The old names ("system" and "application" compression) were not really
accurate in all cases, so the compression types are now referred to by
their number.
The non-stream-based APIs still exist as before and are not deprecated,
they just act as thin wrappers around the stream-based API.
The main rsrcfork module doesn't use the stream-based APIs yet, because
it reads each resource's data all at once and not incrementally.
This is a little more complex than with the other decompressors,
because .dcmp2 has to behave differently when at the byte before EOF.
Checking whether this is the case requires lookahead, which is not easy
to do with a plain IO stream.
Some buffered IO streams provide a peek method for lookahead, but
others don't (such as io.BytesIO). There is no standard way to wrap an
already buffered IO stream to add a peek method, so we need a custom
wrapper class and helper function for this purpose.
The decompression code is more readable this way, because the
compressed data needs to be processed sequentially. It also allows
moving the length check and some debug logging into an outer generator.
This also allows incremental decompression, but this doesn't have any
practical advantage, because the compressed resource data is all read
at once (there is no API for opening resources as streams), and
resources are not very large anyway.
The leading underscore is meant to distinguish private (for internal
use only) APIs from public (for external use) APIs. One can argue about
where the line between public and private should be, but if something
is used from other modules (as with read_variable_length_integer) it's
not really private IMHO.
In scripts (like __main__) it also doesn't make much sense to use
leading underscores, because the entire file is never meant to be used
by external code.
Previously, the types of instance attributes were annotated with the
first assignment of each attribute. The standard way to annotate
instance attributes is to do so at class level without assigning any
value.
All decompressors now have exactly the same signature (as a result,
each decompressor now has to check itself that the header type is
correct). This allows the decompressors to be stored in a simple
dictionary, which makes the lookup process much simpler.
In preparation for #3, the compressed resource data headers are parsed
and stored as proper objects. For now these objects are only used
internally by the decompression code, but in the future they can be
exposed.
