• __new__(cls[,...])__init__()之前调用。
• __init__(self[,...])
• __del__(self[,...])
• __repr__(self): repr()，官方 string 表示。尽可能返回能够重建该对象的可运行的合法 Python 表达式字符串，或者返回形如”<module ‘os’ from ‘/usr/lib/python3.5/os.py’>”的字符串。
• __str__(self): str(), format()，print()。返回字符串
• __bytes__(self):
.. _specialnames:

Special method names
====================

.. index::
single: __getitem__() (mapping object method)

A class can implement certain operations that are invoked by special syntax
(such as arithmetic operations or subscripting and slicing) by defining methods
with special names. This is Python's approach to :dfn:operator overloading,
allowing classes to define their own behavior with respect to language
operators.  For instance, if a class defines a method named :meth:__getitem__,
and x is an instance of this class, then x[i] is roughly equivalent
to type(x).__getitem__(x, i).  Except where mentioned, attempts to execute an
operation raise an exception when no appropriate method is defined (typically
:exc:AttributeError or :exc:TypeError).

Setting a special method to None indicates that the corresponding
operation is not available.  For example, if a class sets
:meth:__iter__ to None, the class is not iterable, so calling
:func:iter on its instances will raise a :exc:TypeError (without
falling back to :meth:__getitem__). [#]_

When implementing a class that emulates any built-in type, it is important that
the emulation only be implemented to the degree that it makes sense for the
object being modelled.  For example, some sequences may work well with retrieval
of individual elements, but extracting a slice may not make sense.  (One example
of this is the :class:~xml.dom.NodeList interface in the W3C's Document
Object Model.)

.. _customization:

Basic customization
-------------------

.. method:: object.__new__(cls[, ...])

.. index:: pair: subclassing; immutable types

Called to create a new instance of class *cls*.  :meth:__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 :meth:__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 :meth:__new__ method using super(currentclass,
cls).__new__(cls[, ...]) with appropriate arguments and then modifying the
newly-created instance as necessary before returning it.

If :meth:__new__ returns an instance of *cls*, then the new instance's
:meth:__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 :meth:__new__.

If :meth:__new__ does not return an instance of *cls*, then the new instance's
:meth:__init__ method will not be invoked.

:meth:__new__ is intended mainly to allow subclasses of immutable types (like
int, str, or tuple) to customize instance creation.  It is also commonly
overridden in custom metaclasses in order to customize class creation.

.. method:: object.__init__(self[, ...])

.. index:: pair: class; constructor

Called after the instance has been created (by :meth:__new__), but before
it is returned to the caller.  The arguments are those passed to the
class constructor expression.  If a base class has an :meth:__init__
method, the derived class's :meth:__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...]).

Because :meth:__new__ and :meth:__init__ work together in constructing
objects (:meth:__new__ to create it, and :meth:__init__ to customize it),
no non-None value may be returned by :meth:__init__; doing so will
cause a :exc:TypeError to be raised at runtime.

.. method:: object.__del__(self)

.. index::
single: destructor
statement: del

Called when the instance is about to be destroyed.  This is also called a
destructor.  If a base class has a :meth:__del__ method, the derived class's
:meth:__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 :meth:__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
:meth:__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_info()[2] 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 second can be
resolved by freeing the reference to the traceback object when it is no
longer useful, and the third can be resolved by storing None in
sys.last_traceback.
Circular references which are garbage are detected and cleaned up when
the cyclic garbage collector is enabled (it's on by default). Refer to the
documentation for the :mod:gc module for more information about this
topic.

.. warning::

Due to the precarious circumstances under which :meth:__del__ methods are
invoked, exceptions that occur during their execution are ignored, and a warning
is printed to sys.stderr instead.  Also, when :meth:__del__ is invoked in
response to a module being deleted (e.g., when execution of the program is
done), other globals referenced by the :meth:__del__ method may already have
been deleted or in the process of being torn down (e.g. the import
machinery shutting down).  For this reason, :meth:__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 :meth:__del__ method is
called.

.. index::
single: repr() (built-in function); __repr__() (object method)

.. method:: object.__repr__(self)

Called by the :func:repr built-in function 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 :meth:__repr__
but not :meth:__str__, then :meth:__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.

.. index::
single: string; __str__() (object method)
single: format() (built-in function); __str__() (object method)
single: print() (built-in function); __str__() (object method)

.. method:: object.__str__(self)

Called by :func:str(object) <str> and the built-in functions
:func:format and :func:print to compute the "informal" or nicely
printable string representation of an object.  The return value must be a
:ref:string <textseq> object.

This method differs from :meth:object.__repr__ in that there is no
expectation that :meth:__str__ return a valid Python expression: a more
convenient or concise representation can be used.

The default implementation defined by the built-in type :class:object
calls :meth:object.__repr__.

.. XXX what about subclasses of string?

.. method:: object.__bytes__(self)

.. index:: builtin: bytes

Called by :func:bytes to compute a byte-string representation of an
object. This should return a bytes object.

.. index::
single: string; __format__() (object method)
pair: string; conversion
builtin: print

.. method:: object.__format__(self, format_spec)

Called by the :func:format built-in function,
and by extension, evaluation of :ref:formatted string literals
<f-strings> and the :meth:str.format method, to produce a "formatted"
string representation of an object. The format_spec argument is
a string that contains a description of the formatting options desired.
The interpretation of the format_spec argument is up to the type
implementing :meth:__format__, however most classes will either
delegate formatting to one of the built-in types, or use a similar
formatting option syntax.

See :ref:formatspec for a description of the standard formatting syntax.

The return value must be a string object.

.. versionchanged:: 3.4
The __format__ method of object itself raises a :exc:TypeError
if passed any non-empty string.

.. _richcmpfuncs:
.. method:: object.__lt__(self, other)
object.__le__(self, other)
object.__eq__(self, other)
object.__ne__(self, other)
object.__gt__(self, other)
object.__ge__(self, other)

.. index::
single: comparisons

These are the so-called "rich comparison" methods. 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 calls
x.__ne__(y), x>y calls x.__gt__(y), and x>=y calls
x.__ge__(y).

A rich comparison method may return the singleton NotImplemented if it does
not implement the operation for a given pair of arguments. By convention,
False and True are returned for a successful comparison. However, these
methods can return any value, so if the comparison operator is used in a Boolean
context (e.g., in the condition of an if statement), Python will call
:func:bool on the value to determine if the result is true or false.

By default, :meth:__ne__ delegates to :meth:__eq__ and
inverts the result unless it is NotImplemented.  There are no other
implied relationships among the comparison operators, for example,
the truth of (x<y or x==y) does not imply x<=y.
To automatically generate ordering operations from a single root operation,
see :func:functools.total_ordering.

See the paragraph on :meth:__hash__ for
some important notes on creating :term:hashable objects which support
custom comparison operations and are usable as dictionary keys.

There are no swapped-argument versions of these methods (to be used when the
left argument does not support the operation but the right argument does);
rather, :meth:__lt__ and :meth:__gt__ are each other's reflection,
:meth:__le__ and :meth:__ge__ are each other's reflection, and
:meth:__eq__ and :meth:__ne__ are their own reflection.
If the operands are of different types, and right operand's type is
a direct or indirect subclass of the left operand's type,
the reflected method of the right operand has priority, otherwise
the left operand's method has priority.  Virtual subclassing is
not considered.

.. method:: object.__hash__(self)

.. index::
object: dictionary
builtin: hash

Called by built-in function :func:hash and for operations on members of
hashed collections including :class:set, :class:frozenset, and
:class:dict.  :meth:__hash__ should return an integer. The only required
property is that objects which compare equal have the same hash value; it is
advised to mix together the hash values of the components of the object that
also play a part in comparison of objects by packing them into a tuple and
hashing the tuple. Example::

def __hash__(self):
return hash((self.name, self.nick, self.color))

.. note::

:func:hash truncates the value returned from an object's custom
:meth:__hash__ method to the size of a :c:type:Py_ssize_t.  This is
typically 8 bytes on 64-bit builds and 4 bytes on 32-bit builds.  If an
object's   :meth:__hash__ must interoperate on builds of different bit
sizes, be sure to check the width on all supported builds.  An easy way
to do this is with
python -c "import sys; print(sys.hash_info.width)".

If a class does not define an :meth:__eq__ method it should not define a
:meth:__hash__ operation either; if it defines :meth:__eq__ but not
:meth:__hash__, its instances will not be usable as items in hashable
collections.  If a class defines mutable objects and implements an
:meth:__eq__ method, it should not implement :meth:__hash__, since the
implementation of hashable collections requires that a key's hash value is
immutable (if the object's hash value changes, it will be in the wrong hash
bucket).

User-defined classes have :meth:__eq__ and :meth:__hash__ methods
by default; with them, all objects compare unequal (except with themselves)
and x.__hash__() returns an appropriate value such that x == y
implies both that x is y and hash(x) == hash(y).

A class that overrides :meth:__eq__ and does not define :meth:__hash__
will have its :meth:__hash__ implicitly set to None.  When the
:meth:__hash__ method of a class is None, instances of the class will
raise an appropriate :exc:TypeError when a program attempts to retrieve
their hash value, and will also be correctly identified as unhashable when
checking isinstance(obj, collections.Hashable).

If a class that overrides :meth:__eq__ needs to retain the implementation
of :meth:__hash__ from a parent class, the interpreter must be told this
explicitly by setting __hash__ = <ParentClass>.__hash__.

If a class that does not override :meth:__eq__ wishes to suppress hash
support, it should include __hash__ = None in the class definition.
A class which defines its own :meth:__hash__ that explicitly raises
a :exc:TypeError would be incorrectly identified as hashable by
an isinstance(obj, collections.Hashable) call.

.. note::

By default, the :meth:__hash__ values of str, bytes and datetime
objects are "salted" with an unpredictable random value.  Although they
remain constant within an individual Python process, they are not
predictable between repeated invocations of Python.

This is intended to provide protection against a denial-of-service caused
by carefully-chosen inputs that exploit the worst case performance of a
dict insertion, O(n^2) complexity.  See

Changing hash values affects the iteration order of dicts, sets and
(and it typically varies between 32-bit and 64-bit builds).

See also :envvar:PYTHONHASHSEED.

.. versionchanged:: 3.3
Hash randomization is enabled by default.

.. method:: object.__bool__(self)

.. index:: single: __len__() (mapping object method)

Called to implement truth value testing and the built-in operation
bool(); should return False or True.  When this method is not
defined, :meth:__len__ is called, if it is defined, and the object is
considered true if its result is nonzero.  If a class defines neither
:meth:__len__ nor :meth:__bool__, all its instances are considered
true.

.. _attribute-access:

Customizing attribute access
----------------------------

The following methods can be defined to customize the meaning of attribute
access (use of, assignment to, or deletion of x.name) for class instances.

.. XXX explain how descriptors interfere here!

.. method:: object.__getattr__(self, name)

Called when an attribute lookup has not found the attribute in the usual places
(i.e. it is not an instance attribute nor is it found in the class tree for
self).  name is the attribute name. This method should return the
(computed) attribute value or raise an :exc:AttributeError exception.

Note that if the attribute is found through the normal mechanism,
:meth:__getattr__ is not called.  (This is an intentional asymmetry between
:meth:__getattr__ and :meth:__setattr__.) This is done both for efficiency
reasons and because otherwise :meth:__getattr__ would have no way to access
other attributes of the instance.  Note that at least for instance variables,
you can fake total control by not inserting any values in the instance attribute
dictionary (but instead inserting them in another object).  See the
:meth:__getattribute__ method below for a way to actually get total control
over attribute access.

.. method:: object.__getattribute__(self, name)

Called unconditionally to implement attribute accesses for instances of the
class. If the class also defines :meth:__getattr__, the latter will not be
called unless :meth:__getattribute__ either calls it explicitly or raises an
:exc:AttributeError. This method should return the (computed) attribute value
or raise an :exc:AttributeError exception. In order to avoid infinite
recursion in this method, its implementation should always call the base class
method with the same name to access any attributes it needs, for example,
object.__getattribute__(self, name).

.. note::

This method may still be bypassed when looking up special methods as the
result of implicit invocation via language syntax or built-in functions.
See :ref:special-lookup.

.. method:: object.__setattr__(self, name, value)

Called when an attribute assignment is attempted.  This is called instead of
the normal mechanism (i.e. store the value in the instance dictionary).
*name* is the attribute name, *value* is the value to be assigned to it.

If :meth:__setattr__ wants to assign to an instance attribute, it should
call the base class method with the same name, for example,
object.__setattr__(self, name, value).

.. method:: object.__delattr__(self, name)

Like :meth:__setattr__ but for attribute deletion instead of assignment.  This
should only be implemented if del obj.name is meaningful for the object.

.. method:: object.__dir__(self)

Called when :func:dir is called on the object. A sequence must be
returned. :func:dir converts the returned sequence to a list and sorts it.

.. _descriptors:

Implementing Descriptors
^^^^^^^^^^^^^^^^^^^^^^^^

The following methods only apply when an instance of the class containing the
method (a so-called *descriptor* class) appears in an *owner* class (the
descriptor must be in either the owner's class dictionary or in the class
dictionary for one of its parents).  In the examples below, "the attribute"
refers to the attribute whose name is the key of the property in the owner
class' :attr:~object.__dict__.

.. method:: object.__get__(self, instance, owner)

Called to get the attribute of the owner class (class attribute access) or of an
instance of that class (instance attribute access). *owner* is always the owner
class, while *instance* is the instance that the attribute was accessed through,
or None when the attribute is accessed through the *owner*.  This method
should return the (computed) attribute value or raise an :exc:AttributeError
exception.

.. method:: object.__set__(self, instance, value)

Called to set the attribute on an instance *instance* of the owner class to a
new value, *value*.

.. method:: object.__delete__(self, instance)

Called to delete the attribute on an instance *instance* of the owner class.

.. method:: object.__set_name__(self, owner, name)

Called at the time the owning class *owner* is created. The
descriptor has been assigned to *name*.

The attribute :attr:__objclass__ is interpreted by the :mod:inspect module
as specifying the class where this object was defined (setting this
appropriately can assist in runtime introspection of dynamic class attributes).
For callables, it may indicate that an instance of the given type (or a
subclass) is expected or required as the first positional argument (for example,
CPython sets this attribute for unbound methods that are implemented in C).

.. _descriptor-invocation:

Invoking Descriptors
^^^^^^^^^^^^^^^^^^^^

In general, a descriptor is an object attribute with "binding behavior", one
whose attribute access has been overridden by methods in the descriptor
protocol:  :meth:__get__, :meth:__set__, and :meth:__delete__. If any of
those methods are defined for an object, it is said to be a descriptor.

The default behavior for attribute access is to get, set, or delete the
attribute from an object's dictionary. For instance, a.x has a lookup chain
starting with a.__dict__['x'], then type(a).__dict__['x'], and
continuing through the base classes of type(a) excluding metaclasses.

However, if the looked-up value is an object defining one of the descriptor
methods, then Python may override the default behavior and invoke the descriptor
method instead.  Where this occurs in the precedence chain depends on which
descriptor methods were defined and how they were called.

The starting point for descriptor invocation is a binding, a.x. How the
arguments are assembled depends on a:

Direct Call
The simplest and least common call is when user code directly invokes a
descriptor method:    x.__get__(a).

Instance Binding
If binding to an object instance, a.x is transformed into the call:
type(a).__dict__['x'].__get__(a, type(a)).

Class Binding
If binding to a class, A.x is transformed into the call:
A.__dict__['x'].__get__(None, A).

Super Binding
If a is an instance of :class:super, then the binding super(B,
obj).m() searches obj.__class__.__mro__ for the base class A
immediately preceding B and then invokes the descriptor with the call:
A.__dict__['m'].__get__(obj, obj.__class__).

For instance bindings, the precedence of descriptor invocation depends on the
which descriptor methods are defined.  A descriptor can define any combination
of :meth:__get__, :meth:__set__ and :meth:__delete__.  If it does not
define :meth:__get__, then accessing the attribute will return the descriptor
object itself unless there is a value in the object's instance dictionary.  If
the descriptor defines :meth:__set__ and/or :meth:__delete__, it is a data
descriptor; if it defines neither, it is a non-data descriptor.  Normally, data
descriptors define both :meth:__get__ and :meth:__set__, while non-data
descriptors have just the :meth:__get__ method.  Data descriptors with
:meth:__set__ and :meth:__get__ defined always override a redefinition in an
instance dictionary.  In contrast, non-data descriptors can be overridden by
instances.

Python methods (including :func:staticmethod and :func:classmethod) are
implemented as non-data descriptors.  Accordingly, instances can redefine and
override methods.  This allows individual instances to acquire behaviors that
differ from other instances of the same class.

The :func:property function is implemented as a data descriptor. Accordingly,
instances cannot override the behavior of a property.

.. _slots:

__slots__
^^^^^^^^^

By default, instances of classes have a dictionary for attribute storage.  This
wastes space for objects having very few instance variables.  The space
consumption can become acute when creating large numbers of instances.

The default can be overridden by defining *__slots__* in a class definition.
The *__slots__* declaration takes a sequence of instance variables and reserves
just enough space in each instance to hold a value for each variable.  Space is
saved because *__dict__* is not created for each instance.

.. data:: object.__slots__

This class variable can be assigned a string, iterable, or sequence of
strings with variable names used by instances.  *__slots__* reserves space
for the declared variables and prevents the automatic creation of *__dict__*
and *__weakref__* for each instance.

Notes on using *__slots__*
""""""""""""""""""""""""""

* When inheriting from a class without *__slots__*, the *__dict__* attribute of
that class will always be accessible, so a *__slots__* definition in the
subclass is meaningless.

* Without a *__dict__* variable, instances cannot be assigned new variables not
listed in the *__slots__* definition.  Attempts to assign to an unlisted
variable name raises :exc:AttributeError. If dynamic assignment of new
variables is desired, then add '__dict__' to the sequence of strings in
the *__slots__* declaration.

* Without a *__weakref__* variable for each instance, classes defining
*__slots__* do not support weak references to its instances. If weak reference
support is needed, then add '__weakref__' to the sequence of strings in the
*__slots__* declaration.

* *__slots__* are implemented at the class level by creating descriptors
(:ref:descriptors) for each variable name.  As a result, class attributes
cannot be used to set default values for instance variables defined by
*__slots__*; otherwise, the class attribute would overwrite the descriptor
assignment.

* The action of a *__slots__* declaration is limited to the class where it is
defined.  As a result, subclasses will have a *__dict__* unless they also define
*__slots__* (which must only contain names of any *additional* slots).

* If a class defines a slot also defined in a base class, the instance variable
defined by the base class slot is inaccessible (except by retrieving its
descriptor directly from the base class). This renders the meaning of the
program undefined.  In the future, a check may be added to prevent this.

* Nonempty *__slots__* does not work for classes derived from "variable-length"
built-in types such as :class:int, :class:bytes and :class:tuple.

* Any non-string iterable may be assigned to *__slots__*. Mappings may also be
used; however, in the future, special meaning may be assigned to the values
corresponding to each key.

* *__class__* assignment works only if both classes have the same *__slots__*.

.. _class-customization:

Customizing class creation
--------------------------

Whenever a class inherits from another class, *__init_subclass__* is
called on that class. This way, it is possible to write classes which
change the behavior of subclasses. This is closely related to class
decorators, but where class decorators only affect the specific class they're
applied to, __init_subclass__ solely applies to future subclasses of the
class defining the method.

.. classmethod:: object.__init_subclass__(cls)

This method is called whenever the containing class is subclassed.
*cls* is then the new subclass. If defined as a normal instance method,
this method is implicitly converted to a class method.

Keyword arguments which are given to a new class are passed to
the parent's class __init_subclass__. For compatibility with
other classes using __init_subclass__, one should take out the
needed keyword arguments and pass the others over to the base
class, as in::

class Philosopher:
def __init_subclass__(cls, default_name, **kwargs):
super().__init_subclass__(**kwargs)
cls.default_name = default_name

class AustralianPhilosopher(Philosopher, default_name="Bruce"):
pass

The default implementation object.__init_subclass__ does
nothing, but raises an error if it is called with any arguments.

.. note::

The metaclass hint metaclass is consumed by the rest of the type
machinery, and is never passed to __init_subclass__ implementations.
The actual metaclass (rather than the explicit hint) can be accessed as
type(cls).

.. _metaclasses:

Metaclasses
^^^^^^^^^^^

.. index::
single: metaclass
builtin: type

By default, classes are constructed using :func:type. The class body is
executed in a new namespace and the class name is bound locally to the
result of type(name, bases, namespace).

The class creation process can be customized by passing the metaclass
keyword argument in the class definition line, or by inheriting from an
existing class that included such an argument. In the following example,
both MyClass and MySubclass are instances of Meta::

class Meta(type):
pass

class MyClass(metaclass=Meta):
pass

class MySubclass(MyClass):
pass

Any other keyword arguments that are specified in the class definition are
passed through to all metaclass operations described below.

When a class definition is executed, the following steps occur:

* the appropriate metaclass is determined
* the class namespace is prepared
* the class body is executed
* the class object is created

Determining the appropriate metaclass
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
.. index::
single: metaclass hint

The appropriate metaclass for a class definition is determined as follows:

* if no bases and no explicit metaclass are given, then :func:type is used
* if an explicit metaclass is given and it is *not* an instance of
:func:type, then it is used directly as the metaclass
* if an instance of :func:type is given as the explicit metaclass, or
bases are defined, then the most derived metaclass is used

The most derived metaclass is selected from the explicitly specified
metaclass (if any) and the metaclasses (i.e. type(cls)) of all specified
base classes. The most derived metaclass is one which is a subtype of *all*
of these candidate metaclasses. If none of the candidate metaclasses meets
that criterion, then the class definition will fail with TypeError.

.. _prepare:

Preparing the class namespace
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

.. index::
single: __prepare__ (metaclass method)

Once the appropriate metaclass has been identified, then the class namespace
is prepared. If the metaclass has a __prepare__ attribute, it is called
as namespace = metaclass.__prepare__(name, bases, **kwds) (where the
additional keyword arguments, if any, come from the class definition).

If the metaclass has no __prepare__ attribute, then the class namespace
is initialised as an empty ordered mapping.

.. seealso::

:pep:3115 - Metaclasses in Python 3000
Introduced the __prepare__ namespace hook

Executing the class body
^^^^^^^^^^^^^^^^^^^^^^^^

.. index::
single: class; body

The class body is executed (approximately) as
exec(body, globals(), namespace). The key difference from a normal
call to :func:exec is that lexical scoping allows the class body (including
any methods) to reference names from the current and outer scopes when the
class definition occurs inside a function.

However, even when the class definition occurs inside the function, methods
defined inside the class still cannot see names defined at the class scope.
Class variables must be accessed through the first parameter of instance or
class methods, or through the implicit lexically scoped __class__ reference
described in the next section.

.. _class-object-creation:

Creating the class object
^^^^^^^^^^^^^^^^^^^^^^^^^

.. index::
single: __class__ (method cell)
single: __classcell__ (class namespace entry)

Once the class namespace has been populated by executing the class body,
the class object is created by calling
metaclass(name, bases, namespace, **kwds) (the additional keywords
passed here are the same as those passed to __prepare__).

This class object is the one that will be referenced by the zero-argument
form of :func:super. __class__ is an implicit closure reference
created by the compiler if any methods in a class body refer to either
__class__ or super. This allows the zero argument form of
:func:super to correctly identify the class being defined based on
lexical scoping, while the class or instance that was used to make the
current call is identified based on the first argument passed to the method.

.. impl-detail::

In CPython 3.6 and later, the __class__ cell is passed to the metaclass
as a __classcell__ entry in the class namespace. If present, this must
be propagated up to the type.__new__ call in order for the class to be
initialised correctly.
Failing to do so will result in a :exc:DeprecationWarning in Python 3.6,
and a :exc:RuntimeWarning in the future.

When using the default metaclass :class:type, or any metaclass that ultimately
calls type.__new__, the following additional customisation steps are
invoked after creating the class object:

* first, type.__new__ collects all of the descriptors in the class
namespace that define a :meth:~object.__set_name__ method;
* second, all of these __set_name__ methods are called with the class
being defined and the assigned name of that particular descriptor; and
* finally, the :meth:~object.__init_subclass__ hook is called on the
immediate parent of the new class in its method resolution order.

After the class object is created, it is passed to the class decorators
included in the class definition (if any) and the resulting object is bound
in the local namespace as the defined class.

When a new class is created by type.__new__, the object provided as the
namespace parameter is copied to a new ordered mapping and the original
object is discarded. The new copy is wrapped in a read-only proxy, which
becomes the :attr:~object.__dict__ attribute of the class object.

.. seealso::

:pep:3135 - New super
Describes the implicit __class__ closure reference

Metaclass example
^^^^^^^^^^^^^^^^^

The potential uses for metaclasses are boundless. Some ideas that have been
explored include logging, interface checking, automatic delegation, automatic
property creation, proxies, frameworks, and automatic resource
locking/synchronization.

Here is an example of a metaclass that uses an :class:collections.OrderedDict
to remember the order that class variables are defined::

class OrderedClass(type):

@classmethod
def __prepare__(metacls, name, bases, **kwds):
return collections.OrderedDict()

def __new__(cls, name, bases, namespace, **kwds):
result = type.__new__(cls, name, bases, dict(namespace))
result.members = tuple(namespace)
return result

class A(metaclass=OrderedClass):
def one(self): pass
def two(self): pass
def three(self): pass
def four(self): pass

>>> A.members
('__module__', 'one', 'two', 'three', 'four')

When the class definition for *A* gets executed, the process begins with
calling the metaclass's :meth:__prepare__ method which returns an empty
:class:collections.OrderedDict.  That mapping records the methods and
attributes of *A* as they are defined within the body of the class statement.
Once those definitions are executed, the ordered dictionary is fully populated
and the metaclass's :meth:__new__ method gets invoked.  That method builds
the new type and it saves the ordered dictionary keys in an attribute
called members.

Customizing instance and subclass checks
----------------------------------------

The following methods are used to override the default behavior of the
:func:isinstance and :func:issubclass built-in functions.

In particular, the metaclass :class:abc.ABCMeta implements these methods in
order to allow the addition of Abstract Base Classes (ABCs) as "virtual base
classes" to any class or type (including built-in types), including other
ABCs.

.. method:: class.__instancecheck__(self, instance)

Return true if *instance* should be considered a (direct or indirect)
instance of *class*. If defined, called to implement isinstance(instance,
class).

.. method:: class.__subclasscheck__(self, subclass)

Return true if *subclass* should be considered a (direct or indirect)
subclass of *class*.  If defined, called to implement issubclass(subclass,
class).

Note that these methods are looked up on the type (metaclass) of a class.  They
cannot be defined as class methods in the actual class.  This is consistent with
the lookup of special methods that are called on instances, only in this
case the instance is itself a class.

.. seealso::

:pep:3119 - Introducing Abstract Base Classes
Includes the specification for customizing :func:isinstance and
:func:issubclass behavior through :meth:~class.__instancecheck__ and
:meth:~class.__subclasscheck__, with motivation for this functionality
in the context of adding Abstract Base Classes (see the :mod:abc
module) to the language.

.. _callable-types:

Emulating callable objects
--------------------------

.. method:: object.__call__(self[, args...])

.. index:: pair: call; instance

Called when the instance is "called" as a function; if this method is defined,
x(arg1, arg2, ...) is a shorthand for x.__call__(arg1, arg2, ...).

.. _sequence-types:

Emulating container types
-------------------------

The following methods can be defined to implement container objects.  Containers
usually are sequences (such as lists or tuples) or mappings (like dictionaries),
but can represent other containers as well.  The first set of methods is used
either to emulate a sequence or to emulate a mapping; the difference is that for
a sequence, the allowable keys should be the integers *k* for which 0 <= k <
N where *N* is the length of the sequence, or slice objects, which define a
range of items.  It is also recommended that mappings provide the methods
:meth:keys, :meth:values, :meth:items, :meth:get, :meth:clear,
:meth:setdefault, :meth:pop, :meth:popitem, :meth:!copy, and
:meth:update behaving similar to those for Python's standard dictionary
objects.  The :mod:collections module provides a
:class:~collections.abc.MutableMapping
abstract base class to help create those methods from a base set of
:meth:__getitem__, :meth:__setitem__, :meth:__delitem__, and :meth:keys.
Mutable sequences should provide methods :meth:append, :meth:count,
:meth:index, :meth:extend, :meth:insert, :meth:pop, :meth:remove,
:meth:reverse and :meth:sort, like Python standard list objects.  Finally,
sequence types should implement addition (meaning concatenation) and
multiplication (meaning repetition) by defining the methods :meth:__add__,
:meth:__radd__, :meth:__iadd__, :meth:__mul__, :meth:__rmul__ and
:meth:__imul__ described below; they should not define other numerical
operators.  It is recommended that both mappings and sequences implement the
:meth:__contains__ method to allow efficient use of the in operator; for
mappings, in should search the mapping's keys; for sequences, it should
search through the values.  It is further recommended that both mappings and
sequences implement the :meth:__iter__ method to allow efficient iteration
through the container; for mappings, :meth:__iter__ should be the same as
:meth:keys; for sequences, it should iterate through the values.

.. method:: object.__len__(self)

.. index::
builtin: len
single: __bool__() (object method)

Called to implement the built-in function :func:len.  Should return the length
of the object, an integer >= 0.  Also, an object that doesn't define a
:meth:__bool__ method and whose :meth:__len__ method returns zero is
considered to be false in a Boolean context.

.. method:: object.__length_hint__(self)

Called to implement :func:operator.length_hint. Should return an estimated
length for the object (which may be greater or less than the actual length).
The length must be an integer >= 0. This method is purely an
optimization and is never required for correctness.

.. note::

Slicing is done exclusively with the following three methods.  A call like ::

a[1:2] = b

is translated to ::

a[slice(1, 2, None)] = b

and so forth.  Missing slice items are always filled in with None.

.. method:: object.__getitem__(self, key)

.. index:: object: slice

Called to implement evaluation of self[key]. For sequence types, the
accepted keys should be integers and slice objects.  Note that the special
interpretation of negative indexes (if the class wishes to emulate a sequence
type) is up to the :meth:__getitem__ method. If *key* is of an inappropriate
type, :exc:TypeError may be raised; if of a value outside the set of indexes
for the sequence (after any special interpretation of negative values),
:exc:IndexError should be raised. For mapping types, if *key* is missing (not
in the container), :exc:KeyError should be raised.

.. note::

:keyword:for loops expect that an :exc:IndexError will be raised for illegal
indexes to allow proper detection of the end of the sequence.

.. method:: object.__missing__(self, key)

Called by :class:dict\ .\ :meth:__getitem__ to implement self[key] for dict subclasses
when key is not in the dictionary.

.. method:: object.__setitem__(self, key, value)

Called to implement assignment to self[key].  Same note as for
:meth:__getitem__.  This should only be implemented for mappings if the
objects support changes to the values for keys, or if new keys can be added, or
for sequences if elements can be replaced.  The same exceptions should be raised
for improper *key* values as for the :meth:__getitem__ method.

.. method:: object.__delitem__(self, key)

Called to implement deletion of self[key].  Same note as for
:meth:__getitem__.  This should only be implemented for mappings if the
objects support removal of keys, or for sequences if elements can be removed
from the sequence.  The same exceptions should be raised for improper *key*
values as for the :meth:__getitem__ method.

.. method:: object.__iter__(self)

This method is called when an iterator is required for a container. This method
should return a new iterator object that can iterate over all the objects in the
container.  For mappings, it should iterate over the keys of the container.

Iterator objects also need to implement this method; they are required to return
themselves.  For more information on iterator objects, see :ref:typeiter.

.. method:: object.__reversed__(self)

Called (if present) by the :func:reversed built-in to implement
reverse iteration.  It should return a new iterator object that iterates
over all the objects in the container in reverse order.

If the :meth:__reversed__ method is not provided, the :func:reversed
built-in will fall back to using the sequence protocol (:meth:__len__ and
:meth:__getitem__).  Objects that support the sequence protocol should
only provide :meth:__reversed__ if they can provide an implementation
that is more efficient than the one provided by :func:reversed.

The membership test operators (:keyword:in and :keyword:not in) are normally
implemented as an iteration through a sequence.  However, container objects can
supply the following special method with a more efficient implementation, which
also does not require the object be a sequence.

.. method:: object.__contains__(self, item)

Called to implement membership test operators.  Should return true if *item*
is in *self*, false otherwise.  For mapping objects, this should consider the
keys of the mapping rather than the values or the key-item pairs.

For objects that don't define :meth:__contains__, the membership test first
tries iteration via :meth:__iter__, then the old sequence iteration
protocol via :meth:__getitem__, see :ref:this section in the language
reference <membership-test-details>.

.. _numeric-types:

Emulating numeric types
-----------------------

The following methods can be defined to emulate numeric objects. Methods
corresponding to operations that are not supported by the particular kind of
number implemented (e.g., bitwise operations for non-integral numbers) should be
left undefined.

object.__sub__(self, other)
object.__mul__(self, other)
object.__matmul__(self, other)
object.__truediv__(self, other)
object.__floordiv__(self, other)
object.__mod__(self, other)
object.__divmod__(self, other)
object.__pow__(self, other[, modulo])
object.__lshift__(self, other)
object.__rshift__(self, other)
object.__and__(self, other)
object.__xor__(self, other)
object.__or__(self, other)

.. index::
builtin: divmod
builtin: pow
builtin: pow

These methods are called to implement the binary arithmetic operations
(+, -, *, @, /, //, %, :func:divmod,
:func:pow, **, <<, >>, &, ^, |).  For instance, to
evaluate the expression x + y, where *x* is an instance of a class that
has an :meth:__add__ method, x.__add__(y) is called.  The
:meth:__divmod__ method should be the equivalent to using
:meth:__floordiv__ and :meth:__mod__; it should not be related to
:meth:__truediv__.  Note that :meth:__pow__ should be defined to accept
an optional third argument if the ternary version of the built-in :func:pow
function is to be supported.

If one of those methods does not support the operation with the supplied
arguments, it should return NotImplemented.

object.__rsub__(self, other)
object.__rmul__(self, other)
object.__rmatmul__(self, other)
object.__rtruediv__(self, other)
object.__rfloordiv__(self, other)
object.__rmod__(self, other)
object.__rdivmod__(self, other)
object.__rpow__(self, other)
object.__rlshift__(self, other)
object.__rrshift__(self, other)
object.__rand__(self, other)
object.__rxor__(self, other)
object.__ror__(self, other)

.. index::
builtin: divmod
builtin: pow

These methods are called to implement the binary arithmetic operations
(+, -, *, @, /, //, %, :func:divmod,
:func:pow, **, <<, >>, &, ^, |) with reflected
(swapped) operands.  These functions are only called if the left operand does
not support the corresponding operation [#]_ and the operands are of different
types. [#]_ For instance, to evaluate the expression x - y, where *y* is
an instance of a class that has an :meth:__rsub__ method, y.__rsub__(x)
is called if x.__sub__(y) returns *NotImplemented*.

.. index:: builtin: pow

Note that ternary :func:pow will not try calling :meth:__rpow__ (the
coercion rules would become too complicated).

.. note::

If the right operand's type is a subclass of the left operand's type and that
subclass provides the reflected method for the operation, this method will be
called before the left operand's non-reflected method.  This behavior allows
subclasses to override their ancestors' operations.

object.__isub__(self, other)
object.__imul__(self, other)
object.__imatmul__(self, other)
object.__itruediv__(self, other)
object.__ifloordiv__(self, other)
object.__imod__(self, other)
object.__ipow__(self, other[, modulo])
object.__ilshift__(self, other)
object.__irshift__(self, other)
object.__iand__(self, other)
object.__ixor__(self, other)
object.__ior__(self, other)

These methods are called to implement the augmented arithmetic assignments
(+=, -=, *=, @=, /=, //=, %=, **=, <<=,
>>=, &=, ^=, |=).  These methods should attempt to do the
operation in-place (modifying *self*) and return the result (which could be,
but does not have to be, *self*).  If a specific method is not defined, the
augmented assignment falls back to the normal methods.  For instance, if *x*
is an instance of a class with an :meth:__iadd__ method, x += y is
equivalent to x = x.__iadd__(y) . Otherwise, x.__add__(y) and
y.__radd__(x) are considered, as with the evaluation of x + y. In
certain situations, augmented assignment can result in unexpected errors (see
:ref:faq-augmented-assignment-tuple-error), but this behavior is in fact
part of the data model.

.. method:: object.__neg__(self)
object.__pos__(self)
object.__abs__(self)
object.__invert__(self)

.. index:: builtin: abs

Called to implement the unary arithmetic operations (-, +, :func:abs
and ~).

.. method:: object.__complex__(self)
object.__int__(self)
object.__float__(self)
object.__round__(self, [,n])

.. index::
builtin: complex
builtin: int
builtin: float
builtin: round

Called to implement the built-in functions :func:complex,
:func:int, :func:float and :func:round.  Should return a value
of the appropriate type.

.. method:: object.__index__(self)

Called to implement :func:operator.index, and whenever Python needs to
losslessly convert the numeric object to an integer object (such as in
slicing, or in the built-in :func:bin, :func:hex and :func:oct
functions). Presence of this method indicates that the numeric object is
an integer type.  Must return an integer.

.. note::

In order to have a coherent integer type class, when :meth:__index__ is
defined :meth:__int__ should also be defined, and both should return
the same value.

.. _context-managers:

With Statement Context Managers
-------------------------------

A :dfn:context manager is an object that defines the runtime context to be
established when executing a :keyword:with statement. The context manager
handles the entry into, and the exit from, the desired runtime context for the
execution of the block of code.  Context managers are normally invoked using the
:keyword:with statement (described in section :ref:with), but can also be
used by directly invoking their methods.

.. index::
statement: with
single: context manager

Typical uses of context managers include saving and restoring various kinds of
global state, locking and unlocking resources, closing opened files, etc.

For more information on context managers, see :ref:typecontextmanager.

.. method:: object.__enter__(self)

Enter the runtime context related to this object. The :keyword:with statement
will bind this method's return value to the target(s) specified in the
:keyword:as clause of the statement, if any.

.. method:: object.__exit__(self, exc_type, exc_value, traceback)

Exit the runtime context related to this object. The parameters describe the
exception that caused the context to be exited. If the context was exited
without an exception, all three arguments will be :const:None.

If an exception is supplied, and the method wishes to suppress the exception
(i.e., prevent it from being propagated), it should return a true value.
Otherwise, the exception will be processed normally upon exit from this method.

Note that :meth:__exit__ methods should not reraise the passed-in exception;
this is the caller's responsibility.

.. seealso::

:pep:343 - The "with" statement
The specification, background, and examples for the Python :keyword:with
statement.

.. _special-lookup:

Special method lookup
---------------------

For custom classes, implicit invocations of special methods are only guaranteed
to work correctly if defined on an object's type, not in the object's instance
dictionary.  That behaviour is the reason why the following code raises an
exception::

>>> class C:
...     pass
...
>>> c = C()
>>> c.__len__ = lambda: 5
>>> len(c)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: object of type 'C' has no len()

The rationale behind this behaviour lies with a number of special methods such
as :meth:__hash__ and :meth:__repr__ that are implemented by all objects,
including type objects. If the implicit lookup of these methods used the
conventional lookup process, they would fail when invoked on the type object
itself::

>>> 1 .__hash__() == hash(1)
True
>>> int.__hash__() == hash(int)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: descriptor '__hash__' of 'int' object needs an argument

Incorrectly attempting to invoke an unbound method of a class in this way is
sometimes referred to as 'metaclass confusion', and is avoided by bypassing
the instance when looking up special methods::

>>> type(1).__hash__(1) == hash(1)
True
>>> type(int).__hash__(int) == hash(int)
True

In addition to bypassing any instance attributes in the interest of
correctness, implicit special method lookup generally also bypasses the
:meth:__getattribute__ method even of the object's metaclass::

>>> class Meta(type):
...     def __getattribute__(*args):
...         print("Metaclass getattribute invoked")
...         return type.__getattribute__(*args)
...
>>> class C(object, metaclass=Meta):
...     def __len__(self):
...         return 10
...     def __getattribute__(*args):
...         print("Class getattribute invoked")
...         return object.__getattribute__(*args)
...
>>> c = C()
>>> c.__len__()                 # Explicit lookup via instance
Class getattribute invoked
10
>>> type(c).__len__(c)          # Explicit lookup via type
Metaclass getattribute invoked
10
>>> len(c)                      # Implicit lookup
10

Bypassing the :meth:__getattribute__ machinery in this fashion
provides significant scope for speed optimisations within the
interpreter, at the cost of some flexibility in the handling of
special methods (the special method *must* be set on the class
object itself in order to be consistently invoked by the interpreter).