ValueError: only a scalar struct can be returned from MATLAB

I tried to use matlab.engine in python to import a struct from Matlab. my operation system is macOS Sierra, and the error message is :

File "/lib/python2.7/site-packages/matlab/engine/fevalfuture.py", line 82, in result

 self._result = pythonengine.getFEvalResult(self._future,self._nargout, None, out=self._out, err=self._err)

   ValueError: only a scalar struct can be returned from MATLAB

The values in the fields of the struct are cell, logical, single, unit8, and unit16. I am not sure what scalar struct refers to. Has anyone met the same problem before ?

Thanks!

[Walter Roberson]

A scalar struct is one for which size() of the struct itself says [1 1]. It does not matter what size the fields contained in the struct are, just the size of the struct itself. For example,

test1.this = 1:20;
test1.that = 'hello';

test2(1).this = 1;
test2(1).that = char('what','is','the','result','here?');

test3(1).this = 1:20;
test3(1).that = 'hello';
test3(2).this = 1;
test3(2).that = char('what','is','the','result','here?');

With these lines, size(test1) and size(test2) will both be [1 1] -- they are scalar structures, no matter how big the fields they hold are. Referring to test1.that would give you the single result 'hello' .

However, size(test3) will be [1 2]: it is not a scalar structure, it is a structure array. Referring to test3.that would give you a comma-separated list of two results, the first of which was 'hello' and the second of which was the char array, because of "structure expansion".