Let me give you an example.
There are two files. The first is compute.py and has the code:
def sumup(a,b):
return a+b
And there is your program, main.py
import compute
print compute.sumup(2,3)
The question is, how will python know where to find this compute module?
The answer is
here.
So in the most simple case, if you put these two files into the same directory, you go there, and run python main.py , then it will work.
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