pythongh-101517: bdb should not lookup linecache with lineno=None

Author: iritkatriel

Lib/bdb.py

@@ -570,9 +570,10 @@ def format_stack_entry(self, frame_lineno, lprefix=': '):
             rv = frame.f_locals['__return__']
             s += '->'
             s += reprlib.repr(rv)
-        line = linecache.getline(filename, lineno, frame.f_globals)
-        if line:
-            s += lprefix + line.strip()
+        if lineno is not None:
+            line = linecache.getline(filename, lineno, frame.f_globals)
+            if line:
+                s += lprefix + line.strip()
         return s
 
     # The following methods can be called by clients to use

Lib/test/test_bdb.py

@@ -1203,5 +1203,11 @@ def main():
                 tracer.runcall(tfunc_import)
 
 
+class TestRegressions(unittest.TestCase):
+    def test_format_stack_entry_no_lineno(self):
+        # See gh-101517
+        Bdb().format_stack_entry((sys._getframe(), None))
+
+
 if __name__ == "__main__":
     unittest.main()