Python access derived class attributes in parent constructor

Question

I've a class structure like

class Parent:
   value_type = (object,)
   def __init__(self, value):
      if not isinstance(value, value_type):
          raise ValueError
      self.value = value

class Derived(Parent):
    value_type = (str,)

When I create an instance variable of derived class as

d = Derived(555)

It doesn't raises any error. (The behaviour I need is that it should raise an error in this case as 555 is not a str)

Is there any way to make the parent class constructor use the derived class attribute value_type instead of it's own class attribute?

I know passing value_type as a parameter to __init__ function is a solution, but the value_type should be same throughout the class, so I don't want to redundantly add same value_type field everytime to initialize a derived class instance.

This very much does raise an error, namely NameError: name 'value_type' is not defined. — Aran-Fey
– Aran-Fey, Commented Jun 17, 2017 at 9:26
Note you should probably raise a TypeError, and ideally with more information. — jonrsharpe
– jonrsharpe, Commented Jun 17, 2017 at 9:30

Artyer · Accepted Answer · 2017-06-17 09:27:40Z

3

The value_type used in Parent is always the value of value_type in the scope, which is whatever you assigned value_type to be in the global scope.

To fix this, use self.value_type to get the value_type assigned to the created instance:

def __init__(self, value):
    if not isinstance(value, self.value_type):
        raise TypeError
    self.value = value

answered Jun 17, 2017 at 9:27

Artyer

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