How to implement command design pattern in python?

08.12.2021 — Tutorial, Python, Pattern — 1 min read

src/interface.py

1import abc
2
3
4class CalcCmdAbs(metaclass=abc.ABCMeta):
5    def __init__(self, *args, **kwargs):
6        self._args = args
7        self._kwargs = kwargs
8
9    @property
10    @abc.abstractmethod
11    def name(self):
12        raise NotImplemented()
13
14    def execute(self):
15        raise NotImplemented()

src/commands.py

1from .interface import CalcCmdAbs
2
3OPERATORS = list()
4
5
6def register_operator(cls):
7    OPERATORS.append(cls)
8    return cls
9
10
11@register_operator
12class AddCmd(CalcCmdAbs):
13    name = "add"
14
15    def __init__(self, a, b):
16        super().__init__(a, b)
17        self.a = a
18        self.b = b
19
20    def execute(self):
21        return self.a + self.b
22
23
24@register_operator
25class SubtractCmd(CalcCmdAbs):
26    name = "subtract"
27
28    def __init__(self, a, b):
29        super().__init__(a, b)
30        self.a = a
31        self.b = b
32
33    def execute(self):
34        return self.a - self.b
35
36
37class NoneCmd(CalcCmdAbs):
38    name = "None Command"
39
40    def execute(self):
41        print("Invalid Command")

src/utils.py

1from .commands import OPERATORS, NoneCmd
2
3
4def get_commands() -> dict:
5    return dict([cmd.name, cmd] for cmd in OPERATORS)
6
7
8def parse_commands(commands: dict, operation: str, *args):
9    command = commands.setdefault(operation, NoneCmd)
10    return command(*args)

src/tests.py

1import unittest
2
3from .utils import get_commands, parse_commands
4
5
6class TestCase(unittest.TestCase):
7
8    def setUp(self) -> None:
9        self.commands = get_commands()
10
11    def test_addition(self):
12        command = parse_commands(self.commands, "add", 1, 1)
13        self.assertEqual(command.execute(), 2, "Addition fail")
14
15    def test_subtraction(self):
16        command = parse_commands(self.commands, "subtract", 1, 1)
17        self.assertEqual(command.execute(), 0, "Subtraction fail")