Auto_assign Callback

KwargsHelper.auto_assign() method automatically assigns all key-value pairs to class.

Assigns all of the key, value pairs passed into constructor to class instance, unless the event is canceled in BeforeAssignAutoEventArgs via KwargsHelper.add_handler_before_assign_auto() callback.

MyClass._arg_before_cb() callback method reads conditions before attribue and value are assigned. If required conditions are not met then cancel can be set.

from kwhelp import KwargsHelper, AfterAssignAutoEventArgs, BeforeAssignAutoEventArgs

class MyClass:
    def __init__(self, **kwargs):
        kw = KwargsHelper(originator=self, obj_kwargs={**kwargs}, field_prefix="")
        kw.add_handler_before_assign_auto(self._arg_before_cb)
        kw.add_handler_after_assign_auto(self._arg_after_cb)
        kw.auto_assign()

    def _arg_before_cb(self, helper: KwargsHelper,
                    args: BeforeAssignAutoEventArgs) -> None:
        # callback function before value assigned to attribute
        if args.key == 'loop_count' and args.field_value < 0:
            # cancel will raise CancelEventError unless
            # KwargsHelper constructor has cancel_error=False
            args.cancel = True
        if args.key == 'name' and args.field_value == 'unknown':
            args.field_value = 'None'

    def _arg_after_cb(self, helper: KwargsHelper,
                    args: AfterAssignAutoEventArgs) -> None:
        # callback function after value assigned to attribute
        if args.key == 'name' and args.field_value == 'unknown':
            raise ValueError(
                f"{args.key} This should never happen. value was suppose to be reassigned")

In the following case key, value args are automatically assigned to class.

>>> my_class = MyClass(exporter='json', file_name='data.json', loop_count=3)
>>> print(my_class.exporter)
json
>>> print(my_class.file_name)
data.json
>>> print(my_class.loop_count)
3

In the following case my_class.name value is changed from "unknown" to "None" in callback MyClass._arg_before_cb() method.

>>> my_class = MyClass(exporter='json', file_name='data.json', loop_count=3, name="unknown")
>>> print(my_class.exporter)
json
>>> print(my_class.file_name)
data.json
>>> print(my_class.loop_count)
3
>>> print(my_class.name)
None

In the following case loop_count is a negative number which triggers a CancelEventError in MyClass._arg_before_cb().

>>> my_class = MyClass(exporter='json', file_name='data.json', loop_count=-1)
kwhelp.CancelEventError: KwargsHelper.auto_assign() canceled in 'BeforeAssignBlindEventArgs'

Note

If KwargsHelper constructor has cancel_error set to False then no error will be raised when BeforeAssignAutoEventArgs.cancel is set to True.

See also