Better syntax for accessing existing shared types

[dlqqq]

Problem

Currently, one must bind an empty shared type to Ydoc keys before accessing their values:

    ydoc["cells"] = Array()
    assert ydoc["cells"].to_py() == [{"metadata": {"foo": "bar"}, "source": "1 + 2"}]
    #      ^- not equal to the empty Array() assigned to this key immediately before,
    #         but rather the value coming from another provider 

There are two drawbacks to only supporting this way of accessing a shared type:

1. This requires 1 additional line of code per shared type for the assignment statement, and can get verbose if one is using many. However, in Yjs, ydoc.getArray(...) can be used inline.

• The inline assignment operator := (available in Python 3.8+) does not work either:

/Users/dlq/micromamba/envs/rtcdev/lib/python3.11/ast.py:50: in parse
    return compile(source, filename, mode, flags,
E     File "/Volumes/workplace/pycrdt-websocket/tests/test_pycrdt_yjs.py", line 92
E       assert (ydoc["cells"] := Array()).to_py() == [{"metadata": {"foo": "bar"}, "source": "1 + 2"}]
E               ^^^^^^^^^^^^^
E   SyntaxError: cannot use assignment expressions with subscript

• From @davidbrochart:

  BTW you can write doc["my_array"] = my_array = Array() if you want a one-liner.

2. The syntax requires assigning an empty shared type in order to access an existing, non-empty shared type. This is stateful and confusing, because the value of ydoc["cells"] is not the value of ydoc["cells"] that was just set in the immediately preceding line. This generally violates how most programming languages work. I understand that this is permitted by Python, but the public API should not rely on exotic behavior exclusive to Python.

Proposed Solution

TBD.

Additional context

• Example and original discussion sourced from https://github.com/jupyter-server/pycrdt-websocket/pull/11

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[davidbrochart]

See #77.