[Python-ideas] Unpacking a dict

[Michael Selik]

Python's iterable unpacking is what Lispers might call a destructuring bind.

    py> iterable = 1, 2, 3, 4, 5
    py> a, b, *rest = iterable
    py> a, b, rest
    (1, 2, (3, 4, 5))

Clojure also supports mapping destructuring. Let's add that to Python!

    py> mapping = {"a": 1, "b": 2, "c": 3}
    py> {"a": x, "b": y, "c": z} = mapping
    py> x, y, z
    (1, 2, 3)
    py> {"a": x, "b": y} = mapping
    Traceback:
    ValueError: too many keys to unpack


This will be approximately as helpful as iterable unpacking was before PEP
3132 (https://www.python.org/dev/peps/pep-3132/).

I hope to keep discussion in this thread focused on the most basic form of
dict unpacking, but we could extended mapping unpacking similarly to how
PEP 3132 extended iterable unpacking. Just brainstorming...

    py> mapping = {"a": 1, "b": 2, "c": 3}
    py> {"a": x, **rest} = mapping
    py> x, rest
    (1, {"b": 2, "c": 3})
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