AssembledPC: fix sub_mat_type

[pbrubeck]

Description

[connorjward]

This is really obscure. Why are those options disallowed?

[pbrubeck]

If we pass sub_mat_type=None we would like to grab the one from the parent _SNESContext. Basically this enables

sub_mat_type = kwargs.get("sub_mat_type") or self.sub_mat_type

but more generically

[pbrubeck]

The line below kwargs.setdefault("sub_mat_type", self.sub_mat_type) is a no-op if submat_type=None is already in the dictionary.

[pbrubeck]

Should the lines below be changed to something like

kwargs["sub_mat_type"] = kwargs.get("sub_mat_type") or self.sub_mat_type

?

[connorjward]

Yeah I think that's much better

[pbrubeck]

That will potentially cause conflicts with empty dictionaries or empty strings

[connorjward]

How so?

[pbrubeck]

We need

def set_default_kwarg(kwargs, key, default):
    if kwargs.get(key) is None:
        kwargs[key] = default

set_default_kwarg(kwargs, "sub_mat_type", self.sub_mat_type)
...

[pbrubeck]

How so?

reconstruct should keep the original arguments if they are not passed as kwargs or if they are set to None in the kwargs. Sometimes one might need to pass appctx={} or options_prefix="", which are falsy values that we should not treat as None.

[connorjward]

Explicitly filtering Nones like that is also fine. I just don't like the premature filtering of all kwarg options.