It appears to work without list comprehensions as well:

    Python 3.7.1 (default, Nov  6 2018, 18:49:54)
    Type 'copyright', 'credits' or 'license' for more information
    IPython 7.1.1 -- An enhanced Interactive Python. Type '?' for help.

    In [1]: lookup = {}

    In [2]: a, lookup[a] = 'xy'

    In [3]: a
    Out[3]: 'x'

    In [4]: lookup
    Out[4]: {'x': 'y'}

I wasn't previously aware that this was possible, but I guess it's no different from

    a = 'x'
    lookup[a] = 'y'  # i.e. lookup['x'] = y