local bindings from a map

bindings.py

#! /usr/bin/env python3

# This is a Proof-Of-Concept, and only works in Python3.
# A Python2 port is almost certainly possible, haven't even tried.


from inspect import currentframe
from dis import get_instructions


def bindings(source_map):
    """
    Find just the values for the bindings you were going to ask for.
    """

    # find our calling frame, and the index of the op that called us
    caller = currentframe().f_back
    code = caller.f_code
    index = caller.f_lasti

    # disassemble the instructions for the calling frame, and advance
    # past our calling op's index
    iterins = get_instructions(code)
    for instr in iterins:
        if instr.offset > index:
            break

    if instr.opname != "UNPACK_SEQUENCE":
        # someone invoked us without being the right-hand side of an
        # unpack assignment, do let's be a noop
        return source_map

    # this is the number of assignments being unpacked, we'll get that
    # many STORE_ ops from the bytecode
    count = instr.argval

    # each STORE_ op has an argval which is the name it would assign
    # to. This is just a convenience that the dis module fills in!
    dest_keys = (next(iterins).argval for _ in range(0, count))

    # finally, just return a generator that'll provide the values from
    # the source_map that match to the bindings
    return (source_map[dest] for dest in dest_keys)


def test():

    data = {"tacos": 900, "beer": 700, "a": 100, "b": 200, "c": 300, }

    a, b, c = bindings(data)
    print("a:", a)
    print("b:", b)
    print("c:", c)


if __name__ == "__main__":
    test()

Summarizing things you'd need to do to make this more robust:

• check that the instructions after UNPACK_SEQUENCE are all STORE_FAST, STORE_DEREF, STORE_GLOBAL, or STORE_NAME. If they aren't, it would indicate some nested unpacking or a *arg function application. In both of those cases, just return the original map
• this is targeted at Python3. The Python2 dis module kinda sucks, and there's no Instruction data type. You'd have to copy/paste most of dis.dis and have it accumulate and yield the ops and their arguments. Still well within reason!
• collapse it all into one function
• write a bunch of unittests and check across a wide range of pythons (2.5+, 3.3+)

https://github.com/obriencj/python-mapbind