renumbering chapters >= 19

24-class-metaprog/checked/decorator/checkeddeco.py

@@ -0,0 +1,152 @@
+"""
+A ``Checked`` subclass definition requires that keyword arguments are
+used to create an instance, and provides a nice ``__repr__``::
+
+# tag::MOVIE_DEFINITION[]
+
+    >>> @checked
+    ... class Movie:
+    ...     title: str
+    ...     year: int
+    ...     box_office: float
+    ...
+    >>> movie = Movie(title='The Godfather', year=1972, box_office=137)
+    >>> movie.title
+    'The Godfather'
+    >>> movie
+    Movie(title='The Godfather', year=1972, box_office=137.0)
+
+# end::MOVIE_DEFINITION[]
+
+The type of arguments is runtime checked when an attribute is set,
+including during instantiation::
+
+# tag::MOVIE_TYPE_VALIDATION[]
+
+    >>> blockbuster = Movie(title='Avatar', year=2009, box_office='billions')
+    Traceback (most recent call last):
+      ...
+    TypeError: 'billions' is not compatible with box_office:float
+    >>> movie.year = 'MCMLXXII'
+    Traceback (most recent call last):
+      ...
+    TypeError: 'MCMLXXII' is not compatible with year:int
+
+# 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, box_office=0.0)
+
+# end::MOVIE_DEFAULTS[]
+
+Providing extra arguments to the constructor is not allowed::
+
+    >>> blockbuster = Movie(title='Avatar', year=2009, box_office=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 `_asdict` instance method creates a `dict` from the attributes
+of a `Movie` object::
+
+    >>> movie._asdict()
+    {'title': 'The Godfather', 'year': 1972, 'box_office': 137.0}
+
+"""
+
+from collections.abc import Callable  # <1>
+from typing import Any, NoReturn, get_type_hints
+
+class Field:
+    def __init__(self, name: str, constructor: Callable) -> None:  # <2>
+        if not callable(constructor) or constructor is type(None):
+            raise TypeError(f'{name!r} type hint must be callable')
+        self.name = name
+        self.constructor = constructor
+
+    def __set__(self, instance: Any, value: Any) -> None:  # <3>
+        if value is ...:  # <4>
+            value = self.constructor()
+        else:
+            try:
+                value = self.constructor(value)  # <5>
+            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  # <6>
+
+
+# tag::CHECKED_DECORATOR[]
+def checked(cls: type) -> type:  # <1>
+    for name, constructor in _fields(cls).items():    # <2>
+        setattr(cls, name, Field(name, constructor))  # <3>
+
+    cls._fields = classmethod(_fields)  # type: ignore  # <4>
+
+    instance_methods = (  # <5>
+        __init__,
+        __repr__,
+        __setattr__,
+        _asdict,
+        __flag_unknown_attrs,
+    )
+    for method in instance_methods:  # <6>
+        setattr(cls, method.__name__, method)
+
+    return cls  # <7>
+# end::CHECKED_DECORATOR[]
+
+# tag::CHECKED_METHODS[]
+def _fields(cls: type) -> dict[str, type]:
+    return get_type_hints(cls)
+
+def __init__(self: Any, **kwargs: Any) -> None:
+    for name in self._fields():
+        value = kwargs.pop(name, ...)
+        setattr(self, name, value)
+    if kwargs:
+        self.__flag_unknown_attrs(*kwargs)
+
+def __setattr__(self: Any, name: str, value: Any) -> None:
+    if name in self._fields():
+        cls = self.__class__
+        descriptor = getattr(cls, name)
+        descriptor.__set__(self, value)
+    else:
+        self.__flag_unknown_attrs(name)
+
+def __flag_unknown_attrs(self: Any, *names: str) -> NoReturn:
+    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: Any) -> dict[str, Any]:
+    return {
+        name: getattr(self, name)
+        for name, attr in self.__class__.__dict__.items()
+        if isinstance(attr, Field)
+    }
+
+def __repr__(self: Any) -> str:
+    kwargs = ', '.join(
+        f'{key}={value!r}' for key, value in self._asdict().items()
+    )
+    return f'{self.__class__.__name__}({kwargs})'
+# end::CHECKED_METHODS[]

24-class-metaprog/checked/decorator/checkeddeco_demo.py

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+#!/usr/bin/env python3
+
+from checkeddeco import checked
+
+@checked
+class Movie:
+    title: str
+    year: int
+    box_office: float
+
+
+if __name__ == '__main__':
+    # No static type checker can understand this...
+    movie = Movie(title='The Godfather', year=1972, box_office=137)  # type: ignore
+    print(movie.title)
+    print(movie)
+    try:
+        # remove the "type: ignore" comment to see Mypy correctly spot the error
+        movie.year = 'MCMLXXII'  # type: ignore
+    except TypeError as e:
+        print(e)
+    try:
+        # Again, no static type checker can understand this...
+        blockbuster = Movie(title='Avatar', year=2009, box_office='billions')  # type: ignore
+    except TypeError as e:
+        print(e)

24-class-metaprog/checked/decorator/checkeddeco_test.py

@@ -0,0 +1,50 @@
+import pytest
+
+
+from checkeddeco import checked
+
+
+def test_field_descriptor_validation_type_error():
+    @checked
+    class Cat:
+        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():
+    @checked
+    class Cat:
+        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():
+    @checked
+    class Cat:
+        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'"
+
+
+def test_field_invalid_constructor():
+    with pytest.raises(TypeError) as e:
+        @checked
+        class Cat:
+            name: str
+            weight: None
+
+    assert str(e.value) == "'weight' type hint must be callable"