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