AxisError: axis out of bounds for array of dimension N

axis out of bounds for array of dimension

$ python - <<'PY'
import numpy as np
arr = np.zeros((3,))
np.sum(arr, axis=1)
PY
Traceback (most recent call last):
  File "<stdin>", line 4, in <module>
AxisError: axis 1 is out of bounds for array of dimension 1

Why this happens

Arrays have dimensions indexed from 0 to ndim-1. Asking for an axis outside this range triggers AxisError.

Fix

Use the correct axis (or omit it for flat reductions), or reshape the array to include the expected dimensions.

Wrong code

import numpy as np
arr = np.zeros((3,))
np.sum(arr, axis=1)

Fixed code

import numpy as np
arr = np.zeros((3,))
np.sum(arr, axis=0)

# or make it 2D
arr2 = arr[np.newaxis, :]
np.sum(arr2, axis=1)