How do you cast an instance to a derived class?

Question

I am trying to use a little inheritance in a Python program I am working on. I have a base class, User, which implements all of the functionality of a user. I am adding the concept of an unapproved user, which is just like a user, with the addition of a single method. The User class has some methods that return a User object. This will not work when I subclass, since I will end up having an UnapprovedUser return a User, preventing me from calling this method, among other things. ``` class User(object): base_dn = 'ou=Users,dc=example,dc=org' @classmethod def get(cls, uid): ldap_data = LdapUtil.get(uid + ',' + self.base_dn) return User._from_ldap(ldap_data) class UnapprovedUser(User): base_dn = 'ou=UnapprovedUsers,dc=example,dc=org' def approve(self): new_dn = '' # the new DN LdapUtil.move(self.dn, new_dn) ``` The `get()` and `_from_ldap()` methods are the same for both classes, though the `get()` method in UnapprovedUser needs to return an UnapprovedUser object, not a User. How can I c…

Accepted Answer

Rather than "casting", I think you really want to create an `UnapprovedUser` rather than a `User` when invoking `UnapprovedUser.get()`. To do that:

Change `User.get` to actually use the `cls` argument that's passed-in:

```
@classmethod
def get(cls, uid):
    ldap_data = LdapUtil.get(uid + ',' + self.base_dn)
    return cls._from_ldap(ldap_data)
```

You'll need to do something similar in `_from_ldap`. You didn't list the code for `_from_ldap`, but I assume that at some point it does something like:

```
result = User(... blah ...)
```

You want to replace this with:

```
result = cls(... blah ...)
```

Remember: in Python a class object is a callable that constructs instances of that class. So you can use the `cls` parameter of a classmethod to construct instances of the class used to call the classmethod.

How do you cast an instance to a derived class?

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