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?

• juanpa.arrivillaga: exec(f"self.{key}") dont do that!
• juanpa.arrivillaga: 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\
• DarrylG: Check out: Dynamically create class attributes

Answer 1

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)