Test dependency resolver

tests/test__resolver.py

@@ -0,0 +1,165 @@
+# Copyright 2022 Alethea Katherine Flowers
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#    http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+from __future__ import annotations
+
+import pytest
+
+from nox._resolver import CycleError, lazy_stable_topo_sort
+
+
+@pytest.mark.parametrize(
+    ("dependencies", "expected"),
+    [
+        # Assert typical example with the following features:
+        # 1.  Topological sort.
+        # 2.  Ignores nodes outside the subgraph of ``dependencies[root]`` and its
+        #     (recursive) dependencies. (``"f"`` and ``"g"``).
+        # 3.  Lazy (``"e"`` does not occur any earlier than it needs to for ``"d"``).
+        # 4.  Obeys ``dependencies[node]`` order preference when possible (``"a"`` and
+        #     ``"d"``, ``"c"`` and ``"b"``).
+        (
+            {
+                "a": ("c", "b"),
+                "b": (),
+                "c": (),
+                "d": ("e",),
+                "e": ("c",),
+                "f": ("b", "g"),
+                "g": (),
+                "0": ("a", "d"),
+            },
+            ("c", "b", "a", "e", "d"),
+        ),
+        # 1.  Topological sort.
+        # 2.  Ignores (``"f"`` and ``"g"``).
+        # 3.  Lazy (``"b"``).
+        # 4.  Obeys order preference (``"d"`` and ``"a"``).
+        # 4.  Obeys order preference (``"c"`` and ``"b"``).
+        (
+            {
+                "a": ("c", "b"),
+                "b": (),
+                "c": (),
+                "d": ("e",),
+                "e": ("c",),
+                "f": ("b", "g"),
+                "g": (),
+                "0": ("d", "a"),
+            },
+            ("c", "e", "d", "b", "a"),
+        ),
+        # 1.  Topological sort.
+        # 2.  Ignores (``"f"`` and ``"g"``).
+        # 4.  Ignores order preference (``"a"`` and ``"d"``) when it is impossible to
+        #     both satisfy a pair preference and produce a producing a topological and
+        #     lazy sort.
+        (
+            {
+                "a": ("d",),
+                "b": (),
+                "c": (),
+                "d": ("e",),
+                "e": ("c",),
+                "f": ("b", "g"),
+                "g": (),
+                "0": ("a", "d"),
+            },
+            ("c", "e", "d", "a"),
+        ),
+        # 1.  Topological sort.
+        # 2.  Ignores (``"f"`` and ``"g"``).
+        # 3.  Lazy (``"e"``).
+        # 4.  Obeys order preference (``"a"`` and ``"d"``).
+        # 4.  Ignores order preference (``"c"`` and ``"b"``).
+        (
+            {
+                "a": ("c", "b"),
+                "b": (),
+                "c": ("b",),
+                "d": ("e",),
+                "e": ("c",),
+                "f": ("b",),
+                "0": ("a", "d"),
+            },
+            ("b", "c", "a", "e", "d"),
+        ),
+        # 1.  Topological sort.
+        # 2.  Ignores (``"f"``).
+        # 3.  Lazy (``"c"``, ``"h"``, ``"a"``, and ``"e"``).
+        # 4.  Obeys order preference (``"g"``, ``"a"``, and ``"d"``).
+        # 4.  Obeys order preference (``"b"`` and ``"g"``).
+        # 4.  Obeys order preference (``"b"`` and ``"h"``).
+        # 4.  Ignores order preference (``"c"`` and ``"b"``). Note that this is despite
+        #     the fact that the topological order between ``"b"`` and ``"c"`` is
+        #     undefined. In the tests above, we only saw a pair order preference ignored
+        #     because the topological order between that pair was defined. Here,
+        #     however, the ``dependencies["a"]`` order preference between ``"b"`` and
+        #     ``"c"`` is ignored because obeying this order preference cannot be done
+        #     without making the sort non-lazy (here, calling ``"c"`` earlier than is
+        #     required by one of its dependents (``"h"``) would be non-lazy).
+        (
+            {
+                "a": ("c", "b"),
+                "b": (),
+                "c": (),
+                "d": ("e",),
+                "e": ("c",),
+                "f": ("b", "g"),
+                "g": ("b", "h"),
+                "h": ("c",),
+                "0": ("g", "a", "d"),
+            },
+            ("b", "c", "h", "g", "a", "e", "d"),
+        ),
+    ],
+)
+def test_lazy_stable_topo_sort(dependencies, expected):
+    actual = tuple(lazy_stable_topo_sort(dependencies, "0"))
+    actual_with_root = tuple(lazy_stable_topo_sort(dependencies, "0", drop_root=False))
+    assert actual == actual_with_root[:-1] == expected
+
+
+@pytest.mark.parametrize(
+    ("dependencies", "expected_cycle"),
+    [
+        # Note that these cycles are inherent to the dependency graph; they cannot be
+        # resolved by ignoring the order preference for a pair in ``dependencies[node]``
+        # for some ``node``.
+        (
+            {
+                "a": ("b",),
+                "b": ("a",),
+                "0": ("a",),
+            },
+            ("a", "b", "a"),
+        ),
+        (
+            {
+                "a": ("c", "b"),
+                "b": (),
+                "c": ("a", "b"),
+                "0": ("a",),
+            },
+            ("a", "c", "a"),
+        ),
+    ],
+)
+def test_lazy_stable_topo_sort_CycleError(dependencies, expected_cycle):
+    with pytest.raises(CycleError) as exc_info:
+        tuple(lazy_stable_topo_sort(dependencies, "0"))
+    # While the exact cycle reported is not unique and is an implementation detail, this
+    # still serves as a regression test for unexpected changes in the implementation's
+    # behavior.
+    assert exc_info.value.args[1] == expected_cycle