Dynamic Class Creation python with metaclass involved

Question

i am trying to create an instance of a class using arguments, and the problem is my meta class needs some positional args to decide about the bases of class creation.

here is an example to clear it more:

Here are the childs which will be methods of the class (they will be chosen be the parent class object name)

import inspect
import sys

class ParentA:
    pass


class Child1(ParentA):
    def ch1_cmd(self):
        print('This is RAM Telnet IOS Cisco ')


class Child2(ParentA):
    def ch2_cmd(self):
        print('This is Time Telnet IOS Cisco ')

Here is my metaclass which accepts an additional argument CONNECTION_TYPE which is a dictionary, so metaclass will load proper childs according to this Dictionary

class MainMeta(type):
    def __new__(cls, clsname, bases, clsdict, CONNECTION_TYPE):
        if CONNECTION_TYPE != None:
            CONNECTION_TYPE['OBJ'] = getattr(
                sys.modules[__name__], CONNECTION_TYPE['OBJ'])
            if CONNECTION_TYPE['TYPE'] == 'CONDITION1':
                if CONNECTION_TYPE['CONNECTION_TYPE'] == 'CONDITION2':
                    bases = []
                    for name, obj in inspect.getmembers(sys.modules[__name__]):
                        if inspect.isclass(obj):
                            if CONNECTION_TYPE['OBJ'].__subclasscheck__(obj):
                                if CONNECTION_TYPE['OBJ'].__name__ != obj.__name__:
                                    bases.append(obj)
        bases = tuple(bases)
        print(clsname, bases, clsdict) # here Shows that bases are loaded CORRECTLY
        return super().__new__(cls, clsname, bases, clsdict)

this is the Dictionary which will be served to the meta class.

clsdic = {'TYPE': 'CONDITION1',
          'CONNECTION_TYPE': 'CONDITION2', 'OBJ': "ParentA", }

HERE IS The PROBLEM, I use exec to create my class dynamically with my desired CONNECTION_TYPE dictionary, code runs without Err but there are something missing in the created class, and those are __module__ and child classes.

def dev_creator(*args, **kwargs):
    print()
    myclass = f"""class MyMain(metaclass=MainMeta, CONNECTION_TYPE={kwargs}):\n    pass\n"""
    return exec(myclass, globals())

dev = dev_creator(**clsdic)
print(dir(dev))

This is the output :|

MyMain (<class '__main__.Child1'>, <class '__main__.Child2'>) {'__module__': '__main__', '__qualname__': 'MyMain'}
['__bool__', '__class__', '__delattr__', '__dir__', '__doc__', '__eq__', '__format__', '__ge__', '__getattribute__', '__gt__', '__hash__', '__init__', '__init_subclass__', '__le__', '__lt__', '__ne__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__sizeof__', '__str__', '__subclasshook__']

Hossein Dehghanpoor · Accepted Answer · 2022-03-03 08:23:56Z

0

HUM, Solved. it was the global variable definition problem.

here is how we should call an exec and get a return from it.

dev = None
def dev_creator(*args, **kwargs):
    # print(kwargs)
    myclass = f"""class MyMain(metaclass=MainMeta, CONNECTION_TYPE={kwargs}):\n    pass\ndev = MyMain"""
    exec(myclass, globals(), dev)
    return dev

answered Mar 3, 2022 at 8:23

Hossein Dehghanpoor

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Comments

jsbueno · Accepted Answer · 2022-03-08 14:34:19Z

0

You don't needand should not call exec at all.

Simply calling your metaclass passign the needed parameters (name, bases and namespace is enough in this case:

 myclass = metaclass("myclass", (), {}, CONNECTION_TYPE=kwargs)

will create you a new class. (the super() call atthe end of the metaclass __new__ does that).

answered Mar 8, 2022 at 14:34

jsbueno

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