Update different dataframes in imported module from inside a class instance, depending on parameters sent to class instance

Question

I believe my problem is adequately described in the picture:

1. In the first function, I assign one of two imported dataframes as params to be sent to the class instance.

2. In the class instance, these params become dataframes contained in local variables, with no connection to the shared resource I want to update.

3. When I run a concatenate operation on the dataframe, the shared resource in the imported dataframe is not updated. Only the local copy in the class instance is updated.

How to get around this? I cannot use a global declaration, since a variable cannot be both a parameter and global at the same time (throws an error)

This is the part where I find Python quirky compared to most other languages. Any guidance or advice higly appreciated. =)