Calling a python command from a deployed python library

I'm having trouble calling Python functions in a function that I've deployed to a Python library.

My MATLAB function is:

function out = test_python()
out = py.sys.path;
end

This works in MATLAB, but if I deploy and call my function from the interpreter, I get the following:

>>> import test_python
>>> runtime = test_python.initialize()
>>> runtime.test_python()
[stack trace]
MatlabRuntimeError: An error occurred when evaluating the result from a function. Details: 
  File 'C:\...\test_python.m, line 2, in test_python'
Undefined variable "py" or class "py.sys.path".

I know Python can be found, since

function [version, executable, isloaded] = test_python()
[version, executable, isloaded] = pyversion;
end

returns

>>> import test_python
>>> runtime = test_python.initialize()
>>> runtime.test_python(nargout=3)
 ('3.7', 'D:\\Users\\...\\Anaconda3\\python.EXE', False)