Operator overloading

Below are the operators that can be overloaded in classes, along with the method definitions that are required, and an example of the operator in use within an expression.

N.B. The use of other as a variable name is not mandatory, but is considered the norm.

Operator | Method | Expression | –––– | –––– | ––––– |\+ Addition | __add__(self, other) | a1 + a2 |\- Subtraction | __sub__(self, other) | a1 - a2 |\* Multiplication | __mul__(self, other) | a1 * a2 |@ Matrix Multiplication | __matmul__(self, other) | a1 @ a2 (Python 3.5) |/ Division | __div__(self, other) | a1 / a2 (Python 2 only)|/ Division | __truediv__(self, other) | a1 / a2 (Python 3) |// Floor Division | __floordiv__(self, other) | a1 // a2 |% Modulo/Remainder | __mod__(self, other) | a1 % a2 |** Power | __pow__(self, other[, modulo]) | a1 ** a2 |<< Bitwise Left Shift | __lshift__(self, other) | a1 << a2 |>> Bitwise Right Shift | __rshift__(self, other) | a1 >> a2 |& Bitwise AND | __and__(self, other) | a1 & a2 |^ Bitwise XOR | __xor__(self, other) | a1 ^ a2 | | (Bitwise OR) | __or__(self, other) | a1 | a2 |\- Negation (Arithmetic) | __neg__(self) | -a1 |\+ Positive | __pos__(self) | +a1 |~ Bitwise NOT | __invert__(self) | ~a1 |\< Less than | __lt__(self, other) | a1 < a2 |<= Less than or Equal to | __le__(self, other) | a1 <= a2 |== Equal to | __eq__(self, other) | a1 == a2 |!= Not Equal to | __ne__(self, other) | a1 != a2 |\> Greater than | __gt__(self, other) | a1 > a2 |>= Greater than or Equal to | __ge__(self, other) | a1 >= a2 |[index] Index operator | __getitem__(self, index) | a1[index] |in In operator | __contains__(self, other) | a2 in a1 |(*args, ...) Calling | __call__(self, *args, **kwargs) | a1(*args, **kwargs) |

The optional parameter modulo for __pow__ is only used by the pow built-in function.

Each of the methods corresponding to a binary operator has a corresponding “right” method which start with __r, for example __radd__:

class A:
    def __init__(self, a):
        self.a = a
    def __add__(self, other):
        return self.a + other
    def __radd__(self, other):
        print("radd")
        return other + self.a

A(1) + 2  # Out:  3
2 + A(1)  # prints radd. Out: 3

as well as a corresponding inplace version, starting with __i:

class B:
    def __init__(self, b):
        self.b = b
    def __iadd__(self, other):
        self.b += other
        print("iadd")
        return self

b = B(2)
b.b       # Out: 2
b += 1    # prints iadd
b.b       # Out: 3

Since there’s nothing special about these methods, many other parts of the language, parts of the standard library, and even third-party modules add magic methods on their own, like methods to cast an object to a type or checking properties of the object. For example, the builtin str() function calls the object’s __str__ method, if it exists. Some of these uses are listed below.

Function | Method | Expression | –––– | –––– | ––––– | Casting to int | __int__(self) | int(a1) | Absolute function | __abs__(self) | abs(a1) | Casting to str | __str__(self) | str(a1) | Casting to unicode | __unicode__(self) | unicode(a1) (Python 2 only)| String representation | __repr__(self) | repr(a1) | Casting to bool | __nonzero__(self) | bool(a1) | String formatting | __format__(self, formatstr) | "Hi {:abc}".format(a1) | Hashing | __hash__(self) | hash(a1) | Length | __len__(self) | len(a1) || Rounding | __round__(self) | round(a1) | Reversed | __reversed__(self) | reversed(a1) || Floor | __floor__(self) | math.floor(a1) | Floor | __floor__(self) | math.floor(a1) | Ceiling | __ceil__(self) | math.ceil(a1) |

There are also the special methods __enter__ and __exit__ for context managers, and many more.