overhead of using
objects in Python, so there is no reason to complicate the language with two kinds
of floating point numbers.
- Complex numbers
- These represent complex numbers as a pair of machine-level double precision
floating point numbers. The same caveats apply as for floating point numbers. The
real and imaginary parts of a complex number
z can be retrieved
through the read-only attributes z.real and z.imag.
- Sequences
- These represent finite ordered sets indexed by non-negative numbers. The built-in
function len()
returns the number of items of a sequence. When the length of a sequence is n,
the index set contains the numbers 0, 1, ..., n-1. Item i of
sequence a is selected by
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:
- Immutable sequences
- An object of an immutable sequence type cannot change once it is created. (If
the object contains references to other objects, these other objects may be
mutable and may be changed; however, the collection of objects directly referenced
by an immutable object cannot change.)
The following types are immutable sequences:
- Strings
- The items of a string are characters. There is no separate character type; a
character is represented by a string of one item. Characters represent (at
least) 8-bit bytes. The built-in functions chr()
and ord()
convert between characters and nonnegative integers representing the byte
values. Bytes with the values 0-127 usually represent the corresponding ASCII
values, but the interpretation of values is up to the program. The string data
type is also used to represent arrays of bytes, e.g., to hold data read from a
file.
(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?)
- Unicode
- The items of a Unicode object are Unicode code units. A Unicode code unit is
represented by a Unicode object of one item and can hold either a 16-bit or
32-bit value representing a Unicode ordinal (the maximum value for the ordinal
is given in
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().
- Tuples
- The items of a tuple are arbitrary Python objects. Tuples of two or more
items are formed by comma-separated lists of expressions. A tuple of one item
(a `singleton') can be formed by affixing a comma to an expression (an
expression by itself does not create a tuple, since parentheses must be usable
for grouping of expressions). An empty tuple can be formed by an empty pair of
parentheses.
- Mutable sequences
- Mutable sequences can be changed after they are created. The subscription and
slicing notations can be used as the target of assignment and del
(delete) statements.
There is currently a single intrinsic mutable sequence type:
- Lists
- The items of a list are arbitrary Python objects. Lists are formed by
placing a comma-separated list of expressions in square brackets. (Note that
there are no special cases needed to form lists of length 0 or 1.)
The extension module array
provides an additional example of a mutable sequence type.
- Mappings
- These represent finite sets of objects indexed by arbitrary index sets. The
subscript notation
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:
- Dictionaries
- These
represent finite sets of objects indexed by nearly arbitrary values. The only
types of values not acceptable as keys are values containing lists or dictionaries
or other mutable types that are compared by value rather than by object identity,
the reason being that the efficient implementation of dictionaries requires a
key's hash value to remain constant. Numeric types used for keys obey the normal
rules for numeric comparison: if two numbers compare equal (e.g.,
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.
- Callable types
- These
are the types to which the function call operation (see section 5.3.4, ``Calls'') can be applied:
- User-defined functions
- A user-defined function object is created by a function definition (see section 7.5, ``Function definitions''). It should be
called with an argument list containing the same number of items as the function's
formal parameter list.
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.
- User-defined methods
- A user-defined method object combines a class, a class instance (or
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.
- Generator functions
- A function or method which uses the yield statement
(see section 6.8, ``The yield
statement'') is called a generator function. Such a function,
when called, always returns an iterator object which can be used to execute the
body of the function: calling the iterator's next() method
will cause the function to execute until it provides a value using the yield statement. When the function executes a return statement or falls off the end, a StopIteration exception is raised and the iterator will
have reached the end of the set of values to be returned.
- Built-in functions
- A built-in function object is a wrapper around a C function. Examples of
built-in functions are len() and math.sin()
(math is a standard built-in module). The number and type
of the arguments are determined by the C function. Special read-only attributes: __doc__ is the function's documentation string, or
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.
- Built-in methods
- This is really a different disguise of a built-in function, this time containing
an object passed to the C function as an implicit extra argument. An example of a
built-in method is
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.
- Class Types
- Class types, or ``new-style classes,'' are callable. These objects normally act
as factories for new instances of themselves, but variations are possible for
class types that override __new__(). The arguments of the
call are passed to __new__() and, in the typical case, to __init__() to initialize the new instance.
- Classic Classes
- Class objects are described below. When a class object is called, a new class
instance (also described below) is created and returned. This implies a call to
the class's __init__() method if it has one. Any arguments
are passed on to the __init__() method. If there is no __init__() method, the class must be called without arguments.
- Class instances
- Class instances are described below. Class instances are callable only when the
class has a __call__() method;
x(arguments)
is a shorthand for x.__call__(arguments).
- Modules
- Modules are imported by the import statement (see section 6.12, ``The import
statement'').
A module object has a namespace implemented by a dictionary object (this is the
dictionary referenced by the func_globals attribute of functions defined in the
module). Attribute references are translated to lookups in this dictionary, e.g.,
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.
- Classes
- Class objects are created by class definitions (see section 7.6, ``Class definitions''). A class has a namespace
implemented by a dictionary object. Class attribute references are translated to
lookups in this dictionary, e.g., "C.x" is translated
to "C.__dict__["x"]". When the attribute
name is not found there, the attribute search continues in the base classes. The
search is depth-first, left-to-right in the order of occurrence in the base class
list.
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.
- Class instances
- A class instance is created by calling a class object (see above). A class instance
has a namespace implemented as a dictionary which is the first place in which
attribute references are searched. When an attribute is not found there, and the
instance's class has an attribute by that name, the search continues with the class
attributes. If a class attribute is found that is a user-defined function object or an
unbound user-defined method object whose associated class is the class (call it C) of the instance for which the attribute reference was initiated
or one of its bases, it is transformed into a bound user-defined method object whose im_class attribute is C whose im_self attribute is the instance. Static method and class method
objects are also transformed, as if they had been retrieved from class C; see above under ``Classes''. See section 3.3.2 for another way in which attributes of a
class retrieved via its instances may differ from the objects actually stored in the
class's __dict__. If no class attribute is found, and the
object's class has a __getattr__() method, that is called to
satisfy the lookup.
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.
- Files
- A file
object represents an open file. File objects are created by the open()
built-in function, and also by
os.popen(), os.fdopen(), and
the makefile()
method of socket objects (and perhaps by other functions or methods provided by
extension modules). The objects
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.
- Internal types
- A few types used internally by the interpreter are exposed to the user. Their
definitions may change with future versions of the interpreter, but they are mentioned
here for completeness.
- Code objects
- Code objects represent byte-compiled executable Python code, or bytecode.
The difference between a code object and a function object is that the function
object contains an explicit reference to the function's globals (the module in
which it was defined), while a code object contains no context; also the default
argument values are stored in the function object, not in the code object (because
they represent values calculated at run-time). Unlike function objects, code
objects are immutable and contain no references (directly or indirectly) to
mutable 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.
- Frame objects
- Frame objects represent execution frames. They may occur in traceback objects
(see below).
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.
- Traceback objects
-
Traceback objects represent a stack trace of an exception. A traceback object
is created when an exception occurs. When the search for an exception handler
unwinds the execution stack, at each unwound level a traceback object is inserted
in front of the current traceback. When an exception handler is entered, the stack
trace is made available to the program. (See section 7.4,
``The
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.
- Slice objects
- Slice objects are used to represent slices when extended slice syntax is
used. This is a slice using two colons, or multiple slices or ellipses separated
by commas, e.g.,
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:
-
- This method takes a single integer argument length and computes
information about the extended slice that the slice object would describe if
applied to a sequence of length items. It returns a tuple of three
integers; respectively these are the start and stop
indices and the step or stride length of the slice. Missing or
out-of-bounds indices are handled in a manner consistent with regular slices. New in version 2.3.
- Static method objects
- Static method objects provide a way of defeating the transformation of function
objects to method objects described above. A static method object is a wrapper
around any other object, usually a user-defined method object. When a static
method object is retrieved from a class or a class instance, the object actually
returned is the wrapped object, which is not subject to any further
transformation. Static method objects are not themselves callable, although the
objects they wrap usually are. Static method objects are created by the built-in staticmethod() constructor.
- Class method objects
- A class method object, like a static method object, is a wrapper around another
object that alters the way in which that object is retrieved from classes and
class instances. The behaviour of class method objects upon such retrieval is
described above, under ``User-defined methods''. Class method objects are created
by the built-in classmethod() constructor.
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