[7.4.x] Ensure the docstring is a string

AUTHORS

@@ -376,6 +376,7 @@ Tomer Keren
 Tony Narlock
 Tor Colvin
 Trevor Bekolay
+Tushar Sadhwani
 Tyler Goodlet
 Tyler Smart
 Tzu-ping Chung

changelog/11140.bugfix.rst

@@ -0,0 +1 @@
+Fix non-string constants at the top of file being detected as docstrings on Python>=3.8.

src/_pytest/assertion/rewrite.py

@@ -604,6 +604,13 @@ def _write_and_reset() -> None:
     return ret
 
 
+def _get_ast_constant_value(value: astStr) -> object:
+    if sys.version_info >= (3, 8):
+        return value.value
+    else:
+        return value.s
+
+
 class AssertionRewriter(ast.NodeVisitor):
     """Assertion rewriting implementation.
 
@@ -700,11 +707,10 @@ def run(self, mod: ast.Module) -> None:
                 expect_docstring
                 and isinstance(item, ast.Expr)
                 and isinstance(item.value, astStr)
+                and isinstance(_get_ast_constant_value(item.value), str)
             ):
-                if sys.version_info >= (3, 8):
-                    doc = item.value.value
-                else:
-                    doc = item.value.s
+                doc = _get_ast_constant_value(item.value)
+                assert isinstance(doc, str)
                 if self.is_rewrite_disabled(doc):
                     return
                 expect_docstring = False

testing/test_assertrewrite.py

@@ -2077,3 +2077,17 @@ def test_max_increased_verbosity(self, pytester: Pytester) -> None:
         self.create_test_file(pytester, DEFAULT_REPR_MAX_SIZE * 10)
         result = pytester.runpytest("-vv")
         result.stdout.no_fnmatch_line("*xxx...xxx*")
+
+
+class TestIssue11140:
+    def test_constant_not_picked_as_module_docstring(self, pytester: Pytester) -> None:
+        pytester.makepyfile(
+            """\
+            0
+
+            def test_foo():
+                pass
+            """
+        )
+        result = pytester.runpytest()
+        assert result.ret == 0