past.builtins ImportError

Problem

Calling a Caffe2 Python call gave this error:

$ python -c "from caffe2.python import core"
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/usr/local/lib/python2.7/dist-packages/caffe2/python/core.py", line 9, in <module>
    from past.builtins import basestring
ImportError: No module named past.builtins

Solution

This requires the future package. Installing that solved the problem:

$ pip install --user future

Tried with: Raspbian 9