To create a matrix with only nan values with numpy a solution is to use numpy.empty((), numpy.nan and fill():

`import numpy as np`

`A = np.empty((5,5))`

`A.fill(np.nan)`

returns

`[[nan nan nan nan nan]`

`[nan nan nan nan nan]`

`[nan nan nan nan nan]`

`[nan nan nan nan nan]`

`[nan nan nan nan nan]]`

Another solution is to do

`A[:] = np.nan`

Note: to check of an element of the matrix is nan, a solution is to use numpy.isnan

`np.isnan(A[2,3])`

returns

`True`