3.3.1 Basic customization

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3.3.1 Basic customization

`__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.