NumPy ベクトル化関数はどのようにして NumPy 配列を複数の方向に効率的に位置合わせできるのでしょうか?

import numpy as np

def justify(a, invalid_val=0, axis=1, side='left'):    
    if invalid_val is np.nan:
        mask = ~np.isnan(a)
    else:
        mask = a!=invalid_val
    justified_mask = np.sort(mask,axis=axis)
    if (side=='up') | (side=='left'):
        justified_mask = np.flip(justified_mask,axis=axis)
    out = np.full(a.shape, invalid_val) 
    if axis==1:
        out[justified_mask] = a[mask]
    else:
        out.T[justified_mask.T] = a.T[mask.T]
    return out

a = np.array([[1, 0, 2, 0],
               [3, 0, 4, 0],
               [5, 0, 6, 0],
               [0, 7, 0, 8]])

justified_array = justify(a, side='left')

print(justified_array)
# Output:
# [[1, 2, 0, 0],
#  [3, 4, 0, 0],
#  [5, 6, 0, 0],
#  [7, 8, 0, 0]]

justified_array = justify(a, axis=0, side='up')

print(justified_array)
# Output:
# [[1, 7, 2, 8],
#  [3, 0, 4, 0],
#  [5, 0, 6, 0],
#  [6, 0, 0, 0]]