ValueError: Unknown label type: 'unknown'

ValueError: unknown label type

$ python -c "from sklearn.metrics import accuracy_score; accuracy_score(['a','b'], ['a','b'])"
Traceback (most recent call last):
  File "<string>", line 1, in <module>
ValueError: Unknown label type: 'unknown'

Why this happens

• Labels not in expected format (e.g., strings with unsupported estimator/metric).

Fix

• Encode labels or pick suitable metrics/estimators.

Wrong code

accuracy_score(['a','b'], ['a','b'])

Fixed code

from sklearn.preprocessing import LabelEncoder
import numpy as np
le = LabelEncoder()
y_true = le.fit_transform(np.array(['a','b']))
y_pred = le.transform(np.array(['a','b']))
from sklearn.metrics import accuracy_score
print(accuracy_score(y_true, y_pred))