目标是创建一个行为类似db结果集的模拟类。
例如，如果一个数据库查询返回，使用dict表达式，{'ab':100， 'cd':200}，那么我想看到:
>>> dummy.ab
100
一开始我想也许我可以这样做:
ks = ['ab', 'cd']
vs = [12, 34]
class C(dict):
def __init__(self, ks, vs):
for i, k in enumerate(ks):
self[k] = vs[i]
setattr(self, k, property(lambda x: vs[i], self.fn_readyonly))
def fn_readonly(self, v)
raise "It is ready only"
if __name__ == "__main__":
c = C(ks, vs)
print c.ab
但是cab返回一个属性对象。
用k = property(lambda x: vs[i])替换setattr行根本没有用。
那么，在运行时创建实例属性的正确方法是什么呢?
附注:我知道在如何使用__getattribute__方法中提出了一个替代方案?
你不需要使用属性。只需重写__setattr__，使其为只读。
class C(object):
def __init__(self, keys, values):
for (key, value) in zip(keys, values):
self.__dict__[key] = value
def __setattr__(self, name, value):
raise Exception("It is read only!")
Tada。
>>> c = C('abc', [1,2,3])
>>> c.a
1
>>> c.b
2
>>> c.c
3
>>> c.d
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
AttributeError: 'C' object has no attribute 'd'
>>> c.d = 42
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "<stdin>", line 6, in __setattr__
Exception: It is read only!
>>> c.a = 'blah'
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "<stdin>", line 6, in __setattr__
Exception: It is read only!
我最近遇到了一个类似的问题，我提出的解决方案使用__getattr__和__setattr__为我想要它处理的属性，其他一切都传递给原始。
class C(object):
def __init__(self, properties):
self.existing = "Still Here"
self.properties = properties
def __getattr__(self, name):
if "properties" in self.__dict__ and name in self.properties:
return self.properties[name] # Or call a function, etc
return self.__dict__[name]
def __setattr__(self, name, value):
if "properties" in self.__dict__ and name in self.properties:
self.properties[name] = value
else:
self.__dict__[name] = value
if __name__ == "__main__":
my_properties = {'a':1, 'b':2, 'c':3}
c = C(my_properties)
assert c.a == 1
assert c.existing == "Still Here"
c.b = 10
assert c.properties['b'] == 10
这和OP想要的有点不同，但我绞尽脑汁，直到我得到了一个可行的解决方案，所以我把它放在这里给下一个男人/女孩
我需要一种方法来指定动态setter和getter。
class X:
def __init__(self, a=0, b=0, c=0):
self.a = a
self.b = b
self.c = c
@classmethod
def _make_properties(cls, field_name, inc):
_inc = inc
def _get_properties(self):
if not hasattr(self, '_%s_inc' % field_name):
setattr(self, '_%s_inc' % field_name, _inc)
inc = _inc
else:
inc = getattr(self, '_%s_inc' % field_name)
return getattr(self, field_name) + inc
def _set_properties(self, value):
setattr(self, '_%s_inc' % field_name, value)
return property(_get_properties, _set_properties)
我知道我的字段提前，所以我要创建我的属性。注意:你不能在每个实例中这样做，这些属性将存在于类中!!
for inc, field in enumerate(['a', 'b', 'c']):
setattr(X, '%s_summed' % field, X._make_properties(field, inc))
现在让我们来测试一下。
x = X()
assert x.a == 0
assert x.b == 0
assert x.c == 0
assert x.a_summed == 0 # enumerate() set inc to 0 + 0 = 0
assert x.b_summed == 1 # enumerate() set inc to 1 + 0 = 1
assert x.c_summed == 2 # enumerate() set inc to 2 + 0 = 2
# we set the variables to something
x.a = 1
x.b = 2
x.c = 3
assert x.a_summed == 1 # enumerate() set inc to 0 + 1 = 1
assert x.b_summed == 3 # enumerate() set inc to 1 + 2 = 3
assert x.c_summed == 5 # enumerate() set inc to 2 + 3 = 5
# we're changing the inc now
x.a_summed = 1
x.b_summed = 3
x.c_summed = 5
assert x.a_summed == 2 # we set inc to 1 + the property was 1 = 2
assert x.b_summed == 5 # we set inc to 3 + the property was 2 = 5
assert x.c_summed == 8 # we set inc to 5 + the property was 3 = 8
会让人困惑吗?是的，很抱歉我无法举出任何有意义的现实例子。另外，这也不适合轻松的人。
如果需求是基于某些实例属性动态生成属性，那么下面的代码可能会有用:
import random
class Foo:
def __init__(self, prop_names: List[str], should_property_be_zero: bool = False) -> None:
self.prop_names = prop_names
self.should_property_be_zero = should_property_be_zero
def create_properties(self):
for name in self.prop_names:
setattr(self.__class__, name, property(fget=lambda self: 0 if self.should_property_be_zero else random.randint(1, 100)))
需要注意的重要一点是使用setattr(self。__class__进行名称,…),而不是setattr(自我、名称…)
使用的例子:
In [261]: prop_names = ['a', 'b']
In [262]: ff = Foo(prop_names=prop_names, should_property_be_zero=False)
In [263]: ff.create_properties()
In [264]: ff.a
Out[264]: 10
In [265]: ff.b
Out[265]: 37
In [266]: ft = Foo(prop_names=prop_names, should_property_be_zero=True)
In [267]: ft.create_properties()
In [268]: ft.a
Out[268]: 0
In [269]: ft.b
Out[269]: 0
设置属性将引发AttributeError:不能按预期设置属性:
In [270]: ff.a = 5
---------------------------------------------------------------------------
AttributeError Traceback (most recent call last)
<ipython-input-270-5f9cad5b617d> in <module>
----> 1 ff.a = 5
AttributeError: can't set attribute
In [271]: ft.a = 5
---------------------------------------------------------------------------
AttributeError Traceback (most recent call last)
<ipython-input-271-65e7b8e25b67> in <module>
----> 1 ft.a = 5
AttributeError: can't set attribute
