You cannot use the Symbolic Math Toolbox and the Python Libraries together directly as while using the Symbolic Math Toolbox, MATLAB utilizes symbolic variables for calculations that cannot be interpreted by Python functions.
To avoid this issue, you need to provide numeric values to the Python functions by performing symbolic calculations in MATLAB and then converting them to numeric values before passing them to Python.
syms x
expr = x^2 + 2*x + 1;
val = subs(expr, x, 3); % Substitute x=3
% val_num = double(val); % Convert symbolic to numerical value double
% Now pass val_num to Python
pyFuncResult = py.yourFunction(val_num);
Another alternative as mentioned in the question is using the 'sympy' library. However to use the results from the Python object created using 'sympy', you would need to convert the result to string so that it can be used in MATLAB.
For more information regarding importing symbolic expressions from Python to MATLAB, refer to the following MATLAB Answer: https://www.mathworks.com/matlabcentral/answers/597118-how-to-import-symbolic-expressions-from-python-to-matlab
