How to dynamically change signatures of method in subclass?

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)

来源：https://stackoverflow.com/questions/45626647/how-to-dynamically-change-signatures-of-method-in-subclass

标签

python

class

methods

signature