Add __repr__ to OrderedSet, for debugging.

api/src/opentrons/ordered_set.py

@@ -130,3 +130,9 @@ def __sub__(
         The elements that aren't removed retain their original relative order.
         """
         return OrderedSet(e for e in self if e not in other)
+
+    def __repr__(self) -> str:  # noqa: D105
+        # Use repr() on the keys view in case it's super long and Python is smart
+        # enough to abbreviate it.
+        elements_str = repr(self._elements.keys())
+        return f"OrderedSet({elements_str})"

api/tests/opentrons/test_ordered_set.py

@@ -145,3 +145,13 @@ def test_difference() -> None:
     b = {1, 9}
 
     assert (a - OrderedSet(b)) == (a - b) == OrderedSet([3, 4, 5, 2, 6, 5, 3, 5, 8, 7])
+
+
+def test_repr() -> None:
+    """It should return a meaningful repr string."""
+    subject = OrderedSet([1, 2, 3])
+    result = repr(subject)
+    assert "OrderedSet" in result
+    assert "1" in result
+    assert "2" in result
+    assert "3" in result