As of now, the method
is_mountable is hard-coded to return True:
This doesn't pose problems in normal situation, since
pip install doesn't normally mount wheels on installation anyway, but it leads to a problem when 3rdparty libraries trust this flag, e.g. like humpty does. That lib takes
is_mountable=True as the fact that the resulting egg would come out
zip-safe, which is not true for packages containing arbitrary data, like jinja templates.
is_mountable=False is a safer stub to have, as it should not create false expectations.