#!/usr/bin/env python3
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from functools import wraps


def cached(f):
    """
    Decorator to convert a function to cache its return value when it's called by the first time.

    :param f: The function to decorate.
    :return: The caching function.
    """
    cache_key = '_once_' + f.__name__

    @wraps(f)
    def once_f(self):
        try:
            v = object.__getattribute__(self, cache_key)  # can not use getattr, will trigger __getattr__ wrongly
        except AttributeError:
            v = f(self)
            setattr(self, cache_key, v)
        return v

    return once_f


def cached_property(f):
    """
    Decorator for declaring a property that caches its value.

    :param f: The property getter function.

    :return: The caching property.
    """
    return property(cached(f))