Question

I’m working on a dispatcher for RPC calls via a webserver. The webserver class has some methods like rpc_echo, rpc_add, … (prefixed with rpc_) which should be accessable from remote. In the dispatcher method I can find the corresponding method and call it with the prepared arguments in the dictionary :

try:
  handler = getattr(self, 'rpc_' + request['method'])  # identify handler
  response['result'] = handler(**params)  # assign arguments and call handler
except (AttributeError, KeyError):
  # exceptions: requested method -> key, call method -> attr, callable -> attr
  raise JSONRPCError('unknown method.')
except TypeError:
  raise JSONRPCError('parameters don\'t match method prototype.')

This is working fine: But if there is thrown an exception inside the handler the error checking is disturbed and leads to wrong conclusions. How can I find out if the exception was thrown inside handler or not? Thus a wrong call or a server error?

Answer 1

you probably want to spend some time with traceback module

here is a simplified example:

import sys, traceback

def outer(b):
    def inner(b):
        return [0,2,99][b]
    return "abcd"[inner(b)]

# "abcd"[[0,2,99][1]] => "abcd"[2] => "c"
print(outer(1))

try:
    # "abcd"[[0,2,99][2]] => "abcd"[99] => IndexError
    print(outer(2))
except IndexError:
    fname = traceback.extract_tb(sys.exc_info()[2])[-1][2]
    print("Exception from: {}".format(fname))

try:
    # "abcd"[[0,2,99][3]] => IndexError
    print(outer(3))
except IndexError:
    fname = traceback.extract_tb(sys.exc_info()[2])[-1][2]
    print("Exception from: {}".format(fname))

Output:

c
Exception from: outer
Exception from: inner