cregox cregox - 2 months ago 15
Python Question

Python's equivalent of .Net's sealed class

Does python have anything similar to a sealed class? I believe it's also known as final class, in java.

In other words, in python, can we mark a class so it can never be inherited or expanded upon? Did python ever considered having such a feature? Why?

Disclaimers




Actually trying to understand why sealed classes even exist. Answer here (and in many, many, many, many, many, really many other places) did not satisfy me at all, so I'm trying to look from a different angle. Please, avoid theoretical answers to this question, and focus on the title! Or, if you'd insist, at least please give one very good and practical example of a sealed class in csharp, pointing what would break big time if it was unsealed.



I'm no expert in either language, but I do know a bit of both. Just yesterday while coding on csharp I got to know about the existence of sealed classes. And now I'm wondering if python has anything equivalent to that. I believe there is a very good reason for its existence, but I'm really not getting it.

Answer

You can use a metaclass to prevent subclassing:

class Final(type):
    def __new__(cls, name, bases, classdict):
        for b in bases:
            if isinstance(b, Final):
                raise TypeError("type '{0}' is not an acceptable base type".format(b.__name__))
        return type.__new__(cls, name, bases, dict(classdict))

class Foo:
    __metaclass__ = Final

class Bar(Foo):
    pass

gives:

>>> class Bar(Foo):
...     pass
... 
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 5, in __new__
TypeError: type 'Foo' is not an acceptable base type

The __metaclass__ = Final line makes the Foo class 'sealed'.

Note that you'd use a sealed class in .NET as a performance measure; since there won't be any subclassing methods can be addressed directly. Python method lookups work very differently, and there is no advantage or disadvantage, when it comes to method lookups, to using a metaclass like the above example.

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