cohen_kappa.py

# Copyright The Lightning team.
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# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
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#     http://www.apache.org/licenses/LICENSE-2.0
#
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# See the License for the specific language governing permissions and
# limitations under the License.
from typing import Any, Optional, Sequence, Union

from torch import Tensor
from typing_extensions import Literal

from torchmetrics.classification.base import _ClassificationTaskWrapper
from torchmetrics.classification.confusion_matrix import BinaryConfusionMatrix, MulticlassConfusionMatrix
from torchmetrics.functional.classification.cohen_kappa import (
    _binary_cohen_kappa_arg_validation,
    _cohen_kappa_reduce,
    _multiclass_cohen_kappa_arg_validation,
)
from torchmetrics.metric import Metric
from torchmetrics.utilities.enums import ClassificationTaskNoMultilabel
from torchmetrics.utilities.imports import _MATPLOTLIB_AVAILABLE
from torchmetrics.utilities.plot import _AX_TYPE, _PLOT_OUT_TYPE

if not _MATPLOTLIB_AVAILABLE:
    __doctest_skip__ = ["BinaryCohenKappa.plot", "MulticlassCohenKappa.plot"]


class BinaryCohenKappa(BinaryConfusionMatrix):
    r"""Calculate `Cohen's kappa score`_ that measures inter-annotator agreement for binary tasks.

    .. math::
        \kappa = (p_o - p_e) / (1 - p_e)

    where :math:`p_o` is the empirical probability of agreement and :math:`p_e` is
    the expected agreement when both annotators assign labels randomly. Note that
    :math:`p_e` is estimated using a per-annotator empirical prior over the
    class labels.

    As input to ``forward`` and ``update`` the metric accepts the following input:

    - ``preds`` (:class:`~torch.Tensor`): A int or float tensor of shape ``(N, ...)``. If preds is a floating point
      tensor with values outside [0,1] range we consider the input to be logits and will auto apply sigmoid per element.
      Addtionally, we convert to int tensor with thresholding using the value in ``threshold``.
    - ``target`` (:class:`~torch.Tensor`): An int tensor of shape ``(N, ...)``.

    .. note::
       Additional dimension ``...`` will be flattened into the batch dimension.

    As output to ``forward`` and ``compute`` the metric returns the following output:

    - ``bck`` (:class:`~torch.Tensor`): A tensor containing cohen kappa score

    Args:
        threshold: Threshold for transforming probability to binary (0,1) predictions
        ignore_index:
            Specifies a target value that is ignored and does not contribute to the metric calculation
        weights: Weighting type to calculate the score. Choose from:

            - ``None`` or ``'none'``: no weighting
            - ``'linear'``: linear weighting
            - ``'quadratic'``: quadratic weighting

        validate_args: bool indicating if input arguments and tensors should be validated for correctness.
            Set to ``False`` for faster computations.
        kwargs: Additional keyword arguments, see :ref:`Metric kwargs` for more info.

    Example (preds is int tensor):
        >>> from torch import tensor
        >>> from torchmetrics.classification import BinaryCohenKappa
        >>> target = tensor([1, 1, 0, 0])
        >>> preds = tensor([0, 1, 0, 0])
        >>> metric = BinaryCohenKappa()
        >>> metric(preds, target)
        tensor(0.5000)

    Example (preds is float tensor):
        >>> from torchmetrics.classification import BinaryCohenKappa
        >>> target = tensor([1, 1, 0, 0])
        >>> preds = tensor([0.35, 0.85, 0.48, 0.01])
        >>> metric = BinaryCohenKappa()
        >>> metric(preds, target)
        tensor(0.5000)

    """
    is_differentiable: bool = False
    higher_is_better: bool = True
    full_state_update: bool = False
    plot_lower_bound: float = 0.0
    plot_upper_bound: float = 1.0

    def __init__(
        self,
        threshold: float = 0.5,
        ignore_index: Optional[int] = None,
        weights: Optional[Literal["linear", "quadratic", "none"]] = None,
        validate_args: bool = True,
        **kwargs: Any,
    ) -> None:
        super().__init__(threshold, ignore_index, normalize=None, validate_args=False, **kwargs)
        if validate_args:
            _binary_cohen_kappa_arg_validation(threshold, ignore_index, weights)
        self.weights = weights
        self.validate_args = validate_args

    def compute(self) -> Tensor:
        """Compute metric."""
        return _cohen_kappa_reduce(self.confmat, self.weights)

    def plot(  # type: ignore[override]
        self, val: Optional[Union[Tensor, Sequence[Tensor]]] = None, ax: Optional[_AX_TYPE] = None
    ) -> _PLOT_OUT_TYPE:
        """Plot a single or multiple values from the metric.

        Args:
            val: Either a single result from calling `metric.forward` or `metric.compute` or a list of these results.
                If no value is provided, will automatically call `metric.compute` and plot that result.
            ax: An matplotlib axis object. If provided will add plot to that axis

        Returns:
            Figure object and Axes object

        Raises:
            ModuleNotFoundError:
                If `matplotlib` is not installed

        .. plot::
            :scale: 75

            >>> from torch import rand, randint
            >>> # Example plotting a single value
            >>> from torchmetrics.classification import BinaryCohenKappa
            >>> metric = BinaryCohenKappa()
            >>> metric.update(rand(10), randint(2,(10,)))
            >>> fig_, ax_ = metric.plot()

        .. plot::
            :scale: 75

            >>> from torch import rand, randint
            >>> # Example plotting multiple values
            >>> from torchmetrics.classification import BinaryCohenKappa
            >>> metric = BinaryCohenKappa()
            >>> values = [ ]
            >>> for _ in range(10):
            ...     values.append(metric(rand(10), randint(2,(10,))))
            >>> fig_, ax_ = metric.plot(values)

        """
        return self._plot(val, ax)


class MulticlassCohenKappa(MulticlassConfusionMatrix):
    r"""Calculate `Cohen's kappa score`_ that measures inter-annotator agreement for multiclass tasks.

    .. math::
        \kappa = (p_o - p_e) / (1 - p_e)

    where :math:`p_o` is the empirical probability of agreement and :math:`p_e` is
    the expected agreement when both annotators assign labels randomly. Note that
    :math:`p_e` is estimated using a per-annotator empirical prior over the
    class labels.

    As input to ``forward`` and ``update`` the metric accepts the following input:

    - ``preds`` (:class:`~torch.Tensor`): Either an int tensor of shape ``(N, ...)` or float tensor of shape
      ``(N, C, ..)``. If preds is a floating point we apply ``torch.argmax`` along the ``C`` dimension to automatically
      convert probabilities/logits into an int tensor.
    - ``target`` (:class:`~torch.Tensor`): An int tensor of shape ``(N, ...)``.

    .. note::
       Additional dimension ``...`` will be flattened into the batch dimension.

    As output to ``forward`` and ``compute`` the metric returns the following output:

    - ``mcck`` (:class:`~torch.Tensor`): A tensor containing cohen kappa score

    Args:
        num_classes: Integer specifing the number of classes
        ignore_index:
            Specifies a target value that is ignored and does not contribute to the metric calculation
        weights: Weighting type to calculate the score. Choose from:

            - ``None`` or ``'none'``: no weighting
            - ``'linear'``: linear weighting
            - ``'quadratic'``: quadratic weighting

        validate_args: bool indicating if input arguments and tensors should be validated for correctness.
            Set to ``False`` for faster computations.
        kwargs: Additional keyword arguments, see :ref:`Metric kwargs` for more info.

    Example (pred is integer tensor):
        >>> from torch import tensor
        >>> from torchmetrics.classification import MulticlassCohenKappa
        >>> target = tensor([2, 1, 0, 0])
        >>> preds = tensor([2, 1, 0, 1])
        >>> metric = MulticlassCohenKappa(num_classes=3)
        >>> metric(preds, target)
        tensor(0.6364)

    Example (pred is float tensor):
        >>> from torchmetrics.classification import MulticlassCohenKappa
        >>> target = tensor([2, 1, 0, 0])
        >>> preds = tensor([[0.16, 0.26, 0.58],
        ...                 [0.22, 0.61, 0.17],
        ...                 [0.71, 0.09, 0.20],
        ...                 [0.05, 0.82, 0.13]])
        >>> metric = MulticlassCohenKappa(num_classes=3)
        >>> metric(preds, target)
        tensor(0.6364)

    """
    is_differentiable: bool = False
    higher_is_better: bool = True
    full_state_update: bool = False
    plot_lower_bound: float = 0.0
    plot_upper_bound: float = 1.0
    plot_legend_name: str = "Class"

    def __init__(
        self,
        num_classes: int,
        ignore_index: Optional[int] = None,
        weights: Optional[Literal["linear", "quadratic", "none"]] = None,
        validate_args: bool = True,
        **kwargs: Any,
    ) -> None:
        super().__init__(num_classes, ignore_index, normalize=None, validate_args=False, **kwargs)
        if validate_args:
            _multiclass_cohen_kappa_arg_validation(num_classes, ignore_index, weights)
        self.weights = weights
        self.validate_args = validate_args

    def compute(self) -> Tensor:
        """Compute metric."""
        return _cohen_kappa_reduce(self.confmat, self.weights)

    def plot(  # type: ignore[override]
        self, val: Optional[Union[Tensor, Sequence[Tensor]]] = None, ax: Optional[_AX_TYPE] = None
    ) -> _PLOT_OUT_TYPE:
        """Plot a single or multiple values from the metric.

        Args:
            val: Either a single result from calling `metric.forward` or `metric.compute` or a list of these results.
                If no value is provided, will automatically call `metric.compute` and plot that result.
            ax: An matplotlib axis object. If provided will add plot to that axis

        Returns:
            Figure object and Axes object

        Raises:
            ModuleNotFoundError:
                If `matplotlib` is not installed

        .. plot::
            :scale: 75

            >>> from torch import randn, randint
            >>> # Example plotting a single value
            >>> from torchmetrics.classification import MulticlassCohenKappa
            >>> metric = MulticlassCohenKappa(num_classes=3)
            >>> metric.update(randn(20,3).softmax(dim=-1), randint(3, (20,)))
            >>> fig_, ax_ = metric.plot()

        .. plot::
            :scale: 75

            >>> from torch import randn, randint
            >>> # Example plotting a multiple values
            >>> from torchmetrics.classification import MulticlassCohenKappa
            >>> metric = MulticlassCohenKappa(num_classes=3)
            >>> values = []
            >>> for _ in range(20):
            ...     values.append(metric(randn(20,3).softmax(dim=-1), randint(3, (20,))))
            >>> fig_, ax_ = metric.plot(values)

        """
        return self._plot(val, ax)


class CohenKappa(_ClassificationTaskWrapper):
    r"""Calculate `Cohen's kappa score`_ that measures inter-annotator agreement.

    .. math::
        \kappa = (p_o - p_e) / (1 - p_e)

    where :math:`p_o` is the empirical probability of agreement and :math:`p_e` is
    the expected agreement when both annotators assign labels randomly. Note that
    :math:`p_e` is estimated using a per-annotator empirical prior over the
    class labels.

    This function is a simple wrapper to get the task specific versions of this metric, which is done by setting the
    ``task`` argument to either ``'binary'`` or ``'multiclass'``. See the documentation of
    :class:`~torchmetrics.classification.BinaryCohenKappa` and
    :class:`~torchmetrics.classification.MulticlassCohenKappa` for the specific details of each argument influence and
    examples.

    Legacy Example:
        >>> from torch import tensor
        >>> target = tensor([1, 1, 0, 0])
        >>> preds = tensor([0, 1, 0, 0])
        >>> cohenkappa = CohenKappa(task="multiclass", num_classes=2)
        >>> cohenkappa(preds, target)
        tensor(0.5000)

    """

    def __new__(  # type: ignore[misc]
        cls,
        task: Literal["binary", "multiclass"],
        threshold: float = 0.5,
        num_classes: Optional[int] = None,
        weights: Optional[Literal["linear", "quadratic", "none"]] = None,
        ignore_index: Optional[int] = None,
        validate_args: bool = True,
        **kwargs: Any,
    ) -> Metric:
        """Initialize task metric."""
        task = ClassificationTaskNoMultilabel.from_str(task)
        kwargs.update({"weights": weights, "ignore_index": ignore_index, "validate_args": validate_args})
        if task == ClassificationTaskNoMultilabel.BINARY:
            return BinaryCohenKappa(threshold, **kwargs)
        if task == ClassificationTaskNoMultilabel.MULTICLASS:
            if not isinstance(num_classes, int):
                raise ValueError(f"`num_classes` is expected to be `int` but `{type(num_classes)} was passed.`")
            return MulticlassCohenKappa(num_classes, **kwargs)
        raise ValueError(f"Task {task} not supported!")