IndexError: axis is out of bounds for array of dimension

Axis out of range

$ python -c "import numpy as np; a=np.zeros((3,4)); np.sum(a, axis=2)"
Traceback (most recent call last):
  File "<string>", line 1, in <module>
IndexError: axis 2 is out of bounds for array of dimension 2

Why this happens

The axis argument must be between 0 and ndim-1. Asking for an axis >= ndim raises an IndexError.

Fix

Check the array’s ndim or use np.expand_dims / np.newaxis to add dimensions before operating.

Wrong code

import numpy as np
a = np.zeros((3,4))
np.sum(a, axis=2)

Fixed code

import numpy as np
a = np.zeros((3,4))
print(a.sum(axis=1))
# or add a dimension
a3 = a[..., np.newaxis]
print(a3.sum(axis=2))