How to dynamically change signatures of method in

问题:

When using classmethod to dynamic change the method in subclass, how to dynamic change signatures of method?

example

import inspect

class ModelBase(object):

    @classmethod
    def method_one(cls, *args):
        raise NotImplementedError

    @classmethod
    def method_two(cls, *args):
        return cls.method_one(*args) + 1

class SubClass(ModelBase):
    @staticmethod
    def method_one(a, b):
        return a + b

test = SubClass()

try:
    print(inspect.signature(test.method_two))
except AttributeError:
    print(inspect.getargspec(test.method_two).args)

I want test.method_two to get the signatures of test.method_one. How to rewrite parent class ModelBase?

I have read about Preserving signatures of decorated functions. In python3.4 +, functools.wraps helps to preserve signatures of decorated functions. I want to apply it to class method.

when uses functools.wraps, I need to assign decorated method's name. But how to access decorated method outside classmethod in this situation?

from functools import wraps

class ModelBase(object):

    @classmethod
    def method_one(cls, *args):
        raise NotImplementedError

    @classmethod
    def method_two(cls):
        @wraps(cls.method_one)
        def fun(*args):
            return cls.method_one(*args) + 1
        return fun

method_two returns a wrapped function, but I must use it with test.method_two()(*arg). This method is not directly.

回答1:

If this is only for introspection purpose you could override __getattribute__ on ModelBase and every time method_two is accessed we return a function that has the signature of method_one.

import inspect

def copy_signature(frm, to):
    def wrapper(*args, **kwargs):
        return to(*args, **kwargs)
    wrapper.__signature__ = inspect.signature(frm)
    return wrapper


class ModelBase(object):

    @classmethod
    def method_one(cls, *args):
        raise NotImplementedError

    @classmethod
    def method_two(cls, *args):
        return cls.method_one(*args) + 1

    def __getattribute__(self, attr):
        value = object.__getattribute__(self, attr)
        if attr == 'method_two':
            value = copy_signature(frm=self.method_one, to=value)
        return value


class SubClass(ModelBase):
    @staticmethod
    def method_one(a, b):
        return a + b


class SubClass2(ModelBase):
    @staticmethod
    def method_one(a, b, c, *arg):
        return a + b

Demo:

>>> test1 = SubClass()
>>> print(inspect.signature(test1.method_two))
(a, b)
>>> test2 = SubClass2()
>>> print(inspect.signature(test2.method_two))
(a, b, c, *arg)