英文:
Return class mixin after checking a field of the class
问题
我想将一个混合类（MixinClass）添加到我已经拥有的类（A）中，但首先我需要检查该类是否有一个字段（atr）。这是因为类A可以来自两个不同的模块，其中一个模块中有一个字段attr，而另一个模块中没有。
我尝试过类似以下的方法，但不知道如何在方法中获取类A：
def check():
class mixinClass(object):
new_atr = None
if hasatr(A, 'atr'): # 我如何检查类是否具有这样的字段？
return mixinClass
class A(check()):
...
请注意，上述代码中的“hasatr”函数需要您自行实现，用于检查类A是否具有名为“atr”的字段。
英文:
I want to add a mixin class (MixinClass) to a class (A) that I already have but first I need to check if that class has a field (atr) or not. This is because class A can come from two different modules, in one there is a field attr and in the other there is not.
I've tried something like this, but I don't know how I can get the class A into the method
def check():
class mixinClass(object):
new_atr = None
if hasatr(A, 'atr'): # How can I check that the class has such a field?
return mixinClass
class A(check()):
...
答案1
得分: 0
你可以使用元类动态设置一个类的基类，在其中可以将条件基类的逻辑放在__new__方法中：
def mixin_if_hasattr(mixin, attr):
class MixinIfHasattr(type):
def __new__(metacls, name, bases, classdict):
return super().__new__(
metacls,
name,
bases + ((mixin,) if attr in classdict else ()),
classdict
)
return MixinIfHasattr
这样:
class mixinClass:
new_attr = None
class A(metaclass=mixin_if_hasattr(mixinClass, 'foo')):
pass
class B(metaclass=mixin_if_hasattr(mixinClass, 'foo')):
foo = None
print(A.__bases__)
print(B.__bases__)
输出:
(<class 'object'>,)
(<class '__main__.mixinClass'>,)
演示：https://replit.com/@blhsing/JointShabbyBetaversion
英文:
You can dynamically set the base class of a class with a metaclass, where you can put your desired logics of a conditional base class in the __new__ method:
def mixin_if_hasattr(mixin, attr):
class MixinIfHasattr(type):
def __new__(metacls, name, bases, classdict):
return super().__new__(
metacls,
name,
bases + ((mixin,) if attr in classdict else ()),
classdict
)
return MixinIfHasattr
so that:
class mixinClass:
new_atr = None
class A(metaclass=mixin_if_hasattr(mixinClass, 'foo')):
pass
class B(metaclass=mixin_if_hasattr(mixinClass, 'foo')):
foo = None
print(A.__bases__)
print(B.__bases__)
outputs:
(<class 'object'>,)
(<class '__main__.mixinClass'>,)
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