Hint at likely cause of ast parsing failure in error message (#2786)

Co-authored-by: Batuhan Taskaya <isidentical@gmail.com>
Co-authored-by: Jelle Zijlstra <jelle.zijlstra@gmail.com>
Co-authored-by: Richard Si <63936253+ichard26@users.noreply.github.com>

diff --git a/src/black/__init__.py b/src/black/__init__.py
index 67c272e3cc9a5e11ef7fe5419a08d0a413b3c9cd..bdece687e4517666e2fa7217a14f183b322edd44 100644 (file)
--- a/src/black/__init__.py
+++ b/src/black/__init__.py
@@ -1312,7 +1312,10 @@ def assert_equivalent(src: str, dst: str, *, pass_num: int = 1) -> None:
         src_ast = parse_ast(src)
     except Exception as exc:
         raise AssertionError(
-            f"cannot use --safe with this file; failed to parse source file: {exc}"
+            f"cannot use --safe with this file; failed to parse source file AST: "
+            f"{exc}\n"
+            f"This could be caused by running Black with an older Python version "
+            f"that does not support new syntax used in your source file."
         ) from exc
 
     try: