RuntimeWarning: Mean of empty slice

RuntimeWarning: Mean of empty slice

$ python -c "import numpy as np; np.mean(np.array([]))"
/tmp/...: RuntimeWarning: Mean of empty slice.
  np.mean(np.array([]))

Why this happens

Statistical reductions like mean or std are undefined for empty inputs, so NumPy emits a RuntimeWarning and returns NaN.

Fix

Guard against empty inputs or provide initial/where parameters where supported, or handle NaN results explicitly.

Wrong code

import numpy as np
print(np.mean(np.array([])))

Fixed code

import numpy as np
arr = np.array([])
if arr.size == 0:
    print(0.0)  # or appropriate default
else:
    print(np.mean(arr))

# or use np.nanmean and handle NaN
print(np.nanmean(arr))