Argmax of numpy array returning non-flat indices

You could use numpy.unravel_index() on the result of numpy.argmax():

>>> a = numpy.random.random((10, 10))
>>> numpy.unravel_index(a.argmax(), a.shape)
(6, 7)
>>> a[6, 7] == a.max()
True

np.where(a==a.max())

returns coordinates of the maximum element(s), but has to parse the array twice.

>>> a = np.array(((3,4,5),(0,1,2)))
>>> np.where(a==a.max())
(array([0]), array([2]))

This, comparing to argmax, returns coordinates of all elements equal to the maximum. argmax returns just one of them (np.ones(5).argmax() returns 0).

To get the non-flat index of all occurrences of the maximum value, you can modify eumiro's answer slightly by using argwhere instead of where:

np.argwhere(a==a.max())

>>> a = np.array([[1,2,4],[4,3,4]])
>>> np.argwhere(a==a.max())
array([[0, 2],
       [1, 0],
       [1, 2]])