bpo-38689: avoid IDLE hanging when calltip fails getting a signature (GH-17152)

Inspect.signature failed on the test case because its isinstance call raised.

Lib/idlelib/calltip.py

@@ -129,20 +129,22 @@ def get_argspec(ob):
     empty line or _MAX_LINES.    For builtins, this typically includes
     the arguments in addition to the return value.
     '''
-    argspec = default = ""
+    # Determine function object fob to inspect.
     try:
         ob_call = ob.__call__
-    except BaseException:
-        return default
-
+    except BaseException:  # Buggy user object could raise anything.
+        return ''  # No popup for non-callables.
     fob = ob_call if isinstance(ob_call, types.MethodType) else ob
 
+    # Initialize argspec and wrap it to get lines.
     try:
         argspec = str(inspect.signature(fob))
-    except ValueError as err:
+    except Exception as err:
         msg = str(err)
         if msg.startswith(_invalid_method):
             return _invalid_method
+        else:
+            argspec = ''
 
     if '/' in argspec and len(argspec) < _MAX_COLS - len(_argument_positional):
         # Add explanation TODO remove after 3.7, before 3.9.
@@ -154,6 +156,7 @@ def get_argspec(ob):
     lines = (textwrap.wrap(argspec, _MAX_COLS, subsequent_indent=_INDENT)
              if len(argspec) > _MAX_COLS else [argspec] if argspec else [])
 
+    # Augment lines from docstring, if any, and join to get argspec.
     if isinstance(ob_call, types.MethodType):
         doc = ob_call.__doc__
     else:
@@ -167,9 +170,8 @@ def get_argspec(ob):
                 line = line[: _MAX_COLS - 3] + '...'
             lines.append(line)
     argspec = '\n'.join(lines)
-    if not argspec:
-        argspec = _default_callable_argspec
-    return argspec
+
+    return argspec or _default_callable_argspec
 
 
 if __name__ == '__main__':