Move some fields (exclude, only, etc) to private API in `Schema.__init__`

[fuhrysteve]

I'd make exclude and only private API because they are used in _normalize_nested_options which is called only at __init__.

I'd also make load_only and dump_only private because setting them after __init__ conflicts with

        self.load_only = set(load_only) or set(self.opts.load_only)
        self.dump_only = set(dump_only) or set(self.opts.dump_only)

Likewise with unknown and context due to

        self.unknown = unknown or self.opts.unknown
        self.context = context or {}

(setting them to None after __init__ wouldn't be the same as in __init__)

At this point, I'm tempted to make everything private. I mean the attributes above plus many, ordered and partial.

Adding the underscore would break libs using marshmallow (like apispec) but that's life. Or is there another way to signify those are private? Like, just a comment in the docstring?

Originally posted by @lafrech in #1258 (comment)

[lafrech]

Would it make sense to make them read-only rather than strictly private?

It can be useful to access them after init to sort of clone a schema. This is what we do in umongo to generate a marshmallow schema from a (marshmallow-based) model schema.

[sirosen]

Would it make sense to make them read-only rather than strictly private?

I think that would probably make everyone's life easier? Both internally and externally, there would be fewer updates to code that really just wants to read these attributes.

I think all that's needed is

def __init__(self, ...):
    ...
    self._unknown = <init_value>

@property
def unknown(self):
    return self._unknown

And since that will presumably happen a bunch of times, it might be worth doing something like

def __init__(self, ...):
    ...
    readonly_property(self, "unknown", <init_value>)

def readonly_property(...): ...  # something to figure out