Merge pull request #4057 from jeffreyrack/4051-improve-error-messaging

Improve error messaging when invalid syntax is passed to the -m option

src/_pytest/mark/legacy.py

@@ -66,7 +66,10 @@ python_keywords_allowed_list = ["or", "and", "not"]
 
 def matchmark(colitem, markexpr):
     """Tries to match on any marker names, attached to the given colitem."""
-    return eval(markexpr, {}, MarkMapping.from_item(colitem))
+    try:
+        return eval(markexpr, {}, MarkMapping.from_item(colitem))
+    except SyntaxError as e:
+        raise SyntaxError(str(e) + "\nMarker expression must be valid Python!")
 
 
 def matchkeyword(colitem, keywordexpr):