fix: don't confuse a named match with a wildcard match

Author: nedbat

CHANGES.rst

@@ -30,6 +30,9 @@ Unreleased
   the problem.  They are now changed to mention "branch coverage data" and
   "statement coverage data."
 
+- Fixed a minor branch coverage problem with wildcard match/case cases using
+  names or guard clauses.
+
 - Started testing on 3.13 free-threading (nogil) builds of Python.  I'm not
   claiming full support yet.
 

coverage/parser.py

@@ -1165,21 +1165,26 @@ def _handle__Match(self, node: ast.Match) -> set[ArcStart]:
             start = self.line_for_node(node)
             last_start = start
             exits = set()
-            had_wildcard = False
             for case in node.cases:
                 case_start = self.line_for_node(case.pattern)
-                pattern = case.pattern
-                while isinstance(pattern, ast.MatchOr):
-                    pattern = pattern.patterns[-1]
-                if isinstance(pattern, ast.MatchAs):
-                    had_wildcard = True
                 self.add_arc(last_start, case_start, "the pattern on line {lineno} always matched")
                 from_start = ArcStart(
                     case_start,
                     cause="the pattern on line {lineno} never matched",
                 )
                 exits |= self.body_exits(case.body, from_start=from_start)
                 last_start = case_start
+
+            # case is now the last case, check for wildcard match.
+            pattern = case.pattern      # pylint: disable=undefined-loop-variable
+            while isinstance(pattern, ast.MatchOr):
+                pattern = pattern.patterns[-1]
+            had_wildcard = (
+                isinstance(pattern, ast.MatchAs)
+                and pattern.pattern is None
+                and case.guard is None  # pylint: disable=undefined-loop-variable
+            )
+
             if not had_wildcard:
                 exits.add(
                     ArcStart(case_start, cause="the pattern on line {lineno} always matched"),

tests/test_arcs.py

@@ -1375,7 +1375,7 @@ def test_match_case_without_wildcard(self) -> None:
         )
         assert self.stdout() == "None\nno go\ngo: n\n"
 
-    def test_absurd_wildcard(self) -> None:
+    def test_absurd_wildcards(self) -> None:
         # https://github.com/nedbat/coveragepy/issues/1421
         self.check_coverage("""\
             def absurd(x):
@@ -1384,9 +1384,47 @@ def absurd(x):
                         print("default")
             absurd(5)
             """,
+            # No arc from 3 to 5 because 3 always matches.
             arcz=".1 15 5.  .2 23 34 4.",
         )
         assert self.stdout() == "default\n"
+        self.check_coverage("""\
+            def absurd(x):
+                match x:
+                    case (3 | 99 | 999 as y):
+                        print("not default")
+            absurd(5)
+            """,
+            arcz=".1 15 5.  .2 23 34 3. 4.",
+            arcz_missing="34 4.",
+        )
+        assert self.stdout() == ""
+        self.check_coverage("""\
+            def absurd(x):
+                match x:
+                    case (3 | 17 as y):
+                        print("not default")
+                    case 7: # 5
+                        print("also not default")
+            absurd(7)
+            """,
+            arcz=".1 17 7.  .2 23 34 4. 35 56 5. 6.",
+            arcz_missing="34 4. 5.",
+        )
+        assert self.stdout() == "also not default\n"
+        self.check_coverage("""\
+            def absurd(x):
+                match x:
+                    case 3:
+                        print("not default")
+                    case _ if x == 7: # 5
+                        print("also not default")
+            absurd(7)
+            """,
+            arcz=".1 17 7.  .2 23 34 4. 35 56 5. 6.",
+            arcz_missing="34 4. 5.",
+        )
+        assert self.stdout() == "also not default\n"
 
 
 class OptimizedIfTest(CoverageTest):

tests/test_parser.py

@@ -946,7 +946,11 @@ def test_missing_arc_descriptions_bug460(self) -> None:
             """)
         assert parser.missing_arc_description(2, -3) == "line 3 didn't finish the lambda on line 3"
 
-    @pytest.mark.skipif(not env.PYBEHAVIOR.match_case, reason="Match-case is new in 3.10")
+
+@pytest.mark.skipif(not env.PYBEHAVIOR.match_case, reason="Match-case is new in 3.10")
+class MatchCaseMissingArcDescriptionTest(PythonParserTestBase):
+    """Missing arc descriptions for match/case."""
+
     def test_match_case(self) -> None:
         parser = self.parse_text("""\
             match command.split():
@@ -966,6 +970,24 @@ def test_match_case(self) -> None:
             "line 4 didn't jump to line 6 because the pattern on line 4 always matched"
         )
 
+    def test_final_wildcard(self) -> None:
+        parser = self.parse_text("""\
+            match command.split():
+                case ["go", direction] if direction in "nesw":      # 2
+                    match = f"go: {direction}"
+                case _:                                             # 4
+                    match = "no go"
+            print(match)                                            # 6
+            """)
+        assert parser.missing_arc_description(2, 3) == (
+            "line 2 didn't jump to line 3 because the pattern on line 2 never matched"
+        )
+        assert parser.missing_arc_description(2, 4) == (
+            "line 2 didn't jump to line 4 because the pattern on line 2 always matched"
+        )
+        # 4-6 isn't a possible arc, so the description is generic.
+        assert parser.missing_arc_description(4, 6) == "line 4 didn't jump to line 6"
+
 
 class ParserFileTest(CoverageTest):
     """Tests for coverage.py's code parsing from files."""