Add method to detect rows are written to the MockBackend (#292)

See [this
comment](https://github.com/databrickslabs/ucx/pull/2625/files#r1758391830)
on motivation

Author: JCZuurmond

src/databricks/labs/lsql/backends.py

@@ -372,6 +372,18 @@ def rows_written_for(self, full_name: str, mode: str) -> list[DataclassInstance]
             rows += stub_rows
         return rows
 
+    def has_rows_written_for(self, full_name: str) -> bool:
+        """Check if the table has been written to.
+
+        This method allows to differentiate between "never written to the table" (returns False) and "zero rows written
+        to the table" (return True).
+        Otherwise, the check is the same as: `assert (not) mock_backend.rows_written_for(full_name, mode)`
+        """
+        for stub_full_name, _, _ in self._save_table:
+            if stub_full_name == full_name:
+                return True
+        return False
+
     @staticmethod
     def rows(*column_names: str):
         """This method is used to create rows for the mock backend."""

tests/unit/test_backends.py

@@ -451,6 +451,43 @@ def test_mock_backend_overwrite():
     ]
 
 
+@pytest.mark.parametrize("mode", ["append", "overwrite"])
+def test_mock_backend_has_no_rows_written(mode) -> None:
+    mock_backend = MockBackend()
+    # There are no rows written
+    assert not mock_backend.has_rows_written_for("a.b.c")
+    # and the results contains no rows
+    assert not mock_backend.rows_written_for("a.b.c", mode)
+
+
+@pytest.mark.parametrize("mode", ["append", "overwrite"])
+def test_mock_backend_has_zero_rows_written(mode) -> None:
+    mock_backend = MockBackend()
+    mock_backend.save_table("a.b.c", [], Foo, mode)
+    # There are rows written
+    assert mock_backend.has_rows_written_for("a.b.c")
+    # while the results contains no rows
+    assert not mock_backend.rows_written_for("a.b.c", mode)
+
+
+@pytest.mark.parametrize("mode", ["append", "overwrite"])
+def test_mock_backend_has_rows_written_for_after_first_write(mode) -> None:
+    mock_backend = MockBackend()
+    mock_backend.save_table("a.b.c", [Foo("a1", True), Foo("c2", False)], Foo, mode)
+    assert mock_backend.has_rows_written_for("a.b.c")
+    assert not mock_backend.has_rows_written_for("a.b.d")
+
+
+@pytest.mark.parametrize("first_mode", ["append", "overwrite"])
+@pytest.mark.parametrize("second_mode", ["append", "overwrite"])
+def test_mock_backend_has_rows_written_for_after_two_writes(first_mode, second_mode) -> None:
+    mock_backend = MockBackend()
+    mock_backend.save_table("a.b.c", [Foo("a1", True), Foo("c2", False)], Foo, first_mode)
+    mock_backend.save_table("a.b.c", [Foo("aaa", True), Foo("bbb", False)], Foo, second_mode)
+    assert mock_backend.has_rows_written_for("a.b.c")
+    assert not mock_backend.has_rows_written_for("a.b.d")
+
+
 @dataclass
 class Nested:
     foo: Foo