SQL Alchemy: cut parameter on Update

RigthAnswer.py

class WorkflowExecution(mb.MistralSecureModelBase):
    """Contains workflow execution information."""

    __tablename__ = 'executions_v2'

    id = mb.id_column()
    wf_name = sa.Column(sa.String(80))
    wf_spec = sa.Column(st.JsonDictType())
    start_params = sa.Column(st.JsonDictType())
    state = sa.Column(sa.String(20))
    state_info = sa.Column(
        sa.String(1024),
        nullable=True)
    input = sa.Column(st.JsonDictType())
    output = sa.Column(st.JsonDictType())
    context = sa.Column(st.JsonDictType())
    # Can't use ForeignKey constraint here because SqlAlchemy will detect
    # a circular dependency and raise an error.
    parent_task_id = sa.Column(sa.String(36))

    # TODO(nmakhotkin): It's not used now, must be fixed later.
    trust_id = sa.Column(sa.String(80))

event.listen(
    # Catch and trim WorkflowExecution.state_info to always fit allocated size.
    WorkflowExecution.state_info,
    'set',
    lambda t, v, o, i: utils.cut(v, 1020),
    retval=True
)

WrongAnswer.py

class WorkflowExecution(mb.MistralSecureModelBase):
    """Contains workflow execution information."""

    __tablename__ = 'executions_v2'

    id = mb.id_column()
    wf_name = sa.Column(sa.String(80))
    wf_spec = sa.Column(st.JsonDictType())
    start_params = sa.Column(st.JsonDictType())
    state = sa.Column(sa.String(20))
    state_info = sa.Column(
        sa.String(1024),
        nullable=True,
        onupdate=lambda x: utils.cut(x.current_parameters['state_info'], 1))
    input = sa.Column(st.JsonDictType())
    output = sa.Column(st.JsonDictType())