How to call a python file, that needs to import packages?

Question

I'm following a tutorial to call python code from a C++ program from the python docs.

Everything works just fine when trying to call the multiply example. Now if I add a line to the python source code importing a library, lets say openpyxl,

from openpyxl import load_workbook

I receive an error from python

ImportError: No module named openpyxl

I thought if I import a system library, I wouldn't have any problems, but I also get an error if I try to import datetime.

I don't have any error if I import the file from the python console. The openpyxl library is installed in my system.

So my question is: how to import python source code that needs to import packages?

EDIT: Ok, I forgot to mention something, I have not been completely honest with you guys, I'm sorry.

Trying to run the example I run into a problem: I couldn't make python found my multiply.py file, and the line PyImport_Import always return null.

My solution was to add the path in which I knew my python source was by using PySys_SetPath. The problem is that I just realized that this function doesn't append a new directory, it just overwrites the PYTHONPATH. So now python can find multiply.py, but absolutly anything else.

Of course I've deleted that line but now I have another question, why does python can't find my source if the file is just in the same directory of the C++ compiled program?

The I realized that my sys.path from my python console was a little different from the path showed in my embedded python: the first one had at the beginning of the list an empty string ''. I'm not a python expert, but when I add that line to my path I could import the multiply.py so it seems that was the reason I couldn't import modules that were located to relative to my executable was the missing of this empty path -but still don't know what it means-.

I have to thank to @paul-evans who give me the idea of adding the path to find my files.

Answer 1

This is what PYTHONPATH is for. You can set it as an environment variable containing a list module directories, or in the code itself something like:

import sys
sys.path.append("path/to/openpyxl/module")