def check_cv(cv=3, y=None, classifier=False):
    """Input checker utility for building a cross-validator

    Parameters
    ----------
    cv : int, cross-validation generator or an iterable, optional
        Determines the cross-validation splitting strategy.
        Possible inputs for cv are:
          - None, to use the default 3-fold cross-validation,
          - integer, to specify the number of folds.
          - An object to be used as a cross-validation generator.
          - An iterable yielding train/test splits.

        For integer/None inputs, if classifier is True and ``y`` is either
        binary or multiclass, :class:`StratifiedKFold` used. In all other
        cases, :class:`KFold` is used.

        Refer :ref:`User Guide <cross_validation>` for the various
        cross-validation strategies that can be used here.

    y : array-like, optional
        The target variable for supervised learning problems.

    classifier : boolean, optional, default False
        Whether the task is a classification task, in which case
        stratified KFold will be used.

    Returns
    -------
    checked_cv : a cross-validator instance.
        The return value is a cross-validator which generates the train/test
        splits via the ``split`` method.
    """
    if cv is None:
        cv = 3

    if isinstance(cv, numbers.Integral):
        if (classifier and (y is not None) and
                (('binary', 'multiclass') in type_of_target(y))):
            return StratifiedKFold(cv)
        else:
            return KFold(cv)

    if not hasattr(cv, 'split') or isinstance(cv, str):
        if not isinstance(cv, Iterable) or isinstance(cv, str):
            raise ValueError("Expected cv as an integer, cross-validation "
                             "object (from sklearn.model_selection) "
                             "or an iterable. Got %s." % cv)
        return _CVIterableWrapper(cv)

    return cv  # New style cv objects are passed without any modification
dataset/ETHPy150Open scikit-learn/scikit-learn/sklearn/model_selection/_split.py/check_cv/swapped operands of `in`