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Safari | Python Developer's Handbook -> Special Methods





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Special Methods

Python exposes some special methods that are easily highlighted in the code because they start and end with __ (double underscores). These methods override (inherit) built-in functions of the same name that are provided by Python itself. The next list shows some of the most used special methods.

__init__(self) This is the constructor method, which is called during creation of instances. Usually, this is the place where the instance variables are initialized, among other things.

__str__(self) This method is called when str() is called on instances of this type. It specifies how the object must be displayed when it is used as a string (for example, when a print command is applied to an object).

__repr__(self) This method is called when repr() is called on instances of this type. This method provides a readable representation of the object. Usually, it is possible to re-create an object by using this method. Although not guaranteed, and the standard repr of an instance can't be executed to re-create the instance.

__getattr__(self, name) Implement this method to trap or modify the access to nonexisting members, for example, returning the attribute self.name.

__setattr__(self, name, value) This method allows you to control setting of attributes in the instance. It assigns the given value to the self.name instance's attribute. Note that you can also use "self.__dict__['attr'] = " to set attributes from within __setattr__ (if you do it the normal way, you will get infinite recursion).

__delattr__(self,name) Implement this method to delete a specific attribute of an object. It's like saying del self.name.

__del__(self) The __del__ method covers the deletion of the object. Be careful because sometimes it isn't immediately used when an object is destroyed (JPython behavior). CPython's garbage collector destructs objects as soon as their reference count reaches zero.

__cmp__(self,other) Implement this method to compare and return a negative, zero, or positive number.

__hash__(self) Implement this method to generate a 32-bit hash index.

__nonzero__(self) Implement this method to return 0 or 1 for truth-value testing.

__call__(self) Classes that implement the __call__ method are callable, and their instances can be invoked like a function. This is the concept used by the built-in functions. The syntax obj(*args) is equivalent to obj.__call__(*args).

__getitem__(self, index) This method supports list indexing, returning self[index].

							
>>> class Seq:
     def __getitem__(self, i):
        if i < 5:
            return i
        else:
            raise IndexError

>>> s = Seq()
>>> for i in s:
>>>    print i,
0, 1, 2, 3, 4
>>> print s[2]
2
>>> print s[6]
Traceback (innermost last):
  File "<stdin>", line 1, in ?
  File "<stdin>", line 6, in __getitem__
IndexError

						

Next, you have some more special methods that deal with sequence and number-related methods.

__len__(self)This method is called to return the length of the instance when len() is called on an instance of this type.

__add__ (self, other) Implement this method to return self + other.

__sub__ (self, other) Implement this method to return self other.

__mul__ (self, other) Implement this method to return self * other.

__div__ (self, other) Implement this method to return self / other.

__mod__ (self, other) Implement this method to return self % other.

__neg__ (self) Implement this method to return self negated.

__pos__ (self) Implement this method to return self positive.

__abs__ (self) This method is called to return the absolute value of self when abs() is called on instances of this type.

__inv__ (self) Implement this method to return the inverse of self.

__lshift__ (self, other) Implement this method to return self shifted left by other.

__rshift__ (self, other) Implement this method to return self shifted right by other.

__and__ (self, other) Implement this method to return the bitwise and value of self and other.

__or__ (self, other) Implement this method to return the bitwise or value of self and other.

__xor__ (self, other) Implement this method to return the bitwise exclusive or value of self and other.

__not__ (self) Implement this method to return the outcome of not self. (Note that there is no __not__ ... ... (Not finished, please download and read the complete file)

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