Merge pull request #684 from realpython/python-mixins

Materials for Python Mixins

Author: brendaweles

python-mixins/README.md

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+# What Are Mixin Classes in Python?
+
+This folder contains sample code for the Real Python tutorial [What Are Mixin Classes in Python?](https://realpython.com/python-mixin/).

python-mixins/mixins.py

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+import json
+from typing import Self
+
+
+class SerializableMixin:
+    def serialize(self) -> dict:
+        if hasattr(self, "__slots__"):
+            return {name: getattr(self, name) for name in self.__slots__}
+        else:
+            return vars(self)
+
+
+class JSONSerializableMixin:
+    @classmethod
+    def from_json(cls, json_string: str) -> Self:
+        return cls(**json.loads(json_string))
+
+    def as_json(self) -> str:
+        return json.dumps(vars(self))
+
+
+class TypedKeyMixin:
+    def __init__(self, key_type, *args, **kwargs):
+        super().__init__(*args, **kwargs)
+        self.__type = key_type
+
+    def __setitem__(self, key, value):
+        if not isinstance(key, self.__type):
+            raise TypeError(f"key must be {self.__type} but was {type(key)}")
+        super().__setitem__(key, value)
+
+
+class TypedValueMixin:
+    def __init__(self, value_type, *args, **kwargs):
+        super().__init__(*args, **kwargs)
+        self.__type = value_type
+
+    def __setitem__(self, key, value):
+        if not isinstance(value, self.__type):
+            if not isinstance(value, self.__type):
+                raise TypeError(
+                    f"value must be {self.__type} but was {type(value)}"
+                )
+        super().__setitem__(key, value)
+
+
+if __name__ == "__main__":
+    from collections import UserDict
+    from dataclasses import dataclass
+    from pathlib import Path
+    from types import SimpleNamespace
+
+    @dataclass
+    class User(JSONSerializableMixin):
+        user_id: int
+        email: str
+
+    class AppSettings(JSONSerializableMixin, SimpleNamespace):
+        def save(self, filepath: str | Path) -> None:
+            Path(filepath).write_text(self.as_json(), encoding="utf-8")
+
+    class Inventory(TypedKeyMixin, TypedValueMixin, UserDict):
+        pass
+
+    user = User(555, "[email protected]")
+    print(user.as_json())
+
+    settings = AppSettings()
+    settings.host = "localhost"
+    settings.port = 8080
+    settings.debug_mode = True
+    settings.log_file = None
+    settings.urls = (
+        "https://192.168.1.200:8000",
+        "https://192.168.1.201:8000",
+    )
+    settings.save("settings.json")
+
+    fruits = Inventory(str, int)
+    fruits["apples"] = 42
+
+    try:
+        fruits["🍌".encode("utf-8")] = 15
+    except TypeError as ex:
+        print(ex)
+
+    try:
+        fruits["bananas"] = 3.5
+    except TypeError as ex:
+        print(ex)

python-mixins/stateful_v1.py

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+class TypedKeyMixin:
+    key_type = object
+
+    def __setitem__(self, key, value):
+        if not isinstance(key, self.key_type):
+            raise TypeError(f"key must be {self.key_type} but was {type(key)}")
+        super().__setitem__(key, value)
+
+
+class TypedValueMixin:
+    value_type = object
+
+    def __setitem__(self, key, value):
+        if not isinstance(value, self.value_type):
+            raise TypeError(
+                f"value must be {self.value_type} but was {type(value)}"
+            )
+        super().__setitem__(key, value)
+
+
+if __name__ == "__main__":
+    from collections import UserDict
+
+    class Inventory(TypedKeyMixin, TypedValueMixin, UserDict):
+        key_type = str
+        value_type = int
+
+    fruits = Inventory()
+    fruits["apples"] = 42
+
+    try:
+        fruits["🍌".encode("utf-8")] = 15
+    except TypeError as ex:
+        print(ex)
+
+    try:
+        fruits["bananas"] = 3.5
+    except TypeError as ex:
+        print(ex)
+
+    print(f"{vars(fruits) = }")

python-mixins/stateful_v2.py

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+def TypedKeyMixin(key_type=object):
+    class _:
+        def __setitem__(self, key, value):
+            if not isinstance(key, key_type):
+                raise TypeError(f"key must be {key_type} but was {type(key)}")
+            super().__setitem__(key, value)
+
+    return _
+
+
+def TypedValueMixin(value_type=object):
+    class _:
+        def __setitem__(self, key, value):
+            if not isinstance(value, value_type):
+                raise TypeError(
+                    f"value must be {value_type} but was {type(value)}"
+                )
+            super().__setitem__(key, value)
+
+    return _
+
+
+if __name__ == "__main__":
+    from collections import UserDict
+
+    class Inventory(TypedKeyMixin(str), TypedValueMixin(int), UserDict):
+        key_type = "This attribute has nothing to collide with"
+
+    fruits = Inventory()
+    fruits["apples"] = 42
+
+    try:
+        fruits["🍌".encode("utf-8")] = 15
+    except TypeError as ex:
+        print(ex)
+
+    try:
+        fruits["bananas"] = 3.5
+    except TypeError as ex:
+        print(ex)
+
+    print(f"{vars(fruits) = }")
+    print(f"{Inventory.key_type = }")

python-mixins/stateful_v3.py

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+def key_type(expected_type):
+    def class_decorator(cls):
+        setitem = cls.__setitem__
+
+        def __setitem__(self, key, value):
+            if not isinstance(key, expected_type):
+                raise TypeError(
+                    f"key must be {expected_type} but was {type(key)}"
+                )
+            return setitem(self, key, value)
+
+        cls.__setitem__ = __setitem__
+        return cls
+
+    return class_decorator
+
+
+def value_type(expected_type):
+    def class_decorator(cls):
+        setitem = cls.__setitem__
+
+        def __setitem__(self, key, value):
+            if not isinstance(value, expected_type):
+                raise TypeError(
+                    f"value must be {expected_type} but was {type(value)}"
+                )
+            return setitem(self, key, value)
+
+        cls.__setitem__ = __setitem__
+        return cls
+
+    return class_decorator
+
+
+if __name__ == "__main__":
+    from collections import UserDict
+
+    @key_type(str)
+    @value_type(int)
+    class Inventory(UserDict):
+        key_type = "This attribute has nothing to collide with"
+
+    fruits = Inventory()
+    fruits["apples"] = 42
+
+    try:
+        fruits["🍌".encode("utf-8")] = 15
+    except TypeError as ex:
+        print(ex)
+
+    try:
+        fruits["bananas"] = 3.5
+    except TypeError as ex:
+        print(ex)
+
+    print(f"{vars(fruits) = }")
+    print(f"{Inventory.key_type = }")

python-mixins/typed_dict.py

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+def typed_dict(key_type=object, value_type=object):
+    def class_decorator(cls):
+        setitem = cls.__setitem__
+
+        def __setitem__(self, key, value):
+            if not isinstance(key, key_type):
+                raise TypeError(
+                    f"value must be {key_type} but was {type(key)}"
+                )
+
+            if not isinstance(value, value_type):
+                raise TypeError(
+                    f"value must be {value_type} but was {type(value)}"
+                )
+
+            setitem(self, key, value)
+
+        cls.__setitem__ = __setitem__
+
+        return cls
+
+    return class_decorator
+
+
+if __name__ == "__main__":
+    from collections import UserDict
+
+    # Enforce str keys and int values:
+    @typed_dict(str, int)
+    class Inventory(UserDict):
+        pass
+
+    # Enforce str keys, allow any value type:
+    @typed_dict(str)
+    class AppSettings(UserDict):
+        pass
+
+    fruits = Inventory()
+    fruits["apples"] = 42
+
+    try:
+        fruits["🍌".encode("utf-8")] = 15
+    except TypeError as ex:
+        print(ex)
+
+    try:
+        fruits["bananas"] = 3.5
+    except TypeError as ex:
+        print(ex)
+
+    settings = AppSettings()
+    settings["host"] = "localhost"
+    settings["port"] = 8080
+    settings["debug_mode"] = True
+
+    try:
+        settings[b"binary data"] = "nope"
+    except TypeError as ex:
+        print(ex)

python-mixins/utils.py

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+from collections import UserDict
+
+
+class DebugMixin:
+    def __setitem__(self, key, value):
+        super().__setitem__(key, value)
+        print(f"Item set: {key=!r}, {value=!r}")
+
+    def __delitem__(self, key):
+        super().__delitem__(key)
+        print(f"Item deleted: {key=!r}")
+
+
+class CaseInsensitiveDict(DebugMixin, UserDict):
+    def __setitem__(self, key: str, value: str) -> None:
+        super().__setitem__(key.lower(), value)
+
+    def __getitem__(self, key: str) -> str:
+        return super().__getitem__(key.lower())
+
+    def __delitem__(self, key: str) -> None:
+        super().__delitem__(key.lower())
+
+    def __contains__(self, key: str) -> bool:
+        return super().__contains__(key.lower())
+
+    def get(self, key: str, default: str = "") -> str:
+        return super().get(key.lower(), default)
+
+
+if __name__ == "__main__":
+    from pprint import pp
+
+    headers = CaseInsensitiveDict()
+    headers["Content-Type"] = "application/json"
+    headers["Cookie"] = "csrftoken=a4f3c7d28c194e5b; sessionid=f92e4b7c6"
+
+    print(f"{headers["cookie"] = }")
+    print(f"{"CooKIE" in headers = }")
+
+    del headers["Cookie"]
+    print(f"{headers = }")
+
+    pp(CaseInsensitiveDict.__mro__)