[PYTHON] Create an empty array in Numpy to add rows for each loop

I think I'll forget it three months later, so make a note.

I get angry when I try to np.r \ _ on an empty array with numpy.

arr = np.array([])
additional = np.array([[1, 2, 3], [4, 5, 6]])
arr = np.r_[arr, additional]
print(arr)

>>>
ValueError: all the input arrays must have same number of dimensions

However, I don't think this method is Pythonic.

arr = np.zeros([1, 3])
additional = np.array([[1, 2, 3], [4, 5, 6]])
arr = np.r_[arr, additional][1:]
print(arr)

>>>
array([[ 1.,  2.,  3.],
       [ 4.,  5.,  6.]])

This can be solved with numpy.empty.

arr = np.empty([0, 3])
additional = np.array([[1, 2, 3], [4, 5, 6]])
arr = np.r_[arr, additional]
print(arr)

>>>
array([[ 1.,  2.,  3.],
       [ 4.,  5.,  6.]])

Use this method when adding arrays little by little in a for loop.

arr = np.empty([0, 3])
for i in range(3):
  additional = np.ones([1, 3]) * i
  arr = np.r_[arr, additional]
print(arr)

>>>
[[ 0.  0.  0.]
 [ 1.  1.  1.]
 [ 2.  2.  2.]]