[英]How to instantiate a subclass type variable from an existing superclass type object in Python
我有一种情况,我用几个属性扩展了 class:
class SuperClass:
def __init__(self, tediously, many, attributes):
# assign the attributes like "self.attr = attr"
class SubClass:
def __init__(self, id, **kwargs):
self.id = id
super().__init__(**kwargs)
然后我想创建实例,但我知道这会导致子类只能像这样实例化:
super_instance = SuperClass(tediously, many, attributes)
sub_instance = SubClass(id, tediously=super_instance.tediously, many=super_instance.many, attributes=super_instance.attributes)
我的问题是,是否可以通过复制超类实例的属性来实例化一个子类,而无需编写一段香肠代码来手动执行它(在构造函数调用或构造函数的主体中)。 。 就像是:
utopic_sub_instance = SubClass(id, **super_instance)
也许你想要一些关于如何不写这么多代码的具体想法? 所以一种方法是这样的:
class A:
def __init___(self, a, b, c):
self.a = a
self.b = b
self.c = c
class B:
def __init__(self, x, a, b, c):
self.x = x
super().__init__(a, b, c)
a = A(1, 2, 3)
b = B('x', 1, 2, 3)
# so your problem is that you want to avoid passing 1,2,3 manually, right?
# So as a comment suggests, you should use alternative constructors here.
# Alternative constructors are good because people not very familiar with
# Python could also understand them.
# Alternatively, you could use this syntax, but it is a little dangerous and prone to producing
# bugs in the future that are hard to spot
class BDangerous:
def __init__(self, x, a, b, c):
self.x = x
kwargs = dict(locals())
kwargs.pop('x')
kwargs.pop('self')
# This is dangerous because if in the future someone adds a variable in this
# scope, you need to remember to pop that also
# Also, if in the future, the super constructor acquires the same parameter that
# someone else adds as a variable here... maybe you will end up passing an argument
# unwillingly. That might cause a bug
# kwargs.pop(...pop all variable names you don't want to pass)
super().__init__(**kwargs)
class BSafe:
def __init__(self, x, a, b, c):
self.x = x
bad_kwargs = dict(locals())
# This is safer: you are explicit about which arguments you're passing
good_kwargs = {}
for name in 'a,b,c'.split(','):
good_kwargs[name] = bad_kwargs[name]
# but really, this solution is not that much better compared to simply passing all
# parameters explicitly
super().__init__(**good_kwargs)
或者,让我们 go 更疯狂一点。 我们将使用自省来动态构建 dict 以作为 arguments 传递。 我的示例中没有包含仅关键字 arguments、默认值、*args 或 **kwargs 的情况
class A:
def __init__(self, a,b,c):
self.a = a
self.b = b
self.c = c
class B(A):
def __init__(self, x,y,z, super_instance):
import inspect
spec = inspect.getfullargspec(A.__init__)
positional_args = []
super_vars = vars(super_instance)
for arg_name in spec.args[1:]: # to exclude 'self'
positional_args.append(super_vars[arg_name])
# ...but of course, you must have the guarantee that constructor
# arguments will be set as instance attributes with the same names
super().__init__(*positional_args)
我最终成功地使用了 alt 构造函数和 super_instance 的__dict__
属性的组合。
class SuperClass:
def __init__(self, tediously, many, attributes):
self.tediously = tediously
self.many = many
self.attributes = attributes
class SubClass(SuperClass):
def __init__(self, additional_attribute, tediously, many, attributes):
self.additional_attribute = additional_attribute
super().__init__(tediously, many, attributes)
@classmethod
def from_super_instance(cls, additional_attribute, super_instance):
return cls(additional_attribute=additional_attribute, **super_instance.__dict__)
super_instance = SuperClass("tediously", "many", "attributes")
sub_instance = SubClass.from_super_instance("additional_attribute", super_instance)
注意:请记住 python 顺序执行语句,因此如果要覆盖继承属性的值,请将super().__init__()
放在SubClass.__init__
中的其他赋值语句之前。
注意 2: pydantic有这个非常好的特性,他们的BaseModel
class 自动生成一个.__init__()
方法,帮助进行属性类型验证并为此类模型提供一个.dict()
方法(尽管它与.__dict__
基本相同)。
Kinda遇到了同样的问题,只是想一个人可以简单地做:
class SubClass(SuperClass):
def __init__(self, additional_attribute, **args):
self.additional_attribute = additional_attribute
super().__init__(**args)
super_class = SuperClass("tediously", "many", "attributes")
sub_instance = SuperClass("additional_attribute", **super_class.__dict__)
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