3.3.1 Basic customization

##
3.3.1 Basic customization

`__new__`( | cls[, ...])

  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).

`__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.