Create class with properties from config file

Question

I am trying to dynamically create a Python class from a configuration file (I use a dictionary in the example for simplicity). The attributes that I create I would like to be set as properties since I have validation functions that I would like to run when the attributes are set.

I have a code that is sort-of working, but it's having some unexpected results.

def func(x):
    """ Example validation function. """
    if x < 0:
        raise ValueError

    return


config = {
    "x": {
        "type": int,
        "default": 0,
        "validate": func
    },
    "y": {
        "type": int,
        "default": 10,
        "validate": func
    },
    "z": {
        "type": str,
        "default": "bar",
        "validate": None
    }
}


def get_property(prop, validate):
    key = f"_{prop}"

    def get_(self):
        return exec(f"self.{key}")

    def set_(self, value):
        if validate is not None:
            validate(value)
        exec(f"self.{key} = value")

    def del_(self):
        exec(f"del self.{key}")

    return property(get_, set_, del_)


def get_Foo(**kwargs):
    class Foo:
        def __init__(self, **_kwargs):
            for k_, v_ in _kwargs.items():
                setattr(self, "_" + k_, None)

                if v_["type"] == str:
                    exec(f"self.{k_} = '{v_['default']}'")
                else:
                    exec(f"self.{k_} = {v_['default']}")

    # Add properties to Foo class
    for k, v in kwargs.items():
        prop_ = get_property(k, v["validate"])
        setattr(Foo, k, prop_)

    # Create Foo class and set defaults
    return Foo(**kwargs)


foo = get_Foo(**config)

Now when testing it appears that 'setter' is working, but 'getter' is not and 'deleter' is partially working.

print(foo.x)  # prints None
print(foo._x) # prints 0
foo.x = 10
print(foo.x)  # prints None (getter not working)
print(foo._x) # prints 10  (setter works)
foo.x = -1    # raises error (validator in setter works)
del foo.x
print(foo.x)  # AttributeError: 'Foo' object has no attribute '_x' (del sort-of works?)

Could anyone explain the results?

`exec` **always** returns `None`. You'd need to use `eval`, but you shouldn't use *either*, use `getattr` and `setattr`, and `delattr` for all of this\ — juanpa.arrivillaga, Jan 01 '22 at 17:08
Check out: [Dynamically create class attributes](https://stackoverflow.com/questions/2583620/dynamically-create-class-attributes) — DarrylG, Jan 01 '22 at 17:32

score 0 · Accepted Answer · answered Jan 01 '22 at 17:32

Thanks to juanpa.arrivillaga; switching to getattr, setattr, delattr solved the issue.

Here is the solution:

def get_property(prop, validate):
    key = f"_{prop}"

    def get_(self):
        return getattr(self, key)

    def set_(self, value):
        if validate is not None:
            validate(value)
        setattr(self, key, value)

    def del_(self):
        delattr(self, key)

    return property(get_, set_, del_)


def get_Foo(**kwargs):
    class Foo:
        def __init__(self, **_kwargs):
            for k_, v_ in _kwargs.items():
                setattr(self, "_" + k_, None)
                setattr(self, k_, v_['default'])

    # Add properties to Foo class
    for k, v in kwargs.items():
        prop_ = get_property(k, v["validate"])
        setattr(Foo, k, prop_)

    # Create Foo class and set defaults
    return Foo(**kwargs)


foo = get_Foo(**config)

Create class with properties from config file

1 Answers1