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# `sklearn.model_selection`.StratifiedGroupKFold

class sklearn.model_selection.StratifiedGroupKFold(n_splits=5, shuffle=False, random_state=None)

  Stratified K-Folds iterator variant with non-overlapping groups.

  This cross-validation object is a variation of StratifiedKFold attempts to return stratified folds with non-overlapping groups. The folds are made by preserving the percentage of samples for each class.

  Each group will appear exactly once in the test set across all folds (the number of distinct groups has to be at least equal to the number of folds).

  The difference between `GroupKFold` and `StratifiedGroupKFold` is that the former attempts to create balanced folds such that the number of distinct groups is approximately the same in each fold, whereas StratifiedGroupKFold attempts to create folds which preserve the percentage of samples for each class as much as possible given the constraint of non-overlapping groups between splits.

  Read more in the User Guide.

  For visualisation of cross-validation behaviour and comparison between common scikit-learn split methods refer to Visualizing cross-validation behavior in scikit-learn

  Parameters:
    n_splits int, default=5

      Number of folds. Must be at least 2.

    shuffle bool, default=False

      Whether to shuffle each class’s samples before splitting into batches. Note that the samples within each split will not be shuffled. This implementation can only shuffle groups that have approximately the same y distribution, no global shuffle will be performed.

    random_state int or RandomState instance, default=None

      When `shuffle` is True, `random_state` affects the ordering of the indices, which controls the randomness of each fold for each class. Otherwise, leave `random_state` as `None`. Pass an int for reproducible output across multiple function calls. See Glossary.

  See also

  `StratifiedKFold`

    Takes class information into account to build folds which retain class distributions (for binary or multiclass classification tasks).

  `GroupKFold`

    K-fold iterator variant with non-overlapping groups.

  Notes

  The implementation is designed to:

  - Mimic the behavior of StratifiedKFold as much as possible for trivial groups (e.g. when each group contains only one sample).

  - Be invariant to class label: relabelling `y = ["Happy", "Sad"]` to `y = [1, 0]` should not change the indices generated.

  - Stratify based on samples as much as possible while keeping non-overlapping groups constraint. That means that in some cases when there is a small number of groups containing a large number of samples the stratification will not be possible and the behavior will be close to GroupKFold.

  Examples

      >>> import numpy as np
      >>> from sklearn.model_selection import StratifiedGroupKFold
      >>> X = np.ones((17, 2))
      >>> y = np.array([0, 0, 1, 1, 1, 1, 1, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0])
      >>> groups = np.array([1, 1, 2, 2, 3, 3, 3, 4, 5, 5, 5, 5, 6, 6, 7, 8, 8])
      >>> sgkf = StratifiedGroupKFold(n_splits=3)
      >>> sgkf.get_n_splits(X, y)
      3
      >>> print(sgkf)
      StratifiedGroupKFold(n_splits=3, random_state=None, shuffle=False)
      >>> for i, (train_index, test_index) in enumerate(sgkf.split(X, y, groups)):
      ...     print(f"Fold {i}:")
      ...     print(f"  Train: index={train_index}")
      ...     print(f"         group={groups[train_index]}")
      ...     print(f"  Test:  index={test_index}")
      ...     print(f"         group={groups[test_index]}")
      Fold 0:
        Train: index=[ 0  1  2  3  7  8  9 10 11 15 16]
               group=[1 1 2 2 4 5 5 5 5 8 8]
        Test:  index=[ 4  5  6 12 13 14]
               group=[3 3 3 6 6 7]
      Fold 1:
        Train: index=[ 4  5  6  7  8  9 10 11 12 13 14]
               group=[3 3 3 4 5 5 5 5 6 6 7]
        Test:  index=[ 0  1  2  3 15 16]
               group=[1 1 2 2 8 8]
      Fold 2:
        Train: index=[ 0  1  2  3  4  5  6 12 13 14 15 16]
               group=[1 1 2 2 3 3 3 6 6 7 8 8]
        Test:  index=[ 7  8  9 10 11]
               group=[4 5 5 5 5]

  Methods

  `get_metadata_routing`() | Get metadata routing of this object.
  `get_n_splits`([X, y, groups]) | Returns the number of splitting iterations in the cross-validator
  `set_split_request`(*[, groups]) | Request metadata passed to the `split` method.
  `split`(X[, y, groups]) | Generate indices to split data into training and test set.

  get_metadata_routing()

    Get metadata routing of this object.

    Please check User Guide on how the routing mechanism works.

    Returns:
      routing MetadataRequest

        A `MetadataRequest` encapsulating routing information.

  get_n_splits(X=None, y=None, groups=None)

    Returns the number of splitting iterations in the cross-validator

    Parameters:
      X object

        Always ignored, exists for compatibility.

      y object

        Always ignored, exists for compatibility.

      groups object

        Always ignored, exists for compatibility.

    Returns:
      n_splits int

        Returns the number of splitting iterations in the cross-validator.

  set_split_request(*, groups: bool | None | str = '$UNCHANGED$') → StratifiedGroupKFold

    Request metadata passed to the `split` method.

    Note that this method is only relevant if `enable_metadata_routing=True` (see `sklearn.set_config`). Please see User Guide on how the routing mechanism works.

    The options for each parameter are:

    - `True`: metadata is requested, and passed to `split` if provided. The request is ignored if metadata is not provided.

    - `False`: metadata is not requested and the meta-estimator will not pass it to `split`.

    - `None`: metadata is not requested, and the meta-estimator will raise an error if the user provides it.

    - `str`: metadata should be passed to the meta-estimator with this given alias instead of the original name.

    The default (`sklearn.utils.metadata_routing.UNCHANGED`) retains the existing request. This allows you to change the request for some parameters and not others.

    New in version 1.3.

    Note

    This method is only relevant if this estimator is used as a sub-estimator of a meta-estimator, e.g. used inside a `Pipeline`. Otherwise it has no effect.

    Parameters:
      groups str, True, False, or None, default=sklearn.utils.metadata_routing.UNCHANGED

        Metadata routing for `groups` parameter in `split`.

    Returns:
      self object

        The updated object.

  split(X, y=None, groups=None)

    Generate indices to split data into training and test set.

    Parameters:
      X array-like of shape (n_samples, n_features)

        Training data, where `n_samples` is the number of samples and `n_features` is the number of features.

      y array-like of shape (n_samples,), default=None

        The target variable for supervised learning problems.

      groups array-like of shape (n_samples,), default=None

        Group labels for the samples used while splitting the dataset into train/test set.

    Yields:
      train ndarray

        The training set indices for that split.

      test ndarray

        The testing set indices for that split.

## Examples using `sklearn.model_selection.StratifiedGroupKFold`

Visualizing cross-validation behavior in scikit-learn

Visualizing cross-validation behavior in scikit-learn