Callback Usage

Callbacks can be used to hook some of KwArg events.

By using the KwArg.kwargs_helper property which is an instance of KwargsHelper it is possible to use callbacks.

This function has two nested callback functions. arg_before_cb() function is called before key, value is assigned to KwArg instance. arg_after_cb() function is called after key, value is assigned to KwArg instance.

from kwhelp import AfterAssignEventArgs, BeforeAssignEventArgs, KwargsHelper, KwArg

def my_method(**kwargs) -> str:
    def arg_before_cb(helper: KwargsHelper,
                    args: BeforeAssignEventArgs) -> None:
        # callback function before value assigned to KwArg instance
        if args.key == 'msg':
            if args.field_value == "cancel":
                # cancel will raise CancelEventError unless
                # kw.kwargs_helper.cancel_error = False
                args.cancel = True
            elif args.field_value == "":
                args.field_value = "Value:"
        if args.key == 'name' and args.field_value == 'unknown':
            args.field_value = 'None'
    def arg_after_cb(helper: KwargsHelper,
                    args: AfterAssignEventArgs) -> None:
        # callback function after value assigned to KwArg instance
        if args.key == 'msg' and args.field_value == "":
            raise ValueError(
                f"{args.key} This should never happen. value was suppose to be reassigned")
    # main function
    kw = KwArg(**kwargs)
    # assign Callbacks
    kw.kwargs_helper.add_handler_before_assign(arg_before_cb)
    kw.kwargs_helper.add_handler_after_assign(arg_after_cb)
    # assign args
    kw.kw_assign(key='first', require=True, types=[int])
    kw.kw_assign(key='second', require=True, types=[int])
    kw.kw_assign(key='msg', types=[str], default='Result:')
    kw.kw_assign(key='end', types=[str])
    _result = kw.first + kw.second
    if kw.is_attribute_exist('end'):
        return_msg = f'{kw.msg} {_result}{kw.end}'
    else:
        return_msg = f'{kw.msg} {_result}'
    return return_msg

Call back does not make changes to the following result.

>>> result = my_method(first = 10, second = 22)
>>> print(result)
Result: 32

Call back changes msg from empty string to Value:

>>> result = my_method(first = 10, second = 22, msg = "")
>>> print(result)
Value: 32

Call back raises CancelEventError when msg equals cancel.

>>> result = my_method(first = 10, second = 22, msg = "cancel")
kwhelp.CancelEventError: KwargsHelper.assign() canceled in 'BeforeAssignEventArgs'

Note

If KwargsHelper.cancel_error property is set to False then no error will be raised when BeforeAssignEventArgs.cancel is set to True.

If no error is raised then AfterAssignEventArgs.canceled can be used to check if an event was canceled in BeforeAssignEventArgs