idlelib has two files that *might* be helpful to anyone who wants to pursue this: PyParse.py and HyperParser.py. One use of the latter is for calltips. When the user types '(', the HyperParser is used first to determine whether the '(' is preceded by an operator (making it a grouper) or not (making it a call operator). In the later case, it parses backwards to find the beginning of the callable expression, which is then evaluated to get the callable (if it does not contain an embedded call) and hence the signature. Parsing forward and back from a given line might be similar.