> return headers.get('x-foo') == settings.FOO_KEY is not None

I'm a huge Python fanboy and this line is absolutely awful.

Comparison chains are great for stuff like "x < y < z" or "x == y == z", but the operators should never be mixed.

"Readability counts" is, IMO, the most important line in the Zen of Python, and that line is unreadable. I imagine it's equivalent to:

    return headers.get('x-foo') == settings.FOO_KEY and settings.FOO_KEY is not None

Which, admittedly, feels like a clumsy line of code, but it's at least readable.