Martin von Zweigbergk <martinvonz@google.com> [Fri, 12 Jul 2019 23:19:50 -0700] rev 4740
py3: don't depend on map() iterating over its input
map(some_generator()) in py2 returns a list, while in py3 it returns a
generator, so the passed-in generator won't be called unless the
returned one is.