# Source code for monai.metrics.loss_metric

```
# Copyright (c) MONAI Consortium
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
# http://www.apache.org/licenses/LICENSE-2.0
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.
from typing import Union
import torch
from torch.nn.modules.loss import _Loss
from monai.metrics.utils import do_metric_reduction
from monai.utils import MetricReduction
from .metric import CumulativeIterationMetric
[docs]class LossMetric(CumulativeIterationMetric):
"""
A wrapper to make ``loss_fn`` available as a cumulative metric. That is, the loss values computed from
mini-batches can be combined in the ``reduction`` mode across multiple iterations, as a quantitative measurement
of a model.
Example:
.. code-block:: python
import torch
from monai.losses import DiceLoss
from monai.metrics import LossMetric
dice_loss = DiceLoss(include_background=True)
loss_metric = LossMetric(loss_fn=dice_loss)
# first iteration
y_pred = torch.tensor([[[[1.0, 0.0], [0.0, 1.0]]]]) # shape [batch=1, channel=1, 2, 2]
y = torch.tensor([[[[1.0, 0.0], [1.0, 1.0]]]]) # shape [batch=1, channel=1, 2, 2]
loss_metric(y_pred, y)
# second iteration
y_pred = torch.tensor([[[[1.0, 0.0], [0.0, 0.0]]]]) # shape [batch=1, channel=1, 2, 2]
y = torch.tensor([[[[1.0, 0.0], [1.0, 1.0]]]]) # shape [batch=1, channel=1, 2, 2]
loss_metric(y_pred, y)
# aggregate
print(loss_metric.aggregate(reduction="none")) # tensor([[0.2000], [0.5000]]) (shape [batch=2, channel=1])
# reset
loss_metric.reset()
print(loss_metric.aggregate())
Args:
loss_fn: a callable function that takes ``y_pred`` and optionally ``y`` as input (in the "batch-first" format),
returns a "batch-first" tensor of loss values.
reduction: define mode of reduction to the metrics, will only apply reduction on `not-nan` values,
available reduction modes: {``"none"``, ``"mean"``, ``"sum"``, ``"mean_batch"``, ``"sum_batch"``,
``"mean_channel"``, ``"sum_channel"``}, default to ``"mean"``. if "none", will not do reduction.
get_not_nans: whether to return the `not_nans` count, if True, aggregate() returns (metric, not_nans).
Here `not_nans` count the number of not nans for the metric, thus its shape equals to the shape of the metric.
"""
def __init__(
self, loss_fn: _Loss, reduction: Union[MetricReduction, str] = MetricReduction.MEAN, get_not_nans: bool = False
) -> None:
super().__init__()
self.loss_fn = loss_fn
self.reduction = reduction
self.get_not_nans = get_not_nans
[docs] def aggregate(self, reduction: Union[MetricReduction, str, None] = None):
"""
Returns the aggregated loss value across multiple iterations.
Args:
reduction: define mode of reduction to the metrics, will only apply reduction on `not-nan` values,
available reduction modes: {``"none"``, ``"mean"``, ``"sum"``, ``"mean_batch"``, ``"sum_batch"``,
``"mean_channel"``, ``"sum_channel"``}, default to `self.reduction`. if "none", will not do reduction.
"""
data = self.get_buffer()
if data is None:
return (torch.tensor(0.0), torch.tensor(0.0)) if self.get_not_nans else torch.tensor(0.0)
f, not_nans = do_metric_reduction(data, reduction or self.reduction)
return (f, not_nans) if self.get_not_nans else f
def _compute_tensor(self, y_pred: torch.Tensor, y: torch.Tensor = None): # type: ignore
"""
Input `y_pred` is compared with ground truth `y`.
Both `y_pred` and `y` are expected to be a batch-first Tensor (BC[HWD]).
Returns:
a tensor with shape (BC[HWD]), or a list of tensors, each tensor with shape (C[HWD]).
"""
iter_loss = self.loss_fn(y_pred) if y is None else self.loss_fn(y_pred, y)
if isinstance(iter_loss, torch.Tensor):
while iter_loss.dim() < 2:
iter_loss = iter_loss[None]
# to be compatible with `Cumulative`, iter_loss should at least have a batch dim.
# to be compatible with `do_metric_reduction`, iter_loss should at least have a batch and a channel dim.
return iter_loss
```