ENH: allow list/tuple pad_width in pad

src/array_api_extra/_funcs.py

@@ -555,7 +555,7 @@ def sinc(x: Array, /, *, xp: ModuleType | None = None) -> Array:
 
 def pad(
     x: Array,
-    pad_width: int,
+    pad_width: int | tuple[int, int] | list[tuple[int, int],...],
     mode: str = "constant",
     *,
     xp: ModuleType | None = None,
@@ -568,8 +568,12 @@ def pad(
     ----------
     x : array
         Input array.
-    pad_width : int
+    pad_width : int or tuple of ints or list of pairs of ints
         Pad the input array with this many elements from each side.
+        If a list of tuples, ``[(before_0, after_0), ... (before_N, after_N)]``,
+        each pair applies to the corresponding axis of ``x``.
+        A single tuple, ``(before, after)``, is equivalent to a list of ``x.ndim``
+        copies of this tuple.
     mode : str, optional
         Only "constant" mode is currently supported, which pads with
         the value passed to `constant_values`.
@@ -590,16 +594,43 @@ def pad(
 
     value = constant_values
 
+    # make pad_width a list of length-2 tuples of ints
+    if isinstance(pad_width, int):
+        pad_width = [(pad_width, pad_width)] * x.ndim
+
+    if isinstance(pad_width, tuple):
+        pad_width = [pad_width] * x.ndim
+
     if xp is None:
         xp = array_namespace(x)
 
+    slices = []
+    newshape = []
+    for ax, w_tpl in enumerate(pad_width):
+        if len(w_tpl) != 2:
+            msg = f"expect a 2-tuple (before, after), got {w_tpl}."
+            raise ValueError(msg)
+
+        sh = x.shape[ax]
+        if w_tpl[0] == 0 and w_tpl[1] == 0:
+            sl = slice(None, None, None)
+        else:
+            start, stop = w_tpl
+            stop = None if stop == 0 else -stop
+
+            sl = slice(start, stop, None)
+            sh += w_tpl[0] + w_tpl[1]
+
+        newshape.append(sh)
+        slices.append(sl)
+
     padded = xp.full(
-        tuple(x + 2 * pad_width for x in x.shape),
+        tuple(newshape),
         fill_value=value,
         dtype=x.dtype,
         device=_compat.device(x),
     )
-    padded[(slice(pad_width, -pad_width, None),) * x.ndim] = x
+    padded[tuple(slices)] = x
     return padded
 
 

tests/test_funcs.py

@@ -416,3 +416,19 @@ def test_device(self):
 
     def test_xp(self):
         assert_array_equal(pad(xp.asarray(0), 1, xp=xp), xp.zeros(3))
+
+    def test_tuple_width(self):
+        a = xp.reshape(xp.arange(12), (3, 4))
+        padded = pad(a, (1, 0))
+        assert padded.shape == (4, 5)
+
+        padded = pad(a, (1, 2))
+        assert padded.shape == (6, 7)
+
+    def test_list_of_tuples_width(self):
+        a = xp.reshape(xp.arange(12), (3, 4))
+        padded = pad(a, [(1, 0), (0, 2)])
+        assert padded.shape == (4, 6)
+
+        padded = pad(a, [(1, 0), (0, 0)])
+        assert padded.shape == (4, 4)