Add empty attr dictionary to lazy/dataframe

[mcrumiller]

Description

As of #13236 we can no longer dynamically add attributes to DataFrame and LazyFrame objects. However, sometimes it is desirable to add user attributes to frames for later use.

I propose here to add to __slots__ an empty attr dictionary so that users can store user-defined attributes in their tables, as in:

import polars as pl

df = pl.DataFrame()
df.attr["name"] = "my_dataframe"

[stinodego]

This is not reliable, as not all DataFrame methods preserve the class. If you were to df.sort(), the data would be gone.

Also, it kind-of defeats the purpose of __slots__. One of the benefits of __slots__ is that this is no longer possible, and users will not be surprised if the above happens.

Closing in favor of #5117

[mcrumiller]

One of the benefits of __slots__ is that this is no longer possible.

I'd call it more of a side effect. The primary benefit is faster access to the _df variable in almost all of the python methods, and adding another variable to __slots__ does not remove that benefit. I think it's worthwhile to allow a single variable for user-defined parameters. This is prevalent in other languages as well. A good resolution of the other issue would be preferred but that seems substantially more difficult and less likely to be realized.