conditions in ordered_transitions

[xdmiodz]

Hi, it's me again 😄

As far as I understand there is no way at the moment to specify a condition for a single transition in a chain? If you don't mind about the feature I could provide a pull request for the feature.

[tyarkoni]

Hmm... that's an interesting idea, but I'm not sure it's a common enough use case to warrant adding. I think there are two alternative approaches that might work better. One is to just fall back on the standard approach and add all transitions explicitly rather than using the ordered_transitions shorthand. I.e., instead of

machine.add_ordered_transitions(['A', 'C', 'B'], trigger='next_state')

...one can always write:

transitions = [['next_state', 'A', 'C', 'this_must_pass'], ['next_state', 'C', 'B']]

...which will usually not be very verbose. I think for the few cases where an individual condition is needed for an ordered chain, and there aren't very many states, that's probably a reasonable approach.

When there are a lot of states, adding all the transitions explicitly might become a pain. In that case I think a reasonable way to handle this would be to add some kind of update_transition or replace_transition method--or maybe add a boolean 'replace' argument to Machine.add_transition() that overwrites any existing matching transition instead of always appending. Then your problem could be solved by doing one of the following:

# Create the chain
machine.add_ordered_transitions(['A', 'C', 'B'], trigger='next_state')

# Option A: get transition and manually update state 
t = machine.get_transition('next_state', 'A', 'C')
t.add_condition('this_must_pass')

# Option B: replace transition in place
machine.add_transition('next_state', 'A', 'C', conditions='this_must_pass', replace=True)

I'm okay with either approach, but I think the first one might be a better way to go as it would probably be generally useful to let users easily retrieve existing transitions for inspection/modification. Thoughts?

[xdmiodz]

Oh, I completely missed the possibility to use a 'next_state' trigger. For me it will do the job. Thanks!