updated python completer to allow for initial open paren

news/openparen.rst

@@ -0,0 +1,15 @@
+**Added:** None
+
+**Changed:** None
+
+**Deprecated:** None
+
+**Removed:** None
+
+**Fixed:**
+
+* Starting a new command with an open parentheses will no longer
+  throw a traceback when ``$UPDATE_COMPLETIONS_ON_KEYPRESS`` is
+  ``True``.
+
+**Security:** None

xonsh/completers/python.py

@@ -169,7 +169,7 @@ def python_signature_complete(prefix, line, end, ctx, filter_func):
         return set()
     funcname = xt.subexpr_before_unbalanced(front, '(', ')')
     val, _ctx = _safe_eval(funcname, ctx)
-    if val is None and _ctx is None:
+    if val is None:
         return set()
     try:
         sig = inspect.signature(val)