我有一个 Django 模型,需要对自定义用户模型进行一些处理。
我无法在类加载时使用该模型的类,因为类的加载顺序未知。
所以我需要在运行时添加一些类属性,目前我将它们添加到__init__
or __new__
like:
def __new__(cls, *args, **kwargs):
# hack to avoid INSTALLED_APPS initialization conflicts.
# get_user_model() can't be called from this module at class loading time,
# so some class attributes must be added later.
# Metaclasses could me more appropiate but I don't want to override
# dango's metaclasses.
if not hasattr(cls, '_reverse_field_name_to_user'):
cls._find_reverse_field_name_to_user()
return Group.__new__(cls, *args, **kwargs)
它有效,但看起来很糟糕,所以我考虑过使用类似的东西@lazyclassproperty
对于这些属性。
我找到了几个@classproperty
and @lazyproperty
装饰器,但不是一个同时适用于两者的装饰器,而且我不知道如何自己编写一个。
Question:我如何编写这样的装饰器?或者建议我当前愚蠢的实现的另一种更清洁的替代方案。
Pyramid 框架有一个非常好的装饰器,称为reify,但它只适用于实例级别,而您需要类级别,所以让我们稍微修改一下
class class_reify(object):
def __init__(self, wrapped):
self.wrapped = wrapped
try:
self.__doc__ = wrapped.__doc__
except: # pragma: no cover
pass
# original sets the attributes on the instance
# def __get__(self, inst, objtype=None):
# if inst is None:
# return self
# val = self.wrapped(inst)
# setattr(inst, self.wrapped.__name__, val)
# return val
# ignore the instance, and just set them on the class
# if called on a class, inst is None and objtype is the class
# if called on an instance, inst is the instance, and objtype
# the class
def __get__(self, inst, objtype=None):
# ask the value from the wrapped object, giving it
# our class
val = self.wrapped(objtype)
# and set the attribute directly to the class, thereby
# avoiding the descriptor to be called multiple times
setattr(objtype, self.wrapped.__name__, val)
# and return the calculated value
return val
class Test(object):
@class_reify
def foo(cls):
print("foo called for class", cls)
return 42
print(Test.foo)
print(Test.foo)
运行程序并打印
foo called for class <class '__main__.Test'>
42
42
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