Callback Usage

Callbacks can be used to hook some of KwargsHelper events.

This class as two callback methods. MyClass._arg_before_cb() method is called before attribute assignment. MyClass._arg_after_cb() method is called after attribute assignment.

from kwhelp import KwargsHelper, AfterAssignEventArgs, BeforeAssignEventArgs, AssignBuilder

class MyClass:
    def __init__(self, **kwargs):
        self._loop_count = -1
        kw = KwargsHelper(originator=self, obj_kwargs={**kwargs})
        ab = AssignBuilder()
        kw.add_handler_before_assign(self._arg_before_cb)
        kw.add_handler_after_assign(self._arg_after_cb)
        ab.append(key='exporter', require=True, types=[str])
        ab.append(key='name', require=True, types=[str],
                default='unknown')
        ab.append(key='file_name', require=True, types=[str])
        ab.append(key='loop_count', types=[int],
                default=self._loop_count)
        result = True
        # by default assign will raise errors if conditions are not met.
        for arg in ab:
            result = kw.assign_helper(arg)
            if result == False:
                break
        if result == False:
            raise ValueError("Error parsing kwargs")

    def _arg_before_cb(self, helper: KwargsHelper,
                    args: BeforeAssignEventArgs) -> 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: AfterAssignEventArgs) -> 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")

    @property
    def exporter(self) -> str:
        return self._exporter

    @property
    def file_name(self) -> str:
        return self._file_name

    @property
    def name(self) -> str:
        return self._name

    @property
    def loop_count(self) -> int:
        return self._loop_count

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)
>>> print(my_class.exporter)
json
>>> print(my_class.file_name)
data.json
>>> print(my_class.name)
None
>>> print(my_class.loop_count)
3

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.assign() canceled in 'BeforeAssignEventArgs'

Note

If KwargsHelper constructor has cancel_error set to False or 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