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Fix DiceFocalLoss to apply activation before removing background #8947

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Fix DiceFocalLoss to apply activation before removing background #8947

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20 changes: 17 additions & 3 deletions monai/losses/dice.py
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Original file line number Diff line number Diff line change
Expand Up @@ -803,9 +803,9 @@ def __init__(
"""
super().__init__()
self.dice = DiceLoss(
sigmoid=sigmoid,
softmax=softmax,
other_act=other_act,
sigmoid=False,
softmax=False,
other_act=None,
squared_pred=squared_pred,
jaccard=jaccard,
reduction=reduction,
Expand All @@ -822,6 +822,9 @@ def __init__(
self.lambda_focal = lambda_focal
self.to_onehot_y = to_onehot_y
self.include_background = include_background
self.sigmoid = sigmoid
self.softmax = softmax
self.other_act = other_act
Comment on lines +825 to +827

@coderabbitai coderabbitai Bot Jun 24, 2026

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🎯 Functional Correctness | 🟠 Major | ⚡ Quick win

Preserve activation exclusivity validation.

With DiceLoss activation disabled, configs like sigmoid=True, softmax=True are now silently accepted and resolved by if/elif.

Proposed fix 
         self.to_onehot_y = to_onehot_y
         self.include_background = include_background
+        if int(sigmoid) + int(softmax) + int(other_act is not None) > 1:
+            raise ValueError("Only one of sigmoid=True, softmax=True, or other_act may be specified.")
         self.sigmoid = sigmoid
         self.softmax = softmax
         self.other_act = other_act

As per path instructions, "Examine code for logical error or inconsistencies".

📝 Committable suggestion

‼️ IMPORTANT
Carefully review the code before committing. Ensure that it accurately replaces the highlighted code, contains no missing lines, and has no issues with indentation. Thoroughly test & benchmark the code to ensure it meets the requirements.

Suggested change
self.sigmoid = sigmoid
self.softmax = softmax
self.other_act = other_act
self.to_onehot_y = to_onehot_y
self.include_background = include_background
if int(sigmoid) + int(softmax) + int(other_act is not None) > 1:
raise ValueError("Only one of sigmoid=True, softmax=True, or other_act may be specified.")
self.sigmoid = sigmoid
self.softmax = softmax
self.other_act = other_act
🤖 Prompt for AI Agents 
Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

In `@monai/losses/dice.py` around lines 825 - 827, DiceLoss is no longer enforcing
mutually exclusive activation settings, so invalid configs like sigmoid=True and
softmax=True can slip through when the activation path is skipped. Restore the
exclusivity validation in DiceLoss initialization or setup logic by checking the
sigmoid, softmax, and other_act flags together and raising an error when more
than one activation is enabled; use the DiceLoss constructor/validation flow to
locate and fix this.

Source: Path instructions


def forward(self, input: torch.Tensor, target: torch.Tensor) -> torch.Tensor:
"""
Expand All @@ -846,6 +849,17 @@ def forward(self, input: torch.Tensor, target: torch.Tensor) -> torch.Tensor:
else:
target = one_hot(target, num_classes=n_pred_ch)

# Apply activation before removing background to ensure softmax/sigmoid works correctly
if self.sigmoid:
input = torch.sigmoid(input)
elif self.softmax:
if n_pred_ch == 1:
warnings.warn("single channel prediction, `softmax=True` ignored.")

@coderabbitai coderabbitai Bot Jun 24, 2026

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📐 Maintainability & Code Quality | 🟡 Minor | ⚡ Quick win

Set stacklevel on the new warning.

Ruff flags this warnings.warn call; use stacklevel=2 so callers see their call site.

Proposed fix 
-                warnings.warn("single channel prediction, `softmax=True` ignored.")
+                warnings.warn("single channel prediction, `softmax=True` ignored.", stacklevel=2)
📝 Committable suggestion

‼️ IMPORTANT
Carefully review the code before committing. Ensure that it accurately replaces the highlighted code, contains no missing lines, and has no issues with indentation. Thoroughly test & benchmark the code to ensure it meets the requirements.

Suggested change
warnings.warn("single channel prediction, `softmax=True` ignored.")
warnings.warn("single channel prediction, `softmax=True` ignored.", stacklevel=2)
🧰 Tools
🪛 Ruff (0.15.18)

[warning] 857-857: No explicit stacklevel keyword argument found

Set stacklevel=2

(B028)

🤖 Prompt for AI Agents 
Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

In `@monai/losses/dice.py` at line 857, The warning emitted in the single-channel
prediction branch of `DiceLoss` needs an explicit stack level so the caller sees
the correct source location. Update the `warnings.warn` call in `dice.py` to
pass `stacklevel=2`, keeping the existing message intact and making sure the
change is applied in the `DiceLoss` logic where `softmax=True` is ignored.

Source: Linters/SAST tools

else:
input = torch.softmax(input, 1)
elif self.other_act is not None:
input = self.other_act(input)

if not self.include_background:
if n_pred_ch == 1:
warnings.warn("single channel prediction, `include_background=False` ignored.")
Expand Down