numpy.ndarray.flatten

官方：
ndarray.flatten(order=’C’)
Return a copy of the array collapsed into one dimension.

Parameters:
order : {‘C’, ‘F’, ‘A’}, optional
Whether to flatten in row-major (C-style) or column-major (Fortran-style) order or preserve the C/Fortran ordering from a. The default is ‘C’.
Returns:
y : ndarray
A copy of the input array, flattened to one dimension.
See also
ravel
Return a flattened array.
flat
A 1-D flat iterator over the array.
Examples

a = np.array([[1,2], [3,4]])
a.flatten()
array([1, 2, 3, 4])

a.flatten(‘F’)
array([1, 3, 2, 4])