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lazy property implementation in python
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""" | |
See: https://segmentfault.com/a/1190000005818249 | |
(A better implementation: django.utils.functional.cached_property) | |
Usage: | |
class Circle(object): | |
def __init__(self, radius): | |
self.radius = radius | |
@lazy_property | |
def area(self): | |
print 'evalute' | |
return 3.14 * self.radius**2 | |
""" | |
class lazy_property(object): | |
def __init__(self, fget): | |
self.fget = fget | |
def __get__(self, instance, cls): | |
value = self.fget(instance) | |
setattr(instance, self.fget.__name__, value) | |
return value |
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""" | |
See also: sqlalchemy.util.langhelpers.memoized_property | |
""" | |
# Property name to hold all lazy data | |
_data_holder_attr = '_lazy_properties' | |
def clean_lazy_proterties(instance): | |
'''Clean all lazy properties''' | |
setattr(instance, _data_holder_attr, {}) | |
class lazy_property(object): | |
"""lazy property decorator, just like built-in `property`""" | |
def __init__(self, fget): | |
self.fget = fget | |
self.property_name = fget.__name__ | |
def get_lazy_data(self, instance): | |
if not hasattr(instance, _data_holder_attr): | |
setattr(instance, _data_holder_attr, {}) | |
return getattr(instance, _data_holder_attr) | |
def __get__(self, instance, owner): | |
if instance is None: | |
return None | |
lazy_data = self.get_lazy_data(instance) | |
if self.property_name in lazy_data: | |
return lazy_data[self.property_name] | |
value = self.fget(instance) | |
lazy_data[self.property_name] = value | |
return value | |
if __name__ == '__main__': | |
class Test(object): | |
'''Test case for lazy_property''' | |
@lazy_property | |
def somenum(self): | |
import random | |
return random.randrange(0, 1000) | |
t = Test() | |
print(t.somenum, t.somenum, t.somenum) # print a random number three times | |
clean_lazy_proterties(t) | |
print(t.somenum, t.somenum, t.somenum) # print another random number three times |
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Ah! You are right,
__get__
will not be triggered when getting attribute value.