Allow MetaFrame to be extended by inheritance

[kori73]

It would be a useful feature if we allow users to extend MetaFrame with their custom methods. Currently, we are creating a MetaFrame instance instead of a DataFrame. I think changing this to cls(...) should work, but I haven't tested it. Example usage:

class MyDataFrame(MetaFrame):
    def my_method(self):
        ...

[burakyilmaz321]

I did't understand this. Why cannot users extend MetaFrame now?

[kori73]

Sorry for the late reply. Let me elaborate on this.

Following lines indicate that we are returning a MetaFrame insance:

https://github.com/invent-analytics/metaframe/blob/main/metaframe/metaframe.py#L111-L117

Let's say I have created a subclass of MetaFrame, and added some methods.

class MyDataFrame(MetaFrame):
    def my_method(self):
        ...

If I create an instance of MyDataFrame, and perform a join (or some other operation), the result will be a MetaFrame instance instead of a MyDataFrame instance. Then, if I'm not mistaken, I won't be able to use my_method in the subsequent operations.