The relative path in sys.path only works for Python 3.x. The following importing method is path and OS agnostic. The only requirement is that the main script and the imported module files have a common parent folder.
$ take pkgex
$ Python -V
Python 3.6.3
mkdir -p {driver,step/impl}
$ cat << EOF > step/impl/inputstep.py
def myfunc(x, y):
return x * 10 + y
EOF
$ cat << EOF > driver/main.py
import sys, os
sys.path.append(os.path.join(sys.path[0], '..'))
# or this: sys.path.append(os.path.join(os.path.dirname(__file__), '..'))
from step.impl.inputstep import myfunc
print(myfunc(3,5))
EOF
$ python driver/main.py
35
Note 1: for Python 3.x, the init.py file is unnecessary.
Note 2: sys.path.append('..') does not work.
import module, class and function
import module with full qualified name
import step.impl.inputstep
step.impl.inputstep.myfunc(3, 5)
import module
from step.impl import inputstep
inputstep.myfunc(3, 5)
import function
from step.impl.inputstep import myfunc
myfunc(3, 5)