import numpy as np

def view_diagonals(arr: np.ndarray) -> np.ndarray:
    """Returns a view of all the diagonals in the array `arr`
    
    If `arr` has shape (n1, n2, *rest),
    then `view_diagonals(arr)` has shape (ndiags, nelmts, *rest), where
      - `ndiags = max(1, n1 - n2 + 1)`
      - `nelmts = min(n1, n2)`
    """
    ndiags = max(1, arr.shape[0] - (arr.shape[1]-1))
    nelmts = min(arr.shape[0], arr.shape[1])
    return np.lib.stride_tricks.as_strided(
        arr,
        shape=(ndiags, nelmts, *arr.shape[2:]),
        strides=(arr.strides[0], arr.strides[0] + arr.strides[1], *arr.strides[2:])
    )

for L in np.arange(6*3).reshape(6,3), np.arange(6*3).reshape(3,6), np.arange(6*3*2).reshape(6,3,2):
    Ldiags = view_diagonals(L)
    Lantis = view_diagonals(np.fliplr(L))
    Ltdiags = view_diagonals(L.transpose())
    print('L = ', L, sep='\n')
    print('diagonals of L =', Ldiags, sep='\n')
    print('anti-diagonals of L =', Lantis, sep='\n')
    print('horizontal diagonals (diagonals along axes 1,0 instead of 0,1) of L =', Ltdiags, sep='\n')
    print()
    assert(np.shares_memory(L, Ldiags))
    assert(np.shares_memory(L, Lantis))
    assert(np.shares_memory(L, Ltdiags))

L = 
[[ 0  1  2]
 [ 3  4  5]
 [ 6  7  8]
 [ 9 10 11]
 [12 13 14]
 [15 16 17]]
diagonals of L =
[[ 0  4  8]
 [ 3  7 11]
 [ 6 10 14]
 [ 9 13 17]]
anti-diagonals of L =
[[ 2  4  6]
 [ 5  7  9]
 [ 8 10 12]
 [11 13 15]]
horizontal diagonals (diagonals along axes 1,0 instead of 0,1) of L =
[[0 4 8]]

L = 
[[ 0  1  2  3  4  5]
 [ 6  7  8  9 10 11]
 [12 13 14 15 16 17]]
diagonals of L =
[[ 0  7 14]]
anti-diagonals of L =
[[ 5 10 15]]
horizontal diagonals (diagonals along axes 1,0 instead of 0,1) of L =
[[ 0  7 14]
 [ 1  8 15]
 [ 2  9 16]
 [ 3 10 17]]

L = 
[[[ 0  1]
  [ 2  3]
  [ 4  5]]

 [[ 6  7]
  [ 8  9]
  [10 11]]

 [[12 13]
  [14 15]
  [16 17]]

 [[18 19]
  [20 21]
  [22 23]]

 [[24 25]
  [26 27]
  [28 29]]

 [[30 31]
  [32 33]
  [34 35]]]
diagonals of L =
[[[ 0  1]
  [ 8  9]
  [16 17]]

 [[ 6  7]
  [14 15]
  [22 23]]

 [[12 13]
  [20 21]
  [28 29]]

 [[18 19]
  [26 27]
  [34 35]]]
anti-diagonals of L =
[[[ 4  5]
  [ 8  9]
  [12 13]]

 [[10 11]
  [14 15]
  [18 19]]

 [[16 17]
  [20 21]
  [24 25]]

 [[22 23]
  [26 27]
  [30 31]]]
horizontal diagonals (diagonals along axes 1,0 instead of 0,1) of L =
[[[ 0  6 12 18 24 30]
  [ 3  9 15 21 27 33]]]