How to append to .mat file using scipy.io.savemat?

According to savemat docs:

file_name : str or file-like object

So you can open the file in append mode, and write, e.g.

io.savemat('temp.mat',{'data':np.ones(10)})  # write
with open('temp.mat','ab') as f:
    io.savemat(f, {'newdata':np.arange(5)})   # append
print io.loadmat('temp.mat').keys()           # read
# dict_keys(['data', '__globals__', 'newdata', '__header__', '__version__'])

Or the full description:

{'data': array([[ 1.,  1.,  1.,  1.,  1.,  1.,  1.,  1.,  1.,  1.]]),
 '__globals__': [],
 'newdata': array([[0, 1, 2, 3, 4]]),
 '__header__': b'MATLAB 5.0 MAT-file Platform: posix, Created on: Fri Mar 13 14:14:33 2015',
 '__version__': '1.0'}

A note in https://github.com/scipy/scipy/blob/master/scipy/io/matlab/mio5.py#L34 suggests that there is a problem with appending when there's a function in the data file, but this indicates that appending isn't a problem if we are just saving arrays. But maybe further search of the scipy issues is in order.

It only works for one append!If you append twice, Matlab gives an error "File might be currupt".

scipy.io.savemat('temp.mat',{'data':np.ones(10)})  # write
with open('temp.mat','ab') as f:
    scipy.io.savemat(f, {'newdata1':np.arange(5)})   # append
with open('temp.mat','ab') as f:
   scipy.io.savemat(f, {'newdata2':np.arange(5)})   # append