lazy_evaluation.py

"""
Lazily-evaluated property pattern in Python.

https://en.wikipedia.org/wiki/Lazy_evaluation

*References:
bottle
https://github.com/bottlepy/bottle/blob/cafc15419cbb4a6cb748e6ecdccf92893bb25ce5/bottle.py#L270
django
https://github.com/django/django/blob/ffd18732f3ee9e6f0374aff9ccf350d85187fac2/django/utils/functional.py#L19
pip
https://github.com/pypa/pip/blob/cb75cca785629e15efb46c35903827b3eae13481/pip/utils/__init__.py#L821
pyramimd
https://github.com/Pylons/pyramid/blob/7909e9503cdfc6f6e84d2c7ace1d3c03ca1d8b73/pyramid/decorator.py#L4
werkzeug
https://github.com/pallets/werkzeug/blob/5a2bf35441006d832ab1ed5a31963cbc366c99ac/werkzeug/utils.py#L35

*TL;DR
Delays the eval of an expr until its value is needed and avoids repeated evals.
"""

import functools


class lazy_property:
    def __init__(self, function):
        self.function = function
        functools.update_wrapper(self, function)

    def __get__(self, obj, type_):
        if obj is None:
            return self
        val = self.function(obj)
        obj.__dict__[self.function.__name__] = val
        return val


def lazy_property2(fn):
    attr = "_lazy__" + fn.__name__

    @property
    def _lazy_property(self):
        if not hasattr(self, attr):
            setattr(self, attr, fn(self))
        return getattr(self, attr)

    return _lazy_property


class Person:
    def __init__(self, name, occupation):
        self.name = name
        self.occupation = occupation
        self.call_count2 = 0

    @lazy_property
    def relatives(self):
        # Get all relatives, let's assume that it costs much time.
        relatives = "Many relatives."
        return relatives

    @lazy_property2
    def parents(self):
        self.call_count2 += 1
        return "Father and mother"


def main():
    """
    >>> Jhon = Person('Jhon', 'Coder')

    >>> Jhon.name
    'Jhon'
    >>> Jhon.occupation
    'Coder'

    # Before we access `relatives`
    >>> sorted(Jhon.__dict__.items())
    [('call_count2', 0), ('name', 'Jhon'), ('occupation', 'Coder')]

    >>> Jhon.relatives
    'Many relatives.'

    # After we've accessed `relatives`
    >>> sorted(Jhon.__dict__.items())
    [('call_count2', 0), ..., ('relatives', 'Many relatives.')]

    >>> Jhon.parents
    'Father and mother'

    >>> sorted(Jhon.__dict__.items())
    [('_lazy__parents', 'Father and mother'), ('call_count2', 1), ..., ('relatives', 'Many relatives.')]

    >>> Jhon.parents
    'Father and mother'

    >>> Jhon.call_count2
    1
    """


if __name__ == "__main__":
    import doctest

    doctest.testmod(optionflags=doctest.ELLIPSIS)