fix: IndexError on node slicing (#1500)

Fixes #1498 

Python lets you slice out of range; `[0,1][90:99]` returns an empty
list. This PR makes node slicing behave as such.

hugr-py/src/hugr/hugr/node_port.py

@@ -176,22 +176,22 @@ def _index(
                     )
                     raise ValueError(msg)
 
-                start = self._normalize_index(start)
-                stop = self._normalize_index(stop, allow_eq_len=True)
+                start = self._normalize_index(start, allow_overflow=True)
+                stop = self._normalize_index(stop, allow_overflow=True)
                 step = index.step or 1
 
                 return (self[i] for i in range(start, stop, step))
             case tuple(xs):
                 return (self[i] for i in xs)
 
-    def _normalize_index(self, index: int, allow_eq_len: bool = False) -> int:
+    def _normalize_index(self, index: int, allow_overflow: bool = False) -> int:
         """Given an index passed to `__getitem__`, normalize it to be within the
         range of output ports.
 
         Args:
             index: index to normalize.
-            allow_eq_len: whether to allow the index to be equal to the number of
-                output ports.
+            allow_overflow: whether to allow indices beyond the number of outputs.
+                If True, indices over `self._num_out_ports` will be truncated.
 
         Returns:
             Normalized index.
@@ -202,17 +202,17 @@ def _normalize_index(self, index: int, allow_eq_len: bool = False) -> int:
         msg = f"Index {index} out of range"
 
         if self._num_out_ports is not None:
-            if index > self._num_out_ports:
-                raise IndexError(msg)
-            if index == self._num_out_ports and not allow_eq_len:
+            if index >= self._num_out_ports and not allow_overflow:
                 raise IndexError(msg)
             if index < -self._num_out_ports:
                 raise IndexError(msg)
         else:
             if index < 0:
                 raise IndexError(msg)
 
-        if index >= 0:
+        if index >= 0 and self._num_out_ports is not None:
+            return min(index, self._num_out_ports)
+        elif index >= 0:
             return index
         else:
             assert self._num_out_ports is not None

hugr-py/tests/test_nodes.py

@@ -30,8 +30,19 @@ def test_slices():
     assert list(n[0:]) == all_ports
     assert list(n[:3]) == all_ports
     assert list(n[0:3]) == all_ports
+    assert list(n[0:999]) == all_ports
+    assert list(n[999:1000]) == []
     assert list(n[-1:]) == [OutPort(n, 2)]
     assert list(n[-3:]) == all_ports
 
     with pytest.raises(IndexError, match="Index -4 out of range"):
         _ = n[-4:]
+
+    n0 = Node(0, _num_out_ports=0)
+    assert list(n0) == []
+    assert list(n0[:0]) == []
+    assert list(n0[:10]) == []
+    assert list(n0[0:0]) == []
+    assert list(n0[0:]) == []
+    assert list(n0[10:]) == []
+    assert list(n0[:]) == []