Fixes: scikit-learn/scikit-learn#12470
Title: Fix OneHotEncoder
to Safely Handle String Categories for ignore
Unknown Strategy
Problem:
The OneHotEncoder
from scikit-learn raises a ValueError
during the transform
method when handle_unknown='ignore'
is set and the categories are strings. This occurs if the string length of any unknown category being transformed exceeds the length of the strings encountered during fitting. The error arises because OneHotEncoder.categories_[i][0]
(the first category) is being used to replace unknown entries, and if it is a longer string than the target array's dtype allows, this string gets truncated, causing subsequent array operations to fail.
Analysis:
The root cause of the issue is the discrepancy in memory handling between strings of different lengths when dealing with NumPy arrays. Specifically, when the handle_unknown='ignore'
option is used, unknown categories are replaced by a known category from the categories_
array. If this known category string length exceeds that of the array it is replacing, it leads to truncation and eventually raises the ValueError
.
Proposed Changes:
-
Locate and Modify the OneHotEncoder Code:
- We need to adjust the
OneHotEncoder
implementation to ensure that the arrays used for transformation are appropriately sized to handle the data being inserted.
- We need to adjust the
-
Modify the
_transform
Method in OneHotEncoder:- Locate the
_transform
method in thesklearn/preprocessing/_encoders.py
file. - Change the handling of unknown categories to first check the size of the elements in the array. If necessary, cast arrays containing string elements to object dtype.
- Locate the
File: sklearn/preprocessing/_encoders.py
-
Import Necessary Utilities:
- Import
np
(NumPy) if not already imported.
- Import
-
Modify the
_transform
Method:- Add a check to see if the dtype of the array can sufficiently handle the replacement category.
- Cast the array to object dtype if necessary.
By making these changes, we ensure that the replacement category string can fit into the transformed array without truncating, thus avoiding the ValueError
.
- Update the
_transform
method to check the size of elements and cast arrays toobject
dtype if the replacement category string exceeds the array’s allowable string length.
This will prevent errors when unknown string categories are handled during transformation with handle_unknown='ignore'
.