Difficulty with ExternalEvaluate

Question

I am trying to import a Python function or execute a Python script from inside Mathematica, but the ExternalEvaluate command drives me nuts.
I proceed cautiously with the following Python test module, named "test.py":

def test():
   print("Test")
print("Hello")

I checked that the module can be found and tried it successfully from the CMD command line:

python
from test import test

Using Import in Mathematica also works:

Import["!python g:\\p\\python37\\test.py", "String"]

But then with ExternalEvaluate I first made sure that the Python connection works as follows, which works OK:

ExternalEvaluate["Python", "2+3"]

However, the following gives Exception: Import Error. EvaluationFailure:

ExternalEvaluate["Python", "from test import test"]

The same happens if I use a session:

ses = StartExternalSession["Python"]
ExternalEvaluate[ses, "from test import test"]

I would be glad for a hint what I am doing wrong

Answer 1

By default when you start a session, Working directory in Windows, is Documents folder:

ExternalEvaluate["Python", "import os;os.getcwd()"]

(*Out: "C:\\Users\\USERNAME\\Documents" *)

Import a single file

If you want to import a single file that does not import other files (not dependent on others), this solution will work:

ss = StartExternalSession["Python"];

ExternalEvaluate[ss, File["C:\\foo.py"]]

DeleteObject[ss];

Now it's like you had executed from foo.py import * and have access to all of its functions and variables.

Import a file with its dependencies

If you have a file that imports other files, the above solution will raise an error:

File: C:\bar.py
===============

import foo

foo.test()

ss = StartExternalSession["Python"];

ExternalEvaluate[ss, File["C:\\bar.py"]]

DeleteObject[ss];

The solution is to make the file directory known to the python interpreter which have so many solutions (some discussed here):

One of the solutions is to add sys.path.append(...) to the beginning of that file:

File: C:\bar.py
===============

import sys
sys.path.append('C:\\')

import foo

foo.test()

Now you have access to all of the functions and variables + ability to import other files.

All the code tested on Mathematica 12.2.