[Tutor] Circular approach to a dictionary structure

Andrei project5 at redrival.net
Sat Oct 29 01:32:42 CEST 2005

Tim Johnson wrote:
> Hello:
> # I have the following dictionary:
> next_phases = {"open":"review","review":"write","write":"open"}
> # a 'phase is extracted by
> next_phase = next_phases[self.phase
> Note that the value for each of the first keys is the key for
> the next item, and that the value for the last key is the *first* key.

I too would be tempted to use a list for this. Something along the lines of:

   phases = ['open', 'review', 'write']

   nextphase = phases[((1 + phases.index(phase)) % len(phases)]

This is not as friendly as the dictionary option, but I'd wrap that in a 
function call (or object interface). It has some advantages:
   - there's a single instance of every string
   - it's obvious that it's a sequence
   - it's one of the standard ways of doing circular structures
   - there's no chance of introducing a dead branch, breaking the circle
   - it's easier to add/remove things.
     E.g. if you'd add and an "approve" step between review and write,
     the list approach requires a single update, while the dictionary
     requires one new key-value pair and modifications in another two
     places in order to get it working correctly.



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