lazyproperty.py

import functools
from typing import Any, cast, Callable, Generic, TypeVar

T = TypeVar("T")


class lazyproperty(Generic[T]):
    """Decorator like @property, but evaluated only on first access.

    Like @property, this can only be used to decorate methods having only a `self`
    parameter, and is accessed like an attribute on an instance, i.e. trailing
    parentheses are not used. Unlike @property, the decorated method is only evaluated
    on first access; the resulting value is cached and that same value returned on
    second and later access without re-evaluation of the method.

    Like @property, this class produces a *data descriptor* object, which is stored in
    the __dict__ of the *class* under the name of the decorated method ('fget'
    nominally). The cached value is stored in the __dict__ of the *instance* under that
    same name.

    Because it is a data descriptor (as opposed to a *non-data descriptor*), its
    `__get__()` method is executed on each access of the decorated attribute; the
    __dict__ item of the same name is "shadowed" by the descriptor.

    While this may represent a performance improvement over a property, its greater
    benefit may be its other characteristics. One common use is to construct
    collaborator objects, removing that "real work" from the constructor, while still
    only executing once. It also de-couples client code from any sequencing
    considerations; if it's accessed from more than one location, it's assured it will
    be ready whenever needed.

    Loosely based on: https://stackoverflow.com/a/6849299/1902513.

    A lazyproperty is read-only. There is no counterpart to the optional "setter" (or
    deleter) behavior of an @property. This is critically important to maintaining its
    immutability and idempotence guarantees. Attempting to assign to a lazyproperty
    raises AttributeError unconditionally.

    The parameter names in the methods below correspond to this usage example::

        class Obj(object)

            @lazyproperty
            def fget(self):
                return 'some result'

        obj = Obj()

    Not suitable for wrapping a function (as opposed to a method) because it is not
    callable. """

    def __init__(self, fget: Callable[..., T]) -> None:
        """*fget* is the decorated method (a "getter" function).

        A lazyproperty is read-only, so there is only an *fget* function (a regular
        @property can also have an fset and fdel function). This name was chosen for
        consistency with Python's `property` class which uses this name for the
        corresponding parameter.
        """
        # --- maintain a reference to the wrapped getter method
        self._fget = fget
        # --- and store the name of that decorated method
        self._name = fget.__name__
        # --- adopt fget's __name__, __doc__, and other attributes
        functools.update_wrapper(self, fget)

    def __get__(self, obj: Any, type: Any = None) -> T:
        """Called on each access of 'fget' attribute on class or instance.

        *self* is this instance of a lazyproperty descriptor "wrapping" the property
        method it decorates (`fget`, nominally).

        *obj* is the "host" object instance when the attribute is accessed from an
        object instance, e.g. `obj = Obj(); obj.fget`. *obj* is None when accessed on
        the class, e.g. `Obj.fget`.

        *type* is the class hosting the decorated getter method (`fget`) on both class
        and instance attribute access.
        """
        # --- when accessed on class, e.g. Obj.fget, just return this descriptor
        # --- instance (patched above to look like fget).
        if obj is None:
            return self  # type: ignore

        # --- when accessed on instance, start by checking instance __dict__ for
        # --- item with key matching the wrapped function's name
        value = obj.__dict__.get(self._name)
        if value is None:
            # --- on first access, the __dict__ item will be absent. Evaluate fget()
            # --- and store that value in the (otherwise unused) host-object
            # --- __dict__ value of same name ('fget' nominally)
            value = self._fget(obj)
            obj.__dict__[self._name] = value
        return cast(T, value)

    def __set__(self, obj: Any, value: Any) -> None:
        """Raises unconditionally, to preserve read-only behavior.

        This decorator is intended to implement immutable (and idempotent) object
        attributes. For that reason, assignment to this property must be explicitly
        prevented.

        If this __set__ method was not present, this descriptor would become a
        *non-data descriptor*. That would be nice because the cached value would be
        accessed directly once set (__dict__ attrs have precedence over non-data
        descriptors on instance attribute lookup). The problem is, there would be
        nothing to stop assignment to the cached value, which would overwrite the result
        of `fget()` and break both the immutability and idempotence guarantees of this
        decorator.

        The performance with this __set__() method in place was roughly 0.4 usec per
        access when measured on a 2.8GHz development machine; so quite snappy and
        probably not a rich target for optimization efforts.
        """
        raise AttributeError("can't set attribute")