4.4. Operators¶

4.4.1. Rationale¶

Operator Overload
Readable syntax
Simpler operations
Following examples uses dataclasses to focus on action code, not boilerplate

>>> from dataclasses import dataclass
>>>
>>>
>>> @dataclass
... class Vector:
...     x: int = 0
...     y: int = 0
>>>
>>>
>>> Vector(x=1, y=2) + Vector(x=3, y=4)
Traceback (most recent call last):
TypeError: unsupported operand type(s) for +: 'Vector' and 'Vector'

>>> from dataclasses import dataclass
>>>
>>>
>>> @dataclass
... class Vector:
...     x: int = 0
...     y: int = 0
...
...     def __add__(self, other):
...         return Vector(
...             self.x + other.x,
...             self.y + other.y)
>>>
>>>
>>> Vector(x=1, y=2) + Vector(x=3, y=4)
Vector(x=4, y=6)
>>>
>>> Vector(x=1, y=2) + Vector(x=3, y=4) + Vector(x=5, y=6)
Vector(x=9, y=12)

4.4.2. Numerical Operators¶

Table 4.2. Numerical Operator Overload¶
Operator	Method
`obj + other`	`obj.__add__(other)`
`obj - other`	`obj.__sub__(other)`
`obj * other`	`obj.__mul__(other)`
`obj / other`	`obj.__truediv__(other)`
`obj // other`	`obj.__floordiv__(other)`
`obj ** other`	`obj.__pow__(other)`
`obj % other`	`obj.__mod__(other)`
`obj @ other`	`obj.__matmul__(other)`

`obj += other`	`obj.__iadd__(other)`
`obj -= other`	`obj.__isub__(other)`
`obj *= other`	`obj.__imul__(other)`
`obj /= other`	`obj.__idiv__(other)`
`obj //= other`	`obj.__itruediv__(other)`
`obj **= other`	`obj.__ipow__(other)`
`obj %= other`	`obj.__imod__(other)`
`obj @= other`	`obj.__imatmul__(other)`

% (__mod__) operator behavior for int and str:

>>> class Int:
...     def __mod__(self, other):
...         """modulo division"""
>>>
>>> 3 % 2
1
>>> 4 % 2
0

>>> class Str:
...     def __mod__(self, other):
...         """str substitute"""
...
...         if type(other) is str:
...             ...
...         if type(other) is tuple:
...             ...
...         if type(other) is dict:
...             ...

>>> 'Echo' % 2
Traceback (most recent call last):
TypeError: not all arguments converted during string formatting
>>> 'Echo %s' % 2
'Echo 2'
>>> 'Echo %d' % 2
'Echo 2'
>>> 'Echo %f' % 2
'Echo 2.000000'
>>> 'Echo %s %s' % (1, 2)
'Echo 1 2'
>>> 'Echo %s %d %f' % (1, 2, 3)
'Echo 1 2 3.000000'
>>>
>>> 'Echo %(firstname)s %(lastname)s' % {'firstname': 'Mark', 'lastname': 'Watney'}
'Echo Mark Watney'
>>>
>>> 'Echo %(name)s %(age)d' % {'name': 'Mark Watney', 'age': 44}
'Echo Mark Watney 44'

%s, %d, %f is currently deprecated in favor of f'...' string formatting. More information in Builtin Printing

4.4.3. Comparison Operators¶

Table 4.3. Comparison Operators Overload¶
Operator	Method
`obj == other`	`obj.__eq__(other)`
`obj != other`	`obj.__ne__(other)`
`obj < other`	`obj.__lt__(other)`
`obj <= other`	`obj.__le__(other)`
`obj > other`	`obj.__gt__(other)`
`obj >= other`	`obj.__ge__(other)`

>>> from dataclasses import dataclass
>>>
>>>
>>> @dataclass
... class Vector:
...     x: int = 0
...     y: int = 0
...
...     def __eq__(self, other):
...         if (self.x == other.x) and (self.y == other.y):
...             return True
...         else:
...             return False
>>>
>>>
>>> Vector(x=1, y=2) == Vector(x=3, y=4)
False
>>>
>>> Vector(x=1, y=2) == Vector(x=1, y=2)
True

4.4.4. Boolean Operators¶

Table 4.4. Boolean Operators Overload¶
Operator	Method
`-obj`	`obj.__neg__()`
`+obj`	`obj.__pos__()`
`~obj`	`obj.__invert__()`

`obj & other`	`obj.__and__(other)`
`obj \| other`	`obj.__or__(other)`
`obj ^ other`	`obj.__xor__(other)`
`obj << other`	`obj.__lshift__(other)`
`obj >> other`	`obj.__rshift__(other)`

`obj &= other`	`obj.__iand__(other)`
`obj \|= other`	`obj.__ior__(other)`
`obj ^= other`	`obj.__ixor__(other)`
`obj <<= other`	`obj.__ilshift__(other)`
`obj >>= other`	`obj.__irshift__(other)`

& 1 = 1
& 0 = 0
& 1 = 0
& 0 = 0

| 1 = 1
| 0 = 1
| 1 = 1
| 0 = 0

^ 1 = 0
^ 0 = 1
^ 1 = 1
^ 0 = 0

>>> from dataclasses import dataclass
>>>
>>>
>>> @dataclass
... class Digit:
...     value: int
...
...     def __xor__(self, other):
...         return Digit(self.value ** other.value)
>>>
>>>
>>> a = Digit(2)
>>> b = Digit(4)
>>>
>>> a ^ b
Digit(value=16)

4.4.5. Builtin Functions and Keywords¶

Table 4.5. Builtin Functions Overload¶
Function	Method
`abs(obj)`	`obj.__abs__()`
`bool(obj)`	`obj.__bool__()`
`complex(obj)`	`obj.__complex__()`
`del obj`	`obj.__del__()`
`delattr(obj, name)`	`obj.__delattr__(name)`
`dir(obj)`	`obj.__dir__()`
`divmod(obj, other)`	`obj.__divmod__(other)`
`float(obj)`	`obj.__float__()`
`getattr(obj, name, default)`	`obj.__getattr__(name, default)`
`hash(obj)`	`obj.__hash__()`
`hex(obj)`	`obj.__hex__()`
`int(obj)`	`obj.__int__()`
`iter(obj)`	`obj.__iter__()`
`len(obj)`	`obj.__len__()`
`next(obj)`	`obj.__next__()`
`oct(obj)`	`obj.__oct__()`
`pow(obj)`	`obj.__pow__()`
`reversed(obj)`	`obj.__reversed__()`
`round(obj, ndigits)`	`obj.__round__(ndigits)`
`setattr(obj, name)`	`obj.__setattr__(name)`

>>> from math import sqrt
>>> from dataclasses import dataclass
>>>
>>>
>>> @dataclass
... class Vector:
...     x: int = 0
...     y: int = 0
...
...     def __abs__(self):
...         return sqrt(self.x**2 + self.y**2)
>>>
>>>
>>> abs(Vector(x=3, y=4))
5.0

>>> class Astronaut:
...     def __float__(self) -> float:
...         return 1961.0
...
...     def __int__(self) -> int:
...         return 1969
...
...     def __len__(self) -> int:
...         return 170
...
...     def __str__(self) -> str:
...         return 'My name... José Jiménez'
...
...     def __repr__(self) -> str:
...         return f'Astronaut()'
>>>
>>> astro = Astronaut()
>>>
>>> float(astro)
1961.0
>>>
>>> int(astro)
1969
>>>
>>> len(astro)
170
>>>
>>> repr(astro)
'Astronaut()'
>>>
>>> str(astro)
'My name... José Jiménez'
>>>
>>> print(astro)
My name... José Jiménez

4.4.6. Accessors Overload¶

Table 4.6. Operator Overload¶
Operator	Method	Remarks
`obj(x)`	`obj.__call__(x)`
`obj[x]`	`obj.__getitem__(x)`
`obj[x]`	`obj.__missing__(x)`	(when `x` is not in `obj`)
`obj[x] = 10`	`obj.__setitem__(x, 10)`
`del obj[x]`	`obj.__delitem__(x)`
`x in obj`	`obj.__contains__(x)`

>>> data = slice(1, 2, 3)
>>>
>>> data.start
1
>>> data.stop
2
>>> data.step
3

>>> class MyClass:
...     def __getitem__(self, item):
...         print(item)
>>>
>>>
>>> my = MyClass()
>>> my[1:2]
slice(1, 2, None)

>>> data = dict()
>>>
>>> data['a'] = 10  # data.__setitem__('a', 10) -> None
>>> data['a']       # data.__getitem__('a') -> 10
10
>>>
>>> data['x']       # data.__getitem__('x') -> data.__missing__() -> KeyError: 'x'
Traceback (most recent call last):
KeyError: x
>>>
>>> data()          # data.__call__() -> TypeError: 'dict' object is not callable
Traceback (most recent call last):
TypeError: 'dict' object is not callable

Contains in numpy:

>>> import numpy as np
>>>
>>>
>>> data = np.array([[1, 2, 3],
...                  [4, 5, 6]])
>>>
>>> data[1][2]
6
>>>
>>> data[1,2]
6
>>>
>>> data[1:2]
array([[4, 5, 6]])
>>>
>>> data[1:2, 0]
array([4])
>>>
>>> data[1:2, 1:]
array([[5, 6]])

Intuitive implementation of numpy array[row,col] accessor:

>>> class array(list):
...     def __getitem__(key):
...         if isinstance(key, int):
...             return super().__getitem__(key)
...
...         if isinstance(key, tuple):
...             row = key[0]
...             col = key[1]
...             return super().__getitem__(row).__getitem__(col)
...
...         if isinstance(key, slice):
...             start = key[0] if key[0] else 0
...             stop = key[1] if key[0] else len(self)
...             step = key[2] if key[2] else 1
...             return ...
>>>
>>>
>>> data[1]         # data.__getitem__(1)
array([4, 5, 6])
>>>
>>> data[1,2]       # data.__getitem__((1,2))
6
>>>
>>> data[1:2]       # data.__getitem__(1:2)  # data.__getitem__(slice(1,2))
array([[4, 5, 6]])
>>>
>>> data[:, 2]      # data.__getitem__((:, 2))  # data.__getitem__((slice(), 2))
array([3, 6])

4.4.7. Eq Works at Both Sides¶

>>> class Astronaut:
...     def __init__(self, firstname, lastname):
...         self.firstname = firstname
...         self.lastname = lastname
>>>
>>>
>>> class Cosmonaut:
...     def __init__(self, firstname, lastname):
...         self.firstname = firstname
...         self.lastname = lastname
>>>
>>>
>>> a = Astronaut('Mark', 'Watney')
>>> c = Cosmonaut('Mark', 'Watney')
>>>
>>> print(a == c)
False

>>> class Astronaut:
...     def __init__(self, firstname, lastname):
...         self.firstname = firstname
...         self.lastname = lastname
...
...     def __eq__(self, other):
...         return (self.firstname == other.firstname) \
...            and (self.lastname == other.lastname)
>>>
>>>
>>> class Cosmonaut:
...     def __init__(self, firstname, lastname):
...         self.firstname = firstname
...         self.lastname = lastname
>>>
>>>
>>> a = Astronaut('Mark', 'Watney')
>>> c = Cosmonaut('Mark', 'Watney')
>>>
>>> print(a == c)
True

>>> class Astronaut:
...     def __init__(self, firstname, lastname):
...         self.firstname = firstname
...         self.lastname = lastname
>>>
>>>
>>> class Cosmonaut:
...     def __init__(self, firstname, lastname):
...         self.firstname = firstname
...         self.lastname = lastname
...
...     def __eq__(self, other):
...         return (self.firstname == other.firstname) \
...            and (self.lastname == other.lastname)
>>>
>>>
>>> a = Astronaut('Mark', 'Watney')
>>> c = Cosmonaut('Mark', 'Watney')
>>>
>>> print(a == c)
True

4.4.8. Use Cases¶

>>> 
...
... hero @ Position(x=50, y=120)
... hero >> Direction(left=10, up=20)
...
... hero < Damage(20)
... hero > Damage(20)
...
... hero['gold'] += dragon['gold']

>>> class Cache(dict):
...     def __init__(self, func):
...         self.func = func
...
...     def __call__(self, *args):
...         if args not in self:
...             self[args] = self.func(*args)
...         return self[args]
>>>
>>>
>>> @Cache
... def add(a, b):
...     return a + b
>>>
>>>
>>> _ = add(1,2)  # computed
>>> _ = add(1,2)  # fetched from cache
>>> _ = add(1,2)  # fetched from cache
>>> _ = add(1,2)  # fetched from cache
>>> _ = add(2,1)  # computed
>>> _ = add(2,1)  # fetched from cache
>>>
>>> add  
{(1, 2): 3,
 (2, 1): 3}

>>> class Cache(dict):
...     def __init__(self, func):
...         self.func = func
...
...     def __call__(self, *args):
...         return self[args]
...
...     def __missing__(self, key):
...         self[key] = self.func(*key)
...         return self[key]
>>>
>>>
>>> @Cache
... def add(a, b):
...     return a + b
>>>
>>>
>>> _ = add(1,2)  # computed
>>> _ = add(1,2)  # fetched from cache
>>> _ = add(1,2)  # fetched from cache
>>> _ = add(1,2)  # fetched from cache
>>> _ = add(2,1)  # computed
>>> _ = add(2,1)  # fetched from cache
>>>
>>> add  
{(1, 2): 3,
 (2, 1): 3}

4.4.9. Further Reading¶

4.4.10. Assignments¶

Code 4.6. Solution¶

"""
* Assignment: OOP Overload Matmul
* Complexity: easy
* Lines of code: 3 lines
* Time: 3 min

English:
    1. Use code from "Given" section (see below)
    2. Overload `@` operator
    3. Set position based on argument `tuple[int, int]`
    4. Compare result with "Tests" section (see below)

Polish:
    1. Użyj kodu z sekcji "Given" (patrz poniżej)
    2. Przeciąż operator `@`
    3. Ustaw pozycję na podstawie argumentu `tuple[int, int]`
    4. Porównaj wyniki z sekcją "Tests" (patrz poniżej)

Tests:
    >>> position = Position()
    >>> position
    Position(x=0, y=0)
    >>> position @ (1, 2)
    >>> position
    Position(x=1, y=2)
"""


# Given
from dataclasses import dataclass


@dataclass
class Position:
    x: int = 0
    y: int = 0

Code 4.7. Solution¶

"""
* Assignment: OOP Overload IAdd
* Complexity: easy
* Lines of code: 3 lines
* Time: 5 min

English:
    1. Use code from "Given" section (see below)
    2. Override operator `+=` for code to work correctly
    3. Compare result with "Tests" section (see below)

Polish:
    1. Użyj kodu z sekcji "Given" (patrz poniżej)
    2. Nadpisz operatory `+=` aby poniższy kod zadziałał poprawnie
    3. Porównaj wyniki z sekcją "Tests" (patrz poniżej)

Hints:
    * `obj.__iadd__(other) -> self`

Tests:
    >>> astro = Astronaut('Jan Twardowski', missions=[
    ...     Mission(1969, 'Apollo 11'),
    ... ])
    >>> astro += Mission(2024, 'Artemis 3')
    >>> astro += Mission(2035, 'Ares 3')

    >>> print(astro)  # doctest: +NORMALIZE_WHITESPACE
    Astronaut(name='Jan Twardowski',
              missions=[Mission(year=1969, name='Apollo 11'),
                        Mission(year=2024, name='Artemis 3'),
                        Mission(year=2035, name='Ares 3')])
"""


# Given
from dataclasses import dataclass


@dataclass
class Astronaut:
    name: str
    missions: list


@dataclass
class Mission:
    year: int
    name: str

Code 4.8. Solution¶

"""
* Assignment: OOP Overload Equals
* Complexity: easy
* Lines of code: 3 lines
* Time: 5 min

English:
    1. Use code from "Given" section (see below)
    2. Override operator for code to work correctly
    3. Do not use `dataclasses`
    4. Compare result with "Tests" section (see below)

Polish:
    1. Użyj kodu z sekcji "Given" (patrz poniżej)
    2. Nadpisz operator aby poniższy kod zadziałał poprawnie
    3. Nie używaj `dataclasses`
    4. Porównaj wyniki z sekcją "Tests" (patrz poniżej)

Tests:
    >>> Mission(2035, 'Ares 3') == Mission(2035, 'Ares 3')
    True
    >>> Mission(2035, 'Ares 3') == Mission(1973, 'Apollo 18')
    False
    >>> Mission(2035, 'Ares 3') == Mission(2035, 'Apollo 18')
    False
    >>> Mission(2035, 'Ares 3') == Mission(1973, 'Ares 3')
    False
"""


# Given
class Mission:
    def __init__(self, year, name):
        self.year = year
        self.name = name

Code 4.9. Solution¶

"""
* Assignment: OOP Overload Contains
* Complexity: easy
* Lines of code: 5 lines
* Time: 8 min

English:
    1. Use code from "Given" section (see below)
    2. Override operators for code to work correctly
    3. Do not use `dataclasses`
    4. Compare result with "Tests" section (see below)

Polish:
    1. Użyj kodu z sekcji "Given" (patrz poniżej)
    2. Nadpisz operatory aby poniższy kod zadziałał poprawnie
    3. Nie używaj `dataclasses`
    4. Porównaj wyniki z sekcją "Tests" (patrz poniżej)

Tests:
    >>> astro = Astronaut('Jan Twardowski', missions=[
    ...     Mission(1969, 'Apollo 11'),
    ...     Mission(2024, 'Artemis 3'),
    ...     Mission(2035, 'Ares 3'),
    ... ])

    >>> Mission(2035, 'Ares 3') == Mission(2035, 'Ares 3')
    True
    >>> Mission(2035, 'Ares 3') == Mission(1973, 'Apollo 18')
    False
    >>> Mission(2035, 'Ares 3') == Mission(2035, 'Apollo 18')
    False
    >>> Mission(2035, 'Ares 3') == Mission(1973, 'Ares 3')
    False

    >>> Mission(2024, 'Artemis 3') in astro
    True
    >>> Mission(1973, 'Apollo 18') in astro
    False
"""


# Given
class Astronaut:
    def __init__(self, name, missions):
        self.name = name
        self.missions = missions


class Mission:
    def __init__(self, year, name):
        self.year = year
        self.name = name