[Python-ideas] Unpacking a dict

Michael Selik michael.selik at gmail.com
Wed May 25 09:11:35 EDT 2016

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
    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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