Called to create a new instance of class cls. __new__()
is a static method (special-cased so you need not declare it as such)
that takes the class of which an instance was requested as its first
argument. The remaining arguments are those passed to the object
constructor expression (the call to the class). The return value of
__new__() should be the new object instance (usually an
instance of cls).
Typical implementations create a new instance of the class by invoking
the superclass's __new__() method using
"super(currentclass, cls).__new__(cls[, ...])"with appropriate arguments and then modifying the newly-created instance
as necessary before returning it.
If __new__() returns an instance of cls, then the new
instance's __init__() method will be invoked like
"__init__(self[, ...])", where self is the new instance
and the remaining arguments are the same as were passed to
__new__().
If __new__() does not return an instance of cls, then the
new instance's __init__() method will not be invoked.
__new__() is intended mainly to allow subclasses of
immutable types (like int, str, or tuple) to customize instance
creation.
__init__(
self[, ...])
Called when the instance is created. The
arguments are those passed to the class constructor expression. If a
base class has an __init__() method, the derived class's
__init__() method, if any, must explicitly call it to ensure proper
initialization of the base class part of the instance; for example:
"BaseClass.__init__(self, [args...])". As a special
constraint on constructors, no value may be returned; doing so will
cause a TypeError to be raised at runtime.
__del__(
self)
Called when the instance is about to be destroyed. This is also
called a destructor. If a base class
has a __del__() method, the derived class's __del__()
method, if any,
must explicitly call it to ensure proper deletion of the base class
part of the instance. Note that it is possible (though not recommended!)
for the __del__()
method to postpone destruction of the instance by creating a new
reference to it. It may then be called at a later time when this new
reference is deleted. It is not guaranteed that
__del__() methods are called for objects that still exist when
the interpreter exits.
Note:
"del x" doesn't directly call
x.__del__() -- the former decrements the reference count for
x by one, and the latter is only called when x's reference
count reaches zero. Some common situations that may prevent the
reference count of an object from going to zero include: circular
references between objects (e.g., a doubly-linked list or a tree data
structure with parent and child pointers); a reference to the object
on the stack frame of a function that caught an exception (the
traceback stored in sys.exc_traceback keeps the stack frame
alive); or a reference to the object on the stack frame that raised an
unhandled exception in interactive mode (the traceback stored in
sys.last_traceback keeps the stack frame alive). The first
situation can only be remedied by explicitly breaking the cycles; the
latter two situations can be resolved by storing None in
sys.exc_traceback or sys.last_traceback. Circular
references which are garbage are detected when the option cycle
detector is enabled (it's on by default), but can only be cleaned up
if there are no Python-level __del__() methods involved.
Refer to the documentation for the gc
module for more information about how
__del__() methods are handled by the cycle detector,
particularly the description of the garbage value.
Warning:
Due to the precarious circumstances under which
__del__() methods are invoked, exceptions that occur during their
execution are ignored, and a warning is printed to sys.stderr
instead. Also, when __del__() is invoked in response to a module
being deleted (e.g., when execution of the program is done), other
globals referenced by the __del__() method may already have been
deleted. For this reason, __del__() methods should do the
absolute minimum needed to maintain external invariants. Starting with
version 1.5, Python guarantees that globals whose name begins with a single
underscore are deleted from their module before other globals are deleted;
if no other references to such globals exist, this may help in assuring that
imported modules are still available at the time when the
__del__() method is called.
__repr__(
self)
Called by the repr() built-in function
and by string conversions (reverse quotes) to compute the ``official''
string representation of an object. If at all possible, this should
look like a valid Python expression that could be used to recreate an
object with the same value (given an appropriate environment). If
this is not possible, a string of the form "<...some useful
description...>" should be returned. The return value must be a
string object.
If a class defines __repr__() but not __str__(),
then __repr__() is also used when an ``informal'' string
representation of instances of that class is required.
This is typically used for debugging, so it is important that the
representation is information-rich and unambiguous.
__str__(
self)
Called by the str() built-in function and
by the print statement to compute the
``informal'' string representation of an object. This differs from
__repr__() in that it does not have to be a valid Python
expression: a more convenient or concise representation may be used
instead. The return value must be a string object.
__lt__(
self, other)
__le__(
self, other)
__eq__(
self, other)
__ne__(
self, other)
__gt__(
self, other)
__ge__(
self, other)
New in version 2.1.
These are the so-called ``rich comparison'' methods, and are called
for comparison operators in preference to __cmp__() below.
The correspondence between operator symbols and method names is as
follows:
x<y calls x.__lt__(y),
x<=y calls x.__le__(y),
x==y calls x.__eq__(y),
x!=y and x<>y call
x.__ne__(y),
x>y calls x.__gt__(y), and
x>=y calls x.__ge__(y).
These methods can return any value, but if the comparison operator is
used in a Boolean context, the return value should be interpretable as
a Boolean value, else a TypeError will be raised.
By convention, False is used for false and True for true.
There are no implied relationships among the comparison operators.
The truth of x==y does not imply that x!=y
is false. Accordingly, when defining __eq__(), one should also
define __ne__() so that the operators will behave as expected.
There are no reflected (swapped-argument) versions of these methods
(to be used when the left argument does not support the operation but
the right argument does); rather, __lt__() and
__gt__() are each other's reflection, __le__() and
__ge__() are each other's reflection, and __eq__()
and __ne__() are their own reflection.
Arguments to rich comparison methods are never coerced. A rich
comparison method may return NotImplemented if it does not
implement the operation for a given pair of arguments.
__cmp__(
self, other)
Called by comparison operations if rich comparison (see above) is not
defined. Should return a negative integer if self < other,
zero if self == other, a positive integer if self >
other. If no __cmp__(), __eq__() or
__ne__() operation is defined, class instances are compared
by object identity (``address''). See also the description of
__hash__() for some important notes on creating objects which
support custom comparison operations and are usable as dictionary
keys.
(Note: the restriction that exceptions are not propagated by
__cmp__() has been removed since Python 1.5.)
__rcmp__(
self, other)
Changed in version 2.1:
No longer supported.
__hash__(
self)
Called for the key object for dictionary operations, and by the built-in function
hash(). Should return a 32-bit integer
usable as a hash value
for dictionary operations. The only required property is that objects
which compare equal have the same hash value; it is advised to somehow
mix together (e.g., using exclusive or) the hash values for the
components of the object that also play a part in comparison of
objects. If a class does not define a __cmp__() method it should
not define a __hash__() operation either; if it defines
__cmp__() or __eq__() but not __hash__(),
its instances will not be usable as dictionary keys. If a class
defines mutable objects and implements a __cmp__() or
__eq__() method, it should not implement __hash__(),
since the dictionary implementation requires that a key's hash value
is immutable (if the object's hash value changes, it will be in the
wrong hash bucket).
Changed in version 2.5:
__hash__() may now also return a long
integer object; the 32-bit integer is then derived from the hash
of that object.
__nonzero__(
self)
Called to implement truth value testing, and the built-in operation
bool(); should return False or True, or their
integer equivalents 0 or 1.
When this method is not defined, __len__() is
called, if it is defined (see below). If a class defines neither
__len__() nor __nonzero__(), all its instances are
considered true.
__unicode__(
self)
Called to implement unicode() builtin;
should return a Unicode object. When this method is not defined, string
conversion is attempted, and the result of string conversion is converted
to Unicode using the system default encoding.
