ch25: new example with __init_subclas__

25-class-metaprog/checked/checkedlib.py

@@ -0,0 +1,143 @@
+"""
+A ``Checked`` subclass definition requires that keyword arguments are
+used to create an instance, and provides a nice ``__repr__``::
+
+# tag::MOVIE_DEFINITION[]
+
+    >>> class Movie(Checked):  # <1>
+    ...     title: str  # <2>
+    ...     year: int
+    ...     megabucks: float
+    ...
+    >>> movie = Movie(title='The Godfather', year=1972, megabucks=137)  # <3>
+    >>> movie.title
+    'The Godfather'
+    >>> movie  # <4>
+    Movie(title='The Godfather', year=1972, megabucks=137.0)
+
+# end::MOVIE_DEFINITION[]
+
+The type of arguments is runtime checked when an attribute is set,
+including during instantiation::
+
+# tag::MOVIE_TYPE_VALIDATION[]
+
+    >>> movie.year = 'MCMLXXII'  # <1>
+    Traceback (most recent call last):
+      ...
+    TypeError: 'MCMLXXII' is not compatible with year:int
+    >>> blockbuster = Movie(title='Avatar', year=2009, megabucks='billions')  # <2>
+    Traceback (most recent call last):
+      ...
+    TypeError: 'billions' is not compatible with megabucks:float
+
+# end::MOVIE_TYPE_VALIDATION[]
+
+Attributes not passed as arguments to the constructor are initialized with
+default values::
+
+# tag::MOVIE_DEFAULTS[]
+
+    >>> Movie(title='Life of Brian')
+    Movie(title='Life of Brian', year=0, megabucks=0.0)
+
+# end::MOVIE_DEFAULTS[]
+
+Providing extra arguments to the constructor is not allowed::
+
+    >>> blockbuster = Movie(title='Avatar', year=2009, megabucks=2000,
+    ...                     director='James Cameron')
+    Traceback (most recent call last):
+      ...
+    AttributeError: 'Movie' has no attribute 'director'
+
+Creating new attributes at runtime is restricted as well::
+
+    >>> movie.director = 'Francis Ford Coppola'
+    Traceback (most recent call last):
+      ...
+    AttributeError: 'Movie' has no attribute 'director'
+
+The `_as_dict` instance creates a `dict` from the attributes of a `Movie` object::
+
+    >>> movie._asdict()
+    {'title': 'The Godfather', 'year': 1972, 'megabucks': 137.0}
+
+"""
+
+# tag::CHECKED_FIELD[]
+from collections.abc import Callable  # <1>
+from typing import Any, NoReturn, get_type_hints
+
+MISSING = object()  # <2>
+
+
+class Field:
+    def __init__(self, name: str, constructor: Callable) -> None:  # <3>
+        self.name = name
+        self.constructor = constructor
+
+    def __set__(self, instance: 'Checked', value: Any) -> None:  # <4>
+        if value is MISSING:  # <5>
+            value = self.constructor()
+        else:
+            try:
+                value = self.constructor(value)  # <6>
+            except (TypeError, ValueError) as e:
+                type_name = self.constructor.__name__
+                msg = f'{value!r} is not compatible with {self.name}:{type_name}'
+                raise TypeError(msg) from e
+        instance.__dict__[self.name] = value  # <7>
+
+
+# end::CHECKED_FIELD[]
+
+# tag::CHECKED_TOP[]
+class Checked:
+    @classmethod
+    def _fields(cls) -> dict[str, type]:  # <1>
+        return get_type_hints(cls)
+
+    def __init_subclass__(subclass) -> None:  # <2>
+        super().__init_subclass__()           # <3>
+        for name, constructor in subclass._fields().items():   # <4>
+            setattr(subclass, name, Field(name, constructor))  # <5>
+
+    def __init__(self, **kwargs: Any) -> None:
+        for name in self._fields():            # <6>
+            value = kwargs.pop(name, MISSING)  # <7>
+            setattr(self, name, value)         # <8>
+        if kwargs:                             # <9>
+            self.__flag_unknown_attrs(*kwargs)
+
+    def __setattr__(self, name: str, value: Any) -> None:  # <10>
+        if name in self._fields():            # <11>
+            cls = self.__class__
+            descriptor = getattr(cls, name)
+            descriptor.__set__(self, value)   # <12>
+        else:                                 # <13>
+            self.__flag_unknown_attrs(name)
+
+    # end::CHECKED_TOP[]
+
+    # tag::CHECKED_BOTTOM[]
+    def __flag_unknown_attrs(self, *names: str) -> NoReturn:  # <1>
+        plural = 's' if len(names) > 1 else ''
+        extra = ', '.join(f'{name!r}' for name in names)
+        cls_name = repr(self.__class__.__name__)
+        raise AttributeError(f'{cls_name} has no attribute{plural} {extra}')
+
+    def _asdict(self) -> dict[str, Any]:  # <2>
+        return {
+            name: getattr(self, name)
+            for name, attr in self.__class__.__dict__.items()
+            if isinstance(attr, Field)
+        }
+
+    def __repr__(self) -> str:  # <3>
+        kwargs = ', '.join(
+            f'{key}={value!r}' for key, value in self._asdict().items()
+        )
+        return f'{self.__class__.__name__}({kwargs})'
+
+# end::CHECKED_BOTTOM[]

25-class-metaprog/checked/checkedlib_demo.py

@@ -0,0 +1,21 @@
+from checkedlib import Checked
+
+class Movie(Checked):
+    title: str
+    year: int
+    megabucks: float
+
+
+if __name__ == '__main__':
+    movie = Movie(title='The Godfather', year=1972, megabucks=137)
+    print(movie.title)
+    print(movie)
+    try:
+        # remove the "type: ignore" comment to see Mypy error
+        movie.year = 'MCMLXXII'  # type: ignore
+    except TypeError as e:
+        print(e)
+    try:
+        blockbuster = Movie(title='Avatar', year=2009, megabucks='billions')
+    except TypeError as e:
+        print(e)

25-class-metaprog/checked/checkedlib_test.py

@@ -0,0 +1,37 @@
+import pytest
+
+
+from checkedlib import Checked
+
+
+def test_field_descriptor_validation_type_error():
+    class Cat(Checked):
+        name: str
+        weight: float
+
+    with pytest.raises(TypeError) as e:
+        felix = Cat(name='Felix', weight=None)
+
+    assert str(e.value) == 'None is not compatible with weight:float'
+
+
+def test_field_descriptor_validation_value_error():
+    class Cat(Checked):
+        name: str
+        weight: float
+
+    with pytest.raises(TypeError) as e:
+        felix = Cat(name='Felix', weight='half stone')
+
+    assert str(e.value) == "'half stone' is not compatible with weight:float"
+
+
+def test_constructor_attribute_error():
+    class Cat(Checked):
+        name: str
+        weight: float
+
+    with pytest.raises(AttributeError) as e:
+        felix = Cat(name='Felix', weight=3.2, age=7)
+
+    assert str(e.value) == "'Cat' has no attribute 'age'"