How do I add keyword arguments to a derived class's constructor in Python?

问题

I want to add keyword arguments to a derived class, but can't figure out how to go about it. Trying the obvious

class ClassA(some.package.Class):

    def __init__(self, *args, **kwargs):
        super(ClassA, self).__init__(*args, **kwargs)

class ClassB(ClassA):

    def __init__(self, *args, a='A', b='B', c='C', **kwargs):
        super(ClassB, self).__init__(*args, **kwargs)
        self.a=a
        self.b=b
        self.c=c

fails because I can't list parameters like that for ClassB's __init__. And

class ClassB(ClassA):

    def __init__(self, *args, **kwargs):
        super(ClassA, self).__init__(*args, **kwargs)
        self.a=a
        self.b=b
        self.c=c

of course doesn't work because the new keywords aren't specified.

How do I add keyword arguments to the __init__ for a derived class?

回答1:

Try doing it like this:

class ClassA(object):
    def __init__(self, *args, **kwargs):
        pass

class ClassB(ClassA):
    def __init__(self, *args, **kwargs):            
        self.a = kwargs.pop('a', 'A')
        self.b = kwargs.pop('b', 'B')
        self.c = kwargs.pop('c', 'C')
        super(ClassB, self).__init__(*args, **kwargs)

Effectively you add the keyword arguments a, b and c to ClassB, while passing on other keyword arguments to ClassA.

回答2:

All you need do is rearrange the arguments.

def __init__(self, a='A', b='B', c='C', *args, **kwargs):

回答3:

Effect of `def init(self, a='A', b='B', c='C', *args, **kwargs):`

Modifying the OP's code with the child class's constructor having the above signature:

class BaseClass(object):
    def __init__(self, *args, **kwargs):
        self.args = args
        for k, v in kwargs.items():
            setattr(self, k, v)

class ClassA(BaseClass):
    def __init__(self, *args, **kwargs):
        super(ClassA, self).__init__(*args, **kwargs)

class ClassB(ClassA):
    def __init__(self, a='A', b='B', c='C', *args, **kwargs):
        self.a = a
        self.b = b
        self.c = c
        super(ClassA, self).__init__(*args, **kwargs)


A = ClassA('hello', 'world', myname='hal',myemail='hal@hal.hal')
B = ClassB('hello', 'world', myname='hal', myemail='hal@hal.hal')

print("CLASS A:", A.__dict__)
print("CLASS B:", B.__dict__)
# yields the following:
CLASS A: {'args': ('hello', 'world'), 'myname': 'hal', 'myemail': 'hal@hal.hal'}
CLASS B: {'args': (), 'myname': 'hal', 'a': 'hello', 'myemail': 'hal@hal.hal', 'c': 'C', 'b': 'world'}

The code will not throw an exception, however, a, b & c are now positionally the first 3 arguments passed to the method (ignoring self) , as seen from the two object's dicts.

Accessing the attributes a, b & c shows this other effect

>>> B.a # expect to see 'A'
'hello'
>>> B.b # similarly
'world'
>>> B.c # but this retails the default value
'C'

AFAIK, you can't add new keyword only arguments to the method signature. Please correct me if I'm wrong.

However, both @aknuds1 and @Oleh Prypin provided solutions that effectively add new keyword arguments to the child class. Oleh's solution is a bit more clever, but I find aknuds1's version easier to understand.

pop elements from the kwargs dict, coalesced to the default value
assign to the instance attribute using setattr
call parent constructor with args, kwargs

回答4:

Use Python 3.x, which makes keyword-only arguments valid.

Or use this workaround...

class ClassB(ClassA):
    def __init__(self, *args, **kwargs):
        for arg, default in [('a', 'A'), ('b', 'B'), ('c', 'C')]:
            setattr(self, arg, kwargs.pop(arg, default))
        super(ClassA, self).__init__(*args, **kwargs)

来源：https://stackoverflow.com/questions/27472250/how-do-i-add-keyword-arguments-to-a-derived-classs-constructor-in-python

标签

python