why i cannot invoke python script using command line?

[smith jack]

but i can invoke it in eclipse, what's wrong?
the script refered to another python script in eclipse project.

f:\project\src\a.py
f:\project\src\lib\b.py

there is such lines in a.py
from lib import b

i can invoke a.py very well in eclipse

but failed when using python f:\project\src\a.py, what's wrong?
(the error msg shows a.py cannot find b.py) , what should i do in
order to run a.py using command line?