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+---
+id: 359acb3362
+question: 'PerformanceWarning: DataFrame is highly fragmented when one-hot encoding
+  categorical columns in a loop'
+sort_order: 33
+---
+
+pandas raises `PerformanceWarning: DataFrame is highly fragmented` when you add many columns one by one, for example `df[name] = (df[col] == value).astype(int)` inside nested loops. Each insert adds a separate internal block.
+
+Don't build one-hot columns by hand. Encode all categorical columns in one call, as in Module 3:
+
+```python
+from sklearn.feature_extraction import DictVectorizer
+
+dv = DictVectorizer(sparse=False)
+train_dicts = df_train[categorical + numerical].to_dict(orient='records')
+X_train = dv.fit_transform(train_dicts)
+```
+
+`pd.get_dummies(df, columns=categorical)` also works in a single call.
+
+If you do need a loop, collect the new columns in a dict and concatenate once:
+
+```python
+new_cols = {}
+for col in categorical:
+    for value in df[col].unique():
+        new_cols[f'{col}_{value}'] = (df[col] == value).astype(int)
+
+df = pd.concat([df, pd.DataFrame(new_cols)], axis=1)
+```