Right now, the default arguments for sorting-related methods are:

Defaults to axis=-1:

• np.sort
• np.ndarray.sort
• np.argsort
• np.ndarray.argsort
• np.ma.sort
• np.masked_array.sort
• The documentation of np.ma.argsort
• The documentation of np.masked_array.argsort

Defaults to axis=None:

• The implementation of np.ma.argsort
• The implementation of np.masked_array.argsort

For 1D arrays, there is no difference, as None just means flatten and use the only axis.

For 2D arrays, however, code written for ndarrays can fail in unexpected ways when passed a masked_array, because argsort breaks liskov subsitution:

def foo(x):
    i = x.argsort()
    assert(i.shape == x.shape)

>>> foo(som_arr)  # ok
>>> foo(som_arr.view(MaskedArray)) # AssertionError