Did you know Python internally uses magic method for operators e.g. for + operator uses __add__() method
number = 10 print(number + 10) #internally calls __add__() method print(number.__add__(20))
The “+” operator depends on the type of the values been added i.e. int, float,string etc.
Now adding 2 integer or float values will add the values while for string it will concatenate the string.
But what about the objects? See below example when 2 objects are added it throws error.
This can be resolved with the magic method i.e. using __add__()
See below example. Here the “+” operator when used on objects calls the __add__() method.
How to know such magic methods for various types? Use help() method
help() method displays documentation for the
help(int) help(str) help(float) help(bool)
Here is a list of operators to magic method mapping-
Comparision methods–
| Operator | Magic method | Implementation meaning or purpose |
|---|---|---|
| == | __eq__(self, other) | equality operator |
| != | __ne__(self, other) | inequality operator |
| < | __lt__(self, other) | less-than operator |
| > | __gt__(self, other) | greater-than operator |
| <= | __le__(self, other) | less-than-or-equal-to operator |
| >= | __ge__(self, other) | greater-than-or-equal-to operato |
Unary Operators
| Operator | Magic method | Implementation meaning or purpose |
|---|---|---|
| + | __pos__(self) | unary positive, like a = +b |
| – | __neg__(self) | unary negative, like a = -b |
| abs() | __abs__(self) | behavior for abs() function |
| round(a, b) | __round__(self, b) | behavior for round() function |
Binary Operators
| Operator | Magic method | Implementation meaning or purpose |
|---|---|---|
| + | __add__(self, other) | addition operator |
| – | __sub__(self, other) | subtraction operator |
| * | __mul__(self, other) | multiplication operator |
| // | __floordiv__(self, other) | integer division operator |
| / | __div__(self, other) | division operator |
| % | __mod__(self, other) | modulo operator |
| ** | __pow__(self, other) | exponential (power) operator |
Unary Operators and assignments
| Operator | Magic method | Implementation meaning or purpose |
|---|---|---|
| += | __iadd__(self, other) | addition and assignment operator |
| -= | __isub__(self, other) | subtraction and assignment operator |
| *= | __imul__(self, other) | multiplication and assignment operator |
| //= | __ifloordiv__(self, other) | integer division and assignment operator |
| /= | __idiv__(self, other) | division and assignment operator |
| %= | __imod__(self, other) | modulo and assignment operator |
| **= | __ipow__(self, other) | exponential (power) and assignment operator |
Type conversion methods
| Function | Magic method | Implementation meaning or purpose |
|---|---|---|
| int() | __int__(self) | conversion to integer type |
| float() | __float__(self) | conversion to float type |
| oct() | __oct__(self) | conversion to string, containing an octal representation |
| hex() | __hex__(self) | conversion to string, containing a hexadecimal representation |
Object introspection
| Function | Magic method | Implementation meaning or purpose |
|---|---|---|
| str() | __str__(self) | responsible for handling str() function calls |
| repr() | __repr__(self) | responsible for handling repr() function calls |
| format() | __format__(self, formatstr) | called when new-style string formatting is applied to an object |
| hash() | __hash__(self) | responsible for handling hash() function calls |
| dir() | __dir__(self) | responsible for handling dir() function calls |
| bool() | __nonzero__(self) | responsible for handling bool() function calls |
Object retrospection
| Function | Magic method | Implementation meaning or purpose |
|---|---|---|
| isinstance(object, class) | __instancecheck__(self, object) | responsible for handling isinstance() function calls |
| issubclass(subclass, class) | __subclasscheck__(self, subclass) | responsible for handling issubclass() function calls |
Object attribute access
| Expression example | Magic method | Implementation meaning or purpose |
|---|---|---|
| object.attribute | __getattr__(self, attribute) | responsible for handling access to a non-existing attribute |
| object.attribute | __getattribute__(self, attribute) | responsible for handling access to an existing attribute |
| object.attribute = value | __setattr__(self, attribute, value) | responsible for setting an attribute value |
| del object.attribute | __delattr__(self, attribute) | responsible for deleting an attribute |
Methods allowing access to containers
| Expression example | Magic method | Implementation meaning or purpose |
|---|---|---|
| len(container) | __len__(self) | returns the length (number of elements) of the container |
| container[key] | __getitem__(self, key) | responsible for accessing (fetching) an element identified by the key argument |
| container[key] = value | __setitem__(self, key, value) | responsible for setting a value to an element identified by the key argument |
| del container[key] | __delitem__(self, key) | responsible for deleting an element identified by the key argument |
| for element in container | __iter__(self) | returns an iterator for the container |
| item in container | __contains__(self, item) | responds to the question: does the container contain the selected item? |
References – https://docs.python.org/3/reference/datamodel.html#special-method-names