94 lines
2.8 KiB
Python
94 lines
2.8 KiB
Python
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from functools import update_wrapper, wraps
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from types import MethodType
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class _AvailableIfDescriptor:
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"""Implements a conditional property using the descriptor protocol.
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Using this class to create a decorator will raise an ``AttributeError``
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if check(self) returns a falsey value. Note that if check raises an error
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this will also result in hasattr returning false.
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See https://docs.python.org/3/howto/descriptor.html for an explanation of
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descriptors.
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"""
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def __init__(self, fn, check, attribute_name):
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self.fn = fn
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self.check = check
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self.attribute_name = attribute_name
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# update the docstring of the descriptor
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update_wrapper(self, fn)
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def _check(self, obj, owner):
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attr_err_msg = (
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f"This {repr(owner.__name__)} has no attribute {repr(self.attribute_name)}"
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)
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try:
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check_result = self.check(obj)
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except Exception as e:
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raise AttributeError(attr_err_msg) from e
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if not check_result:
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raise AttributeError(attr_err_msg)
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def __get__(self, obj, owner=None):
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if obj is not None:
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# delegate only on instances, not the classes.
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# this is to allow access to the docstrings.
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self._check(obj, owner=owner)
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out = MethodType(self.fn, obj)
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else:
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# This makes it possible to use the decorated method as an unbound method,
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# for instance when monkeypatching.
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@wraps(self.fn)
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def out(*args, **kwargs):
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self._check(args[0], owner=owner)
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return self.fn(*args, **kwargs)
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return out
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def available_if(check):
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"""An attribute that is available only if check returns a truthy value.
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Parameters
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----------
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check : callable
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When passed the object with the decorated method, this should return
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a truthy value if the attribute is available, and either return False
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or raise an AttributeError if not available.
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Returns
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-------
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callable
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Callable makes the decorated method available if `check` returns
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a truthy value, otherwise the decorated method is unavailable.
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Examples
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--------
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>>> from sklearn.utils.metaestimators import available_if
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>>> class HelloIfEven:
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... def __init__(self, x):
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... self.x = x
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...
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... def _x_is_even(self):
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... return self.x % 2 == 0
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...
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... @available_if(_x_is_even)
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... def say_hello(self):
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... print("Hello")
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...
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>>> obj = HelloIfEven(1)
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>>> hasattr(obj, "say_hello")
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False
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>>> obj.x = 2
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>>> hasattr(obj, "say_hello")
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True
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>>> obj.say_hello()
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Hello
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"""
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return lambda fn: _AvailableIfDescriptor(fn, check, attribute_name=fn.__name__)
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