Wrapping of python's incoherent error messages

rpc.py

class RpcException(Exception):
    """ An RPC-specific exception. Details should be in the message """
    pass


def rpc_call(target, data, fn_prefix='_cmd_'):
    """ Common logic for exposing several methods from a class as RPC
    (at the level after the data-parsing).
    Note: likely only works in CPython.  """
    if not isinstance(data, dict):
        raise RpcException('`data` is not a dict')
    try:
        cmd = data.pop('cmd')
    except KeyError as e:
        raise RpcException('`cmd` is not specified in the data', e)
    try:
        fn = getattr(target, '%s%s' % (fn_prefix, cmd))
    except AttributeError as e:
        raise RpcException('command %r not found' % (cmd,), e)
    _log.debug("Calling: %r, %r", fn, data)
    try:
        return fn(**data)
    except TypeError as e:
        # More-sane-than-default processing of a case `parameter ... was not specified` (py2)
        if fn.func_code.co_flags & 0x04:
            # it accepts `*ar`, so not the case
            raise
        f_vars = fn.func_code.co_varnames
        f_defvars_count = len(fn.func_defaults)
        f_posvars = f_vars[:-f_defvars_count]
        extra_args = list(set(data.keys()) - set(f_vars))
        # TODO: `inspect.getargspec`
        # XXX: it catches `self` in a bound method as required. (also, classmethods?)
        missing_args = list(set(f_posvars) - set(data.keys()))
        if missing_args:  # is the case, raise it verbosely.
            msg = "Required argument(s) not specified: %s" % (
                ', '.join(missing_args),)
            if extra_args:
                msg += "; additionally, there are extraneous arguments: %s" % (
                    ', '.join(extra_args))
            raise TypeError(msg, e)
        raise