refactor: check for more kinds of constant tests

Author: nedbat

coverage/parser.py

@@ -654,6 +654,25 @@ def __init__(self, body: Sequence[ast.AST]) -> None:
 # TODO: Shouldn't the cause messages join with "and" instead of "or"?
 
 
+def is_constant_test_expr(node: ast.AST) -> bool:
+    """Is this a compile-time constant test expression?"""
+    node_name = node.__class__.__name__
+    if node_name in [       # in PYVERSIONS:
+        "Constant",         # all
+        "NameConstant",     # 9 10 11, gone in 12
+        "Num",              # 9 10 11, gone in 12
+    ]:
+        return True
+    elif isinstance(node, ast.Name):
+        if node.id in ["True", "False", "None", "__debug__"]:
+            return True
+    elif isinstance(node, ast.UnaryOp) and isinstance(node.op, ast.Not):
+        return is_constant_test_expr(node.operand)
+    elif isinstance(node, ast.BoolOp):
+        return all(is_constant_test_expr(v) for v in node.values)
+    return False
+
+
 class AstArcAnalyzer:
     """Analyze source text with an AST to find executable code paths.
 
@@ -1022,16 +1041,6 @@ def _missing__While(self, node: ast.While) -> ast.AST | None:
         new_while.orelse = []
         return new_while
 
-    def is_constant_expr(self, node: ast.AST) -> bool:
-        """Is this a compile-time constant?"""
-        node_name = node.__class__.__name__
-        if node_name in ["Constant", "NameConstant", "Num"]:
-            return True
-        elif isinstance(node, ast.Name):
-            if node.id in ["True", "False", "None", "__debug__"]:
-                return True
-        return False
-
     # In the fullness of time, these might be good tests to write:
     #   while EXPR:
     #   while False:
@@ -1262,7 +1271,7 @@ def _handle__Try(self, node: ast.Try) -> set[ArcStart]:
 
     def _handle__While(self, node: ast.While) -> set[ArcStart]:
         start = to_top = self.line_for_node(node.test)
-        constant_test = self.is_constant_expr(node.test)
+        constant_test = is_constant_test_expr(node.test)
         top_is_body0 = False
         if constant_test:
             top_is_body0 = True

tests/test_arcs.py

@@ -485,8 +485,6 @@ def test_while_1(self) -> None:
         )
 
     def test_while_true(self) -> None:
-        # With "while True", 2.x thinks it's computation,
-        # 3.x thinks it's constant.
         self.check_coverage("""\
             a, i = 1, 0
             while True:
@@ -500,6 +498,20 @@ def test_while_true(self) -> None:
             branchz_missing="",
         )
 
+    def test_while_not_false(self) -> None:
+        self.check_coverage("""\
+            a, i = 1, 0
+            while not False:
+                if i >= 3:
+                    a = 4
+                    break
+                i += 1
+            assert a == 4 and i == 3
+            """,
+            branchz="34 36",
+            branchz_missing="",
+        )
+
     def test_zero_coverage_while_loop(self) -> None:
         # https://github.com/nedbat/coveragepy/issues/502
         self.make_file("main.py", "print('done')")

tests/test_parser.py

@@ -5,6 +5,7 @@
 
 from __future__ import annotations
 
+import ast
 import re
 import textwrap
 from unittest import mock
@@ -13,7 +14,7 @@
 
 from coverage import env
 from coverage.exceptions import NoSource, NotPython
-from coverage.parser import PythonParser
+from coverage.parser import PythonParser, is_constant_test_expr
 
 from tests.coveragetest import CoverageTest
 from tests.helpers import arcz_to_arcs
@@ -1192,3 +1193,24 @@ def test_os_error(self) -> None:
         with pytest.raises(NoSource, match=re.escape(msg)):
             with mock.patch("coverage.python.read_python_source", side_effect=OSError("Fake!")):
                 PythonParser(filename="cant-read.py")
+
+
+@pytest.mark.parametrize(
+    ["expr", "is_constant"],
+    [
+        ("True", True),
+        ("False", True),
+        ("__debug__", True),
+        ("not __debug__", True),
+        ("not(__debug__)", True),
+        ("-__debug__", False),
+        ("__debug__ or True", True),
+        ("__debug__ + True", False),
+        ("x", False),
+        ("__debug__ or debug", False),
+    ]
+)
+def test_is_constant_test_expr(expr: str, is_constant: bool) -> None:
+    node = ast.parse(expr, mode="eval").body
+    print(ast.dump(node, indent=4))
+    assert is_constant_test_expr(node) == is_constant