Control of flow

Tim Peters at
Wed Aug 1 22:51:38 CEST 2001

> I use python to write telephony apps, and I find that a 'goto' is often
> a nice tool (stop going 'aargh', its properly "useful" sometimes =).

Draw a diagram using boxes showing the states of your dialog.  Draw arrows
between boxes, labelled with the conditions under which you can move from
one state to another ("user said 'yes'"; "user pushed '#'"; "error"; "this
specific error"; etc).  Picture the "quick reference" card that comes with
most fancy phone systems -- that's the idea.  Design a data structure using
a dict to map a state to a pair (or class instance) containing:

1. A function implementing the state's logic, returning "a condition"
   (corresponding to the labels on your arrows).

2. A dict of (condition, new_state) pairs (these are the labelled edges
   on hand diagram).

Then write incredibly simple and uniform code to walk the data structure in
a loop.  Then initialize the main dict.  You're done <wink>.  But I'm not
kidding:  if you had gotos, you'd get hopelessly lost as soon as the dialogs
you're working with now get more complicated.  You can *make* this very
pleasant indeed, but not with masses of twisted control-flow logic.  Highly
irregular problems are much better handled via dead-uniform simple code
marching over a lumpy data structure.

believe-me-now-or-believe-me-later<wink>-ly y'rs  - tim

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