MATLAB Errors in Python

When a MATLAB^® function raises an error, the MATLAB Engine for Python^® stops the function and catches the exception raised by MATLAB. The engine copies the error message to a new Python exception. The engine raises the Python exception.

If the Python interpreter catches the exception, the interpreter displays the error message that came from MATLAB. You also can handle exceptions raised by the engine in your Python code. See the matlab.engine.MatlabEngine and matlab.engine.FutureResult reference pages for the types of exceptions that the engine can raise.

MatlabExecutionError: Undefined Function

Call the MATLAB sqrt function on an integer from Python. (This code sample omits the Python trace back and shows the error message only.)

import matlab.engine
eng = matlab.engine.start_matlab()
print(eng.sqrt(4))

matlab.engine.MatlabExecutionError: Undefined function 'sqrt' for input arguments of type 'int64'.

MATLAB defines a sqrt function, but expects the input argument to be of data type double, not an integer. However, the input argument is 4, and before it is passed to MATLAB, Python interprets 4 as an integer. The engine converts the Python integer to an int64 MATLAB data type.

MATLAB and Python define different default types for numbers. If you type x = 4 at the MATLAB command line, x is a MATLAB double. If you type x = 4 at the Python command line, x is a Python int.

To avoid this error, specify input arguments that are of Python data type float. The engine converts this type to MATLAB double.

print(eng.sqrt(4.0))

2.0

SyntaxError: Expression Not Valid Target

You can call the MATLAB eval function from Python to create MATLAB variables. (This code sample omits the Python trace back and shows the error message only.)

import matlab.engine
eng = matlab.engine.start_matlab()
eng.eval("x = 4;")

SyntaxError: Error: The expression to the left of the equals sign is not a 
valid target for an assignment.

When the engine calls eval, it passes a statement to MATLAB for execution. When you do not specify the nargout input argument, the engine expects one output argument. However, this MATLAB statement returns no output arguments.

To avoid this error, specify nargout as 0 whenever the MATLAB function you call returns no output arguments.

eng.eval("x = 4;",nargout=0)

Cannot Find MATLAB Session in Python

If you override the operating system TEMP or TMP environment variables in MATLAB, Python might not be able to connect to the MATLAB Engine for Python. For example, if you type the following at the Python prompt:

matlab.engine.find_matlab()

Python displays ().

MATLAB Engine for Python uses the temp folder to record information for shared MATLAB sessions. To work around this issue, make the following changes to the environment variables in Python. temp_folder is the path to the folder which you set in MATLAB.

os.environ['TMP'] = r'temp_folder'
os.environ['TEMP'] = r'temp_folder'
eng=matlab.engine.find_matlab()