Source code for caspia.reactive.operations.transition

from ..errors import ValueNotReady
from .cached import cached


class transition(cached):
    async def on_next(self, value, **kwargs):
        if not hasattr(self, 'previous'):
            await super().on_error(ValueNotReady('waiting for transition'))
            self.previous = value
        else:
            await super().on_next((self.previous, value))
            self.previous = value

    def __str__(self):
        return '<transition (last, current) of {}>'.format(self.observable)