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105 lines (77 loc) · 2.55 KB
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"""
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
pyramid
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)