Passing from Python to MATLAB a cell array of structures

Question

Thanks to scipy.io it is fast and easy to pass from Python to MATLAB, objects like stuctures or cell arrays.

MATLAB structure:

in Python:

import scipy.io as sio

titi={'oui': 'Y', 'non': 'N', 'AgeDuCapitaine': 53}
sio.savemat('titi.mat', {'titi': titi})

in Matlab:

load('titi')
titi

    titi = 

    AgeDuCapitaine: 53
               oui: 'Y'
               non: 'N'

titi.AgeDuCapitaine

    ans =

                   53

MATLAB cell array:

in Python:

import scipy.io as sio
import numpy as np

tutu=np.zeros((3,), dtype=np.object)
tutu[0]=1
tutu[1]='omg'
tutu[2]=np.zeros((2,), dtype=np.object)
tutu[2][0]='vrai'
tutu[2][1]=2
sio.savemat('tutu.mat', {'tutu': tutu})

in Matlab:

load('tutu')
tutu

    tutu = 

    [1]    'omg'    {1x2 cell}

tutu{1}

    ans =

                1

tutu{3}{1}

    ans =

                vrai

However, let's say that we want to pass an object corresponding to a mixed MATLAB cell array of structures, as an example a final MATLAB object like:
toto{1}.weapon{2}.Name='fleurs' ...

MATLAB cell array of structures:

in python (test, not yet convincing !):

import scipy.io as sio
import numpy as np

toto = np.zeros((2,), dtype=np.object)
toto[0] = {}
toto[1] = {}
toto[0]['weapon'] = np.zeros((2,), dtype=np.object)
toto[0]['weapon'][0] = {}
toto[0]['weapon'][1] = {}
toto[0]['weapon'][1]['Name'] = 'fleurs'
toto

   array([{'weapon': array([{}, {'Name': 'fleurs'}], dtype=object)}, {}], dtype=object)

sio.savemat('toto.mat', {'toto':toto})

   Traceback (most recent call last):
       File "<stdin>", line 1, in <module>
       File "/usr/lib64/python2.7/site-packages/scipy/io/matlab/mio.py", line 207, in savemat
         MW.put_variables(mdict)
       File "/usr/lib64/python2.7/site-packages/scipy/io/matlab/mio5.py", line 876, in put_variables
         self._matrix_writer.write_top(var, asbytes(name), is_global)
       File "/usr/lib64/python2.7/site-packages/scipy/io/matlab/mio5.py", line 626, in write_top
         self.write(arr)
       File "/usr/lib64/python2.7/site-packages/scipy/io/matlab/mio5.py", line 655, in write
         self.write_cells(narr)
       File "/usr/lib64/python2.7/site-packages/scipy/io/matlab/mio5.py", line 759, in write_cells
         self.write(el)
       File "/usr/lib64/python2.7/site-packages/scipy/io/matlab/mio5.py", line 653, in write
         self.write_struct(narr)
       File "/usr/lib64/python2.7/site-packages/scipy/io/matlab/mio5.py", line 764, in write_struct
         self._write_items(arr)
       File "/usr/lib64/python2.7/site-packages/scipy/io/matlab/mio5.py", line 782, in _write_items
         self.write(el[f])
       File "/usr/lib64/python2.7/site-packages/scipy/io/matlab/mio5.py", line 655, in write
         self.write_cells(narr)
       File "/usr/lib64/python2.7/site-packages/scipy/io/matlab/mio5.py", line 759, in write_cells
         self.write(el)
       File "/usr/lib64/python2.7/site-packages/scipy/io/matlab/mio5.py", line 647, in write
         % (arr, type(arr)))
   TypeError: Could not convert {} (type <type 'dict'>) to array

So, is it possible to create in Python and pass to MATLAB a cell array of structures ?
Did I make a mistake ?

servoz · Accepted Answer · 2017-03-14 15:16:30Z

Ooops I just understood why the TypeError exception was raised ... In fact, this is the right way to pass a cell array of structures from Python to MATLAB but ... It seems that the scipy.io.savemat does not allow empty dictionary. Indeed If I complet all the dictionnaries all is working fine !:

In Python:

>>> import scipy.io as sio   
>>> import numpy as np

>>> toto = np.zeros((2,), dtype=np.object)
>>> toto[0]={}
>>> toto[1]={}
>>> toto[1]['Site']=['Tataouine']
>>> toto[0]['weapon'] = np.zeros((2,), dtype=np.object)
>>> toto[0]['weapon'][0]={}
>>> toto[0]['weapon'][1]={}
>>> toto[0]['weapon'][1]['Name']='fleurs'
>>> toto[0]['weapon'][0]['Name']='bonbons'
>>> toto
array([ {'weapon': array([{'Name': 'bonbons'}, {'Name': 'fleurs'}], dtype=object)},
       {'Site': ['Tataouine']}], dtype=object)
>>> sio.savemat('toto.mat', {'toto':toto})

In MATLAB:

>> load('toto')
>> toto

toto = 

    [1x1 struct]    [1x1 struct]

>> toto{1}.weapon{2}.Name

ans =

fleurs

Hope this will help you to not waste your time with this silly problem !!

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