TypeError: unsupported operand type(s) for +: 'int' and 'list' (example)

unsupported operand types

$ python - <<'PY'
import numpy as np
arr = np.array([1, 2, 3], dtype=object)
arr + [4]
PY
Traceback (most recent call last):
  File "<stdin>", line 3, in <module>
TypeError: unsupported operand type(s) for +: 'int' and 'list'

Why this happens

Object dtype arrays hold arbitrary Python objects. Using ufuncs or operators expects element-level compatibility; adding incompatible types raises TypeError.

Fix

Use numeric dtypes when possible. Convert lists to arrays or ensure broadcastable shapes and compatible element types.

Wrong code

import numpy as np
arr = np.array([1, 2, 3], dtype=object)
arr + [4]

Fixed code

import numpy as np
arr = np.array([1, 2, 3])
arr + np.array([4, 4, 4])

title: “TypeError: unsupported operand type(s) for +: ‘numpy.ndarray’ and ‘str’” description: “Mixing incompatible types in NumPy operations and how to fix them.” date: 2025-12-07 tags: [“python”, “numpy”, “errors”]

TypeError: unsupported operand types for + with numpy.ndarray

$ python -c "import numpy as np; np.array([1,2]) + 'a'"
Traceback (most recent call last):
  File "<string>", line 1, in <module>
TypeError: unsupported operand type(s) for +: 'numpy.ndarray' and 'str'

Why this happens

You’re attempting an element-wise operation between incompatible types (numeric array and string), which NumPy can’t perform.

Fix

Convert types to compatible dtypes (e.g., all strings or all numbers), or perform the intended operation using Python lists or vectorized string methods (np.char).

Wrong code

import numpy as np
print(np.array([1,2]) + 'a')

Fixed code (convert to strings)

import numpy as np
print(np.array([1,2]).astype(str) + 'a')

# or use numeric addition
print(np.array([1,2]) + np.array([3,4]))