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Prefer the PyTorch padding backend when supported and safely fall back
to NumPy on error. Add unit tests to validate backend selection and
ensure output dtype is preserved.

Fixes #7842

Description

This pull request relaxes dtype restrictions in pad_nd and prefers
the PyTorch padding backend when supported, with a safe fallback to
NumPy on error. This enables support for additional dtypes (e.g. bool)
that are already handled correctly by recent PyTorch versions.

Unit tests are added to validate backend selection and ensure dtype
preservation.

Types of changes

  • Non-breaking change (fix or new feature that would not break existing functionality).
  • New tests added to cover the changes.

Prefer the PyTorch padding backend when supported and safely fall back
to NumPy on error. Add unit tests to validate backend selection and
ensure output dtype is preserved.

Signed-off-by: Shubham Chandravanshi <shubham.chandravanshi378@gmail.com>
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📝 Walkthrough

Walkthrough

pad_nd now unconditionally attempts the PyTorch padding implementation for modes {"constant", "reflect", "edge", "replicate", "wrap", "circular"}, removing the previous dtype-based gating. If a non-constant mode is passed with a value kwarg, it raises ValueError. If PyTorch padding raises NotImplementedError, pad_nd falls back to the NumPy implementation; certain ValueError/TypeError/RuntimeError messages also trigger the NumPy fallback, otherwise a detailed ValueError is raised. A new test module adds unit tests for backend selection, dtype preservation across many dtypes and modes, and the value-with-non-constant-mode error.

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~20 minutes

🚥 Pre-merge checks | ✅ 5
✅ Passed checks (5 passed)
Check name Status Explanation
Title check ✅ Passed Title clearly summarizes the main enhancement: relaxing dtype restrictions in pad_nd to support additional types like bool.
Description check ✅ Passed Description follows the template, includes issue reference, clear summary, and appropriate checkboxes for non-breaking change and new tests.
Linked Issues check ✅ Passed Changes fully address #7842 objectives: supports additional dtypes (bool, integers), prefers PyTorch backend, provides NumPy fallback, preserves dtype, and maintains mode contracts.
Out of Scope Changes check ✅ Passed All changes are directly scoped to the linked issue: dtype support enhancement in pad_nd, fallback logic, value parameter validation, and comprehensive tests.
Docstring Coverage ✅ Passed Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.

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Actionable comments posted: 4

Caution

Some comments are outside the diff and can’t be posted inline due to platform limitations.

⚠️ Outside diff range comments (1)
monai/transforms/croppad/functional.py (1)

99-110: Critical: NotImplementedError not caught by except clause.

Line 103 catches (ValueError, TypeError, RuntimeError) but line 104 checks isinstance(err, NotImplementedError). NotImplementedError would propagate uncaught, breaking the fallback mechanism. The test at test_pad_falls_back_to_np_if_pt_raises expects this fallback but would fail in real execution.

🔎 Proposed fix
-    except (ValueError, TypeError, RuntimeError) as err:
+    except (ValueError, TypeError, RuntimeError, NotImplementedError) as err:
         if isinstance(err, NotImplementedError) or any(
             k in str(err) for k in ("supported", "unexpected keyword", "implemented", "value")
         ):
🧹 Nitpick comments (1)
tests/transforms/croppad/test_pad_nd_dtypes.py (1)

49-58: Consider testing additional modes.

Current tests only cover "constant" mode. The updated code supports {"reflect", "edge", "replicate", "wrap", "circular"} via PyTorch. Testing dtype preservation across these modes would strengthen coverage.

Optional enhancement
@pytest.mark.parametrize(
    "mode", ["constant", "reflect", "replicate"]
)
@pytest.mark.parametrize(
    "dtype", [torch.bool, torch.int8, torch.float32]
)
def test_pad_modes_with_dtypes(mode, dtype):
    """Test that pad_nd handles various modes and dtypes correctly."""
    img = torch.ones((1, 4, 4), dtype=dtype)
    to_pad = [(0, 0), (1, 1), (2, 2)]
    out = pad_nd(img, to_pad, mode=mode, value=0)
    
    assert out.shape == (1, 6, 8)
    assert out.dtype == img.dtype
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Strip value for non-constant modes for both PyTorch and NumPy backends
and ensure reliable fallback behavior. Update tests to cover multiple
padding modes and dtype preservation.

Signed-off-by: Shubham Chandravanshi <shubham.chandravanshi378@gmail.com>
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Actionable comments posted: 1

♻️ Duplicate comments (4)
tests/transforms/croppad/test_pad_nd_dtypes.py (4)

1-20: Module docstring and imports look good.

Past review comments about missing module docstring have been addressed.


22-31: Test correctly validates PyTorch backend usage and dtype preservation.

Past docstring feedback addressed.


33-45: Fallback test is well-structured.

Mocking _pt_pad to raise NotImplementedError and verifying _np_pad is called covers the new fallback path. Past docstring feedback addressed.


47-57: Good dtype coverage.

Tests bool, int8/16/32/64, uint8, float32. Past docstring feedback addressed.

🧹 Nitpick comments (2)
monai/transforms/croppad/functional.py (1)

99-112: Minor formatting: double spaces before **call_kwargs.

Lines 107 and 112 have two spaces before **call_kwargs.

🔎 Fix spacing
     except NotImplementedError:
-        return _np_pad(img, pad_width=to_pad, mode=mode,  **call_kwargs)
+        return _np_pad(img, pad_width=to_pad, mode=mode, **call_kwargs)
     except (ValueError, TypeError, RuntimeError) as err:
         if any(
             k in str(err) for k in ("supported", "unexpected keyword", "implemented", "value")
         ):
-            return _np_pad(img, pad_width=to_pad, mode=mode,  **call_kwargs)
+            return _np_pad(img, pad_width=to_pad, mode=mode, **call_kwargs)
tests/transforms/croppad/test_pad_nd_dtypes.py (1)

59-69: Consider omitting value=0 for non-constant modes.

Line 66 passes value=0 for all modes, but "reflect" and "replicate" don't use this parameter. While the implementation strips it, omitting it in tests makes the intent clearer.

🔎 Suggested change
-    out = pad_nd(img, to_pad, mode=mode, value=0)
+    kwargs = {"value": 0} if mode == "constant" else {}
+    out = pad_nd(img, to_pad, mode=mode, **kwargs)
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🔇 Additional comments (1)
monai/transforms/croppad/functional.py (1)

102-105: Logic looks correct for stripping value in non-constant modes.

Copying kwargs and removing "value" when mode != "constant" prevents passing unsupported arguments to reflect/replicate/etc. modes.

shubham-61969 and others added 2 commits December 27, 2025 19:49
Signed-off-by: Shubham Chandravanshi <shubham.chandravanshi378@gmail.com>
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Actionable comments posted: 0

🧹 Nitpick comments (4)
tests/transforms/croppad/test_pad_nd_dtypes.py (4)

15-20: Import ordering could follow PEP8 more strictly.

Group imports: standard library, third-party, local. Consider:

 from __future__ import annotations
+
 from unittest.mock import Mock, patch
+
 import pytest
 import torch
+
 import monai.transforms.croppad.functional as F
 from monai.transforms.croppad.functional import pad_nd

22-31: Consider verifying output shape and values.

Test confirms backend selection and dtype preservation but doesn't check padding correctness. Add assertions:

assert out.shape == (1, 6, 8)
assert out[0, 1, 2].item() == True  # verify padded region if needed

33-45: Expand fallback testing to cover other exception types.

The implementation also catches ValueError, TypeError, and RuntimeError with message matching. Test these paths:

🔎 Additional test cases
@pytest.mark.parametrize("error_type,message", [
    (ValueError, "not supported"),
    (TypeError, "unexpected keyword argument"),
    (RuntimeError, "not implemented"),
])
def test_pad_falls_back_on_other_errors(error_type, message):
    """Test fallback when PyTorch raises ValueError/TypeError/RuntimeError."""
    img = torch.ones((1, 4, 4), dtype=torch.bool)
    to_pad = [(0, 0), (1, 1), (2, 2)]
    with (
        patch.object(F, "_pt_pad", new=Mock(side_effect=error_type(message))),
        patch.object(F, "_np_pad", wraps=F._np_pad) as mock_np,
    ):
        out = pad_nd(img, to_pad, mode="constant", value=0)
    assert mock_np.called
    assert out.dtype == img.dtype

59-70: LGTM. Consider expanding mode coverage.

Correctly handles value kwarg for constant mode only. For more comprehensive testing, add modes like "edge", "wrap", "circular".

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🔇 Additional comments (1)
tests/transforms/croppad/test_pad_nd_dtypes.py (1)

47-57: LGTM.

Good dtype coverage. Shape and dtype assertions are appropriate for this test.

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Hi @shubham-61969 thanks for the contribution. I think the change itself is fine with a few comments, the tests do need to be reformulated with unittest in particular. I think the previous implementation with its limitations was a result of older PyTorch versions so it's good to get this fix in. Please have a look again and then we can rereview.

Comment on lines 103 to 104
if mode != "constant":
call_kwargs.pop("value", None)
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If this condition isn't present, the effect of providing a value argument and not use "constant" mode is to raise an exception in the pad routine used, I think this is intended behaviour. Here if the value argument is removed this silently allows unintended arguments to be ignored, it's better to raise an exception instead.

shubham-61969 and others added 5 commits January 21, 2026 23:04
- Raise an explicit error when �alue is provided with non-constant modes.
- Rewrite tests using unittest + parameterized to match MONAI style.

Signed-off-by: Shubham Chandravanshi <shubham.chandravanshi378@gmail.com>
Signed-off-by: Shubham Chandravanshi <shubham.chandravanshi378@gmail.com>
Signed-off-by: Shubham Chandravanshi <shubham.chandravanshi378@gmail.com>
@shubham-61969
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Hi @ericspod , thanks for the feedback.

I’ve updated the implementation to explicitly raise an error when value is provided with modes other than "constant", so the original contract is preserved. I also reworked the tests to use unittest + parameterized and aligned them with MONAI’s existing test style.

All GitHub CI checks are now passing.

Could you please take another look? If there are any further changes you’d like, I’m happy to address them otherwise I believe this should be ready to merge.

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Looks good to me now, I had some minor comments only. We'll try to get this merge shortly when other things have been resolved.

Signed-off-by: Shubham Chandravanshi <shubham.chandravanshi378@gmail.com>
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Actionable comments posted: 2

Caution

Some comments are outside the diff and can’t be posted inline due to platform limitations.

⚠️ Outside diff range comments (2)
monai/transforms/croppad/functional.py (2)

95-113: Guarded ValueError is swallowed by fallback logic.

The new ValueError is raised inside the try, then caught and treated as a fallback because "value" matches the substring filter. This bypasses the intended contract and can allow value for non-constant modes (especially callable modes), or change the error path/message.

🐛 Proposed fix
-    try:
-        _pad = _np_pad
-        if mode in {"constant", "reflect", "edge", "replicate", "wrap", "circular"}:
-            # Try PyTorch pad for these modes; fallback to NumPy on error.
-            _pad = _pt_pad
-        if mode != "constant" and "value" in kwargs:
-            raise ValueError("'value' argument is only valid when mode='constant'")
-        return _pad(img, pad_width=to_pad, mode=mode, **kwargs)
+    if mode != "constant" and "value" in kwargs:
+        raise ValueError("'value' argument is only valid when mode='constant'")
+    try:
+        _pad = _np_pad
+        if mode in {"constant", "reflect", "edge", "replicate", "wrap", "circular"}:
+            # Try PyTorch pad for these modes; fallback to NumPy on error.
+            _pad = _pt_pad
+        return _pad(img, pad_width=to_pad, mode=mode, **kwargs)

73-93: Document the new ValueError in the docstring.

The docstring doesn’t describe the new exception. Add a Raises section per the project’s docstring rules.

📝 Suggested docstring update
     Args:
         img: data to be transformed, assuming `img` is channel-first and padding doesn't apply to the channel dim.
         to_pad: the amount to be padded in each dimension [(low_H, high_H), (low_W, high_W), ...].
             default to `self.to_pad`.
         mode: available modes: (Numpy) {``"constant"``, ``"edge"``, ``"linear_ramp"``, ``"maximum"``,
             ``"mean"``, ``"median"``, ``"minimum"``, ``"reflect"``, ``"symmetric"``, ``"wrap"``, ``"empty"``}
             (PyTorch) {``"constant"``, ``"reflect"``, ``"replicate"``, ``"circular"``}.
             One of the listed string values or a user supplied function. Defaults to ``"constant"``.
             See also: https://numpy.org/doc/stable/reference/generated/numpy.pad.html
             https://pytorch.org/docs/stable/generated/torch.nn.functional.pad.html
         kwargs: other arguments for the `np.pad` or `torch.pad` function.
             note that `np.pad` treats channel dimension as the first dimension.
+    Raises:
+        ValueError: If `value` is provided when `mode` is not ``"constant"``.

As per coding guidelines, ...

🤖 Fix all issues with AI agents
In `@tests/transforms/croppad/test_pad_nd_dtypes.py`:
- Around line 71-79: The test docstring for
test_pad_dtype_no_error_and_dtype_preserved is missing a Google-style Args
section documenting the parameter `dtype`; update the docstring of
test_pad_dtype_no_error_and_dtype_preserved to include an Args block that
describes the `dtype` parameter (e.g., "dtype: torch dtype to test that pad_nd
preserves input dtype") so the parameterized test using DTYPES is properly
documented; keep the description concise and consistent with other test
docstrings and retain references to pad_nd, DTYPES, and the asserted behavior.
- Around line 81-91: Update the docstring for the test method
test_pad_multiple_modes_dtype_preserved to include a Google-style Args section
documenting the two parameters `mode` and `dtype`; keep the existing one-line
summary and add an Args block that briefly describes `mode` (padding mode under
test) and `dtype` (tensor dtype used for the input image) so the parameterized
test's inputs are documented.

Signed-off-by: Shubham Chandravanshi <shubham.chandravanshi378@gmail.com>
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Support more dtypes in pad_nd

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