export PYTHONPATH=/home/dev/python-files

$ python ~/python-files/py-vgdl/examples/gridphysics/frogs.py
Traceback (most recent call last):
    File "/home/dev/python-files/py-vgdl/examples/gridphysics/frogs.py", line 67, in <module>
        from vgdl.core import VGDLParser
ImportError: No module named vgdl.core

>>> import os
>>> os.environ["PYTHONPATH"]
'/home/dev/python-files'

export PYTHONPATH=$PYTHONPATH:/home/dev/python-files

PYTHONPATH=/home/dev/python-files/py-vgdl

PYTHONPATH="/home/dev/python-files/py-vgdl:/home/dev/python-files/something:$PYTHONPATH"