include the requirement flag on the thrift spec

[tmehlinger]

This amends the parser to include the requirement flag so it can be used for later validation.

I've used this in my own implementation of TProcessor (not included in these changes) to accomplish something like this:

struct Thing {
    1: required string required_field;
}

class ValidatingProcessor(TProcessor):
    def validate(self, result):
        obj = result.success
        for _, s in obj.thrift_spec.items():
            k = s[1]
            r = s[-1] # requirement will be at the end of the tuple
            v = getattr(obj, k)
            if v is None and r:
                raise TApplicationException(
                        message='{} is required'.format(k))

    def process(self, iprot, oprot):
        api, seqid, result, call = self.process_in(iprot)
        if isinstance(result, TApplicationException):
            self.send_exception(oprot, api, result, seqid)
            return
        try:
            result.success = call()
            self.validate(result)
        except TApplicationException as e:
            self.send_exception(oprot, api, e, seqid)
        except Exception as e:
            # raise if api don't have throws
            self.handle_exception(e, result)
        else:
            self.send_result(oprot, api, result, seqid)

Which, when called, produces:

$ python failing_client.py
<backtrace snip>
thriftpy.thrift.TApplicationException: `required_field` is required

[tmehlinger]

Tests are passing but this doesn't work right with complex (map, list, etc.) types. Please don't merge.

[tmehlinger]

Should be fixed now.

[hit9]

Parser has been rebuit, but this feature was included.