Called to create and return a new instance of class cls. Receives constructor arguments passed to the class. If this returns an instance of the class, the instance’s __init__ method is invoked with the same constructor arguments. Not used in normal classes; intended to allow subclasses of immutable types to customize instance creation, and to allow custom metaclasses to customize class creation.

Invoked on class(args...). This is the constructor that initializes the new instance, self. When run for calls to a class name, self is provided automatically; arg is the arguments passed to the class name, and may be any function-definition argument form (see The Expression Statement and The def Statement). Must return no value, and call superclass __init__ manually if needed, passing the instance to self.

Invoked on instance garbage collection. This destructor method cleans up when an instance is freed. Embedded objects are automatically freed when the parent is (unless referenced from elsewhere). Exceptions during this method’s run are ignored, and simply print messages to sys.stderr. The try/finally statement allows more predictable termination actions for a code block.

Invoked on repr(self), and interactive echoes, (and `self` in Python 2.X only). Also invoked on str(self) and print(self) if there is no __str__. This method returns a low-level “as code” string representation of self.

Invoked on str(self) and print(self) (or uses __repr__ as a backup if defined). This method returns a high-level “user friendly” string representation of self.

Called by the format() built-in function (and by extension, the str.format() method of str strings) to produce a “formatted” string representation of an object. See Strings and Built-in Functions. New in Python 2.6 and 3.0.

Invoked on dictionary[self] and hash(self), and other hashed collection operations, including those of the set object type. This method returns a unique and unchanging integer hash-key.

Called for truth value testing and the built-in bool() function; returns False or True. When __bool__() is not defined, __len__() is called if it is defined and designates a true value with a nonzero length. If neither __len__() nor __bool__() is defined, all its instances are considered true. New in Python 3.0; in Python 2.X, this method is named __nonzero__ instead of __bool__, but works the same way.

Invoked on self(args...), when an instance is called like a function. arg may take any function-definition argument form (see The Expression Statement and The def Statement). For example, __call__(self, a, b, c, d=5) and __call__(self, *pargs, **kargs) both match calls self(1, 2, 3, 4) and self(1, *(2,), c=3, **dict(d=4)).

Invoked on self.name, when name is an undefined attribute access (this method is not called if name exists in or is inherited by self). name is a string. This method returns an object or raises AttributeError.

In Python 3.0, this is no longer run for __X__ attributes implicitly fetched by built-in operations; redefine such names in wrapper/proxy classes.

Invoked on self.name=value (all attribute assignments). Hint—assign through __dict__ key to avoid recursive loops: self.attr=x statement within a __setattr__ calls __setattr__ again, but self.__dict__['attr']=x does not. Recursion may also be avoided by calling the superclass version explicitly: object.__setattr__(self, name, value).

Invoked on del self.name (all attribute deletions). Hint: must avoid recursive loops by routing attribute deletions through __dict__ or a superclass, much like __setattr__.

Called unconditionally to implement attribute accesses for instances of the class. If the class also defines __getattr__, it will never be called (unless it is called explicitly). This method should return the (computed) attribute value or raise an AttributeError exception. To avoid infinite recursion in this method, its implementation should always call the superclass method with the same name to access any attributes it needs (e.g., object.__getattribute__(self, name).

In Python 3.0, this is no longer run for __X__ attributes implicitly fetched by built-in operations; redefine such names in wrapper/proxy classes.

Respectively, used on self < other, self <= other, self == other, self != other, and self > other, self >= other. Added in version 2.1, these are known as rich comparison methods and are called for all comparison expressions in Python 3.0. For example, X < Y calls X.__lt__(Y) if defined. In Python 2.X only, these methods are called in preference to __cmp__, and __ne__ is also run for self <> other.

These methods can return any value, but if the comparison operator is used in a Boolean context, the return value is interpreted as a Boolean result for the operator. These methods can also return (not raise) the special object NotImplemented if not supported for the operands (which works as though the method were not defined at all, and which forces Python 2.X to revert to the general __cmp__ method if defined).

There are no implied relationships among comparison operators. For example, x==y being true does not imply that x!=y is false: __ne__ should be defined along with __eq__ if the operators are expected to behave symmetrically. There are also no right-side (swapped-argument) versions of these methods to be used when the left argument does not support the operation but the right argument does. __lt__ and __gt__ are each other’s reflection, __le__ and __ge__ are each other’s reflection, and __eq__ and __ne__ are their own reflections. Use __lt__ for sorting in Python 3.0.

This class attribute can be assigned a string, iterable, or sequence of strings giving the names of attributes of instances of the class. If defined in a new-style class (including all classes in Python 3), __slots__ reserves space for the declared attributes, and prevents the automatic creation of __dict__ for each instance (unless string '__dict__' is included in __slots__, in which case instances also have a __dict__ and attributes not named in __slots__ may be added dynamically).

To support classes with __slots__, tools that generically list attributes or access them by string name must use storage-neutral tools such as the getattr(), setattr(), and dir(), which apply to both __dict__ and __slots__ attribute storage. Both attribute sources may need to be queried.

Called on dir(self). Returns a list of attribute names. New in Python 3.0.

Invoked on len(self) and possibly for truth-value tests. This method returns a collection’s size. For Boolean tests, Python looks for __bool__ first, then __len__, and then considers the object true (__bool__ is named __nonzero__ in Python 2). Zero length means false.

Invoked on item in self for custom membership tests (otherwise, membership uses __iter__, if defined, or __getitem__). This method returns a true or false result.

Invoked on iter(self). New in version 2.2, this method is part of the iteration protocol. It returns an object with a __next__() method (possibly self). The result object’s __next__() method is then called repeatedly in all iteration contexts (e.g., for loops), and should return the next result or raise StopIteration to terminate the results progression. See also sections The for Statement and The yield Statement. If no __iter__ is defined, iteration falls back on __getitem__. In Python 2.X, __next__() is named next().

Invoked by the next(self) built-in function, and by all iteration contexts to advance through results. See __iter__() for usage details. New in Python 3.0; in Python 2.X, this method is named next(), but works the same way.

Invoked on self[key], self[i:j:k], x in self, and for x in self (and all iteration contexts). This method implements all indexing-related operations. Iteration contexts (e.g., in and for) repeatedly index from 0 until IndexError, unless __iter__ is defined.

In Python 3.0, this and the following two methods are also called for slice operations, in which case key is a slice object. Slice objects may be propagated to another slice expression, and have attributes start, stop, and step, any of which can be None. See also slice() in Built-in Functions.

Invoked on self[key]=value, self[i:j:k]=value. This method is called for assignment to a collection key or index, or to a sequence’s slice.

Invoked on del self[key], del self[i:j:k]. This method called is for index/key and sequence slice deletion.

Called if defined by the reversed() built-in function to implement custom reverse iteration. Returns a new iterator object that iterates over all the objects in the container in reverse order. If no __reversed__ is defined, reversed() expects and uses sequence protocol (methods __len__() and __getitem__()).

Invoked on -self.

Invoked on +self.

Invoked on abs(self).

Invoked on ˜self.

Invoked on complex(self).

Invoked on int(self).

Invoked on float(self).

Invoked on round(self [, n]). New in Python 3.0.

Called to implement operator.index(). Also called in other contexts where Python requires an integer object. This includes instance appearances as indexes, as slice bounds, and as arguments to the built-in bin(), hex(), and oct() functions. Must return an integer.

Similar in Python 3.0 and 2.6, but not called for hex() and oct() in 2.6 (these require __hex__ and __oct__ methods in 2.6). In Python 3.0, __index__ subsumes and replaces the __oct__ and __hex__ methods of Python 2.X, and the returned integer is formatted automatically.

Called to get the attribute of the owner class or of an instance of that class. owner is always the owner class; instance is the instance the attribute was accessed through, or None when the attribute is accessed through the owner class directly. Return the attribute value or raise AttributeError.

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

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

Enter the runtime context related to this object. The with statement assigns this method’s return value to the target specified in the as clause of the statement (if any).

Exit the runtime context related to this object. The parameters describe the exception that caused the context to be exited. If the context exited without an exception, all three arguments are None. Return a true value to prevent a raised exception from being propagated by the caller.