TypeError: Cannot cast array from dtype('object') to dtype('float64') according to the rule 'safe'

cannot cast array

$ python - <<'PY'
import numpy as np
arr = np.array(['a', 'b', 'c'], dtype=object)
arr.astype(float)
PY
Traceback (most recent call last):
  File "<stdin>", line 3, in <module>
TypeError: float() argument must be a string or a number, not 'NoneType'

Why this happens

NumPy uses rules for safe casting. Converting from one dtype to another can fail when values are incompatible or when attempting an unsafe implicit cast.

Fix

Clean or pre-validate data before casting. Use np.asarray(..., dtype=...) or pd.to_numeric for robust conversion, or specify errors='coerce' in pandas. For explicit unsafe casts use astype(new_dtype, casting='unsafe') but be cautious.

Wrong code

import numpy as np
arr = np.array(['a', '2', None], dtype=object)
arr.astype(float)

Fixed code

import numpy as np
arr = np.array(['1.0', '2.5', '3.0'])
float_arr = arr.astype(float)

# or sanitize before cast
import math
obj = np.array(['1', '2', None], dtype=object)
clean = [float(x) for x in obj if x is not None]
arr2 = np.array(clean, dtype=float)