foo.py

#!/usr/bin/env python
# vim: set ts=4 sw=4 et foldmethod=indent paste:
#
# ---

people = [{'name': 'Mary', 'height': 160},
          {'name': 'Isla', 'height': 80},
          {'name': 'Sam'}]

hs = map(lambda o: o['height'], filter(lambda o: 'height' in o, people))
sum_h = reduce(lambda a, o: a + o, hs, 0)

if sum_h > 0:
    print str(sum_h/len(hs))