ENH: add pad

src/array_api_extra/__init__.py

@@ -1,6 +1,6 @@
 """Extra array functions built on top of the array API standard."""
 
-from ._funcs import atleast_nd, cov, create_diagonal, expand_dims, kron, setdiff1d, sinc
+from ._funcs import atleast_nd, cov, create_diagonal, expand_dims, kron, setdiff1d, sinc, pad
 
 __version__ = "0.4.1.dev0"
 
@@ -14,4 +14,5 @@
     "kron",
     "setdiff1d",
     "sinc",
+    "pad",
 ]

src/array_api_extra/_funcs.py

@@ -3,7 +3,9 @@
 import warnings
 
 from ._lib import _compat, _utils
-from ._lib._compat import array_namespace
+from ._lib._compat import (
+    array_namespace, is_torch_namespace, is_array_api_strict_namespace
+)
 from ._lib._typing import Array, ModuleType
 
 __all__ = [
@@ -14,6 +16,7 @@
     "kron",
     "setdiff1d",
     "sinc",
+    "pad",
 ]
 
 
@@ -538,3 +541,54 @@ def sinc(x: Array, /, *, xp: ModuleType | None = None) -> Array:
         xp.asarray(xp.finfo(x.dtype).eps, dtype=x.dtype, device=_compat.device(x)),
     )
     return xp.sin(y) / y
+
+
+def pad(x: Array, pad_width: int, mode: str = 'constant', *, xp: ModuleType = None, **kwargs):
+    """
+    Pad the input array.
+
+    Parameters
+    ----------
+    x : array
+        Input array
+    pad_width: int
+        Pad the input array with this many elements from each side
+    mode: str, optional
+        Only "constant" mode is currently supported.
+    xp : array_namespace, optional
+        The standard-compatible namespace for `x`. Default: infer.
+    constant_values: python scalar, optional
+        Use this value to pad the input. Default is zero.
+
+    Returns
+    -------
+    array
+        The input array, padded with ``pad_width`` elements equal to ``constant_values``
+    """
+    # xp.pad is available on numpy, cupy and jax.numpy; on torch, reuse
+    # http://github.com/pytorch/pytorch/blob/main/torch/_numpy/_funcs_impl.py#L2045
+
+    if mode != 'constant':
+        raise NotImplementedError()
+
+    value = kwargs.get("constant_values", 0)
+    if kwargs and list(kwargs.keys()) != ['constant_values']:
+        raise ValueError(f"Unknown kwargs: {kwargs}")
+
+    if xp is None:
+        xp = array_namespace(x)
+
+    if is_array_api_strict_namespace(xp):
+        padded = xp.full(
+            tuple(x + 2*pad_width for x in x.shape), fill_value=value, dtype=x.dtype
+        )
+        padded[(slice(pad_width, -pad_width, None),)*x.ndim] = x
+        return padded
+    elif is_torch_namespace(xp):
+        pad_width = xp.asarray(pad_width)
+        pad_width = xp.broadcast_to(pad_width, (x.ndim, 2))
+        pad_width = xp.flip(pad_width, axis=(0,)).flatten()
+        return xp.nn.functional.pad(x, tuple(pad_width), value=value)
+
+    else:
+        return xp.pad(x, pad_width, mode=mode, **kwargs)

src/array_api_extra/_lib/_compat.py

@@ -11,6 +11,8 @@
     from array_api_compat import (  # pyright: ignore[reportMissingTypeStubs]
         array_namespace,  # pyright: ignore[reportUnknownVariableType]
         device,
+        is_torch_namespace,
+        is_array_api_strict_namespace,
     )
 
 __all__ = [

tests/test_funcs.py

@@ -15,6 +15,7 @@
     kron,
     setdiff1d,
     sinc,
+    pad,
 )
 from array_api_extra._lib._typing import Array
 
@@ -385,3 +386,24 @@ def test_device(self):
 
     def test_xp(self):
         assert_array_equal(sinc(xp.asarray(0.0), xp=xp), xp.asarray(1.0))
+
+
+class TestPad:
+    def test_simple(self):
+        a = xp.arange(1, 4)
+        padded = pad(a, 2)
+        assert xp.all(padded == xp.asarray([0, 0, 1, 2, 3, 0, 0]))
+
+    def test_fill_value(self):
+        a = xp.arange(1, 4)
+        padded = pad(a, 2, constant_values=42)
+        assert xp.all(padded == xp.asarray([42, 42, 1, 2, 3, 42, 42]))
+
+    def test_ndim(self):
+        a = xp.reshape(xp.arange(2*3*4), (2, 3, 4))
+        padded = pad(a, 2)
+        assert padded.shape == (6, 7, 8)
+
+    def test_typo(self):
+        with pytest.raises(ValueError, match="Unknown"):
+            pad(xp.arange(2), pad_width=3, oops=3)