That version (and the one in the article) are somewhat confusing to parse and detract from the point of the article.

This seems more "Pythonic":

    def lookup(d, key):
        return Just(d[key]) if key in d else None

    def values_in(keys, d):
        values = []
        for k in keys:
            x = lookup(d, k)
            if x:
                values.append(x.value)
        return values

If you really want a one-liner, I think this is easier to understand:

    def values_in(keys, d):
        return [x.value for x in (lookup(d, k) for k in keys) if x]