[Python-checkins] r47149 - peps/trunk/pep-3103.txt

[guido.van.rossum]

Author: guido.van.rossum
Date: Wed Jun 28 16:41:23 2006
New Revision: 47149

Modified:
   peps/trunk/pep-3103.txt
Log:
Introducing school IIb -- duplicate cases resolved by case order, not errors.


Modified: peps/trunk/pep-3103.txt
==============================================================================
--- peps/trunk/pep-3103.txt	(original)
+++ peps/trunk/pep-3103.txt	Wed Jun 28 16:41:23 2006
@@ -390,10 +390,38 @@
 unflagged, when the dict-based dispatch implementation makes it so
 easy to trap this.
 
+However, there are some use cases for overlapping/duplicate cases.
+Suppose you're switching on some OS-specific constants (e.g. exported
+by the os module or some module like that).  You have a case for each.
+But on some OS, two different constants have the same value (since on
+that OS they are implemented the same way -- like O_TEXT and O_BINARY
+on Unix).  If duplicate cases are flagged as errors, your switch
+wouldn't work at all on that OS.  It would be much better if you could
+arrange the cases so that one case has preference over another.
+
+There's also the (more likely) use case where you have a set of cases
+to be treated the same, but one member of the set must be treated
+differently.  It would be convenient to put the exception in an
+earlier case and be done with it.
+
+(Yes, it seems a shame not to be able to diagnose dead code due to
+accidental case duplication.  Maybe that's less important, and
+pychecker can deal with it?  After all we don't diagnose duplicate
+method definitions either.)
+
+This suggests school IIb: like school II but redundant cases must be
+resolved by choosing the first match.  This is trivial to implement
+when building the dispatch dict (skip keys already present).
+
+(An alternative would be to introduce new syntax to indicate "okay to
+have overlapping cases" or "ok if this case is dead code" but I find
+that overkill.)
+
 Personally, I'm in school II: I believe that the dict-based dispatch
 is the one true implementation for switch statements and that we
 should face the limitiations up front, so that we can reap maximal
-benefits.
+benefits.  I'm leaning towards school IIb -- duplicate cases should be
+resolved by the ordering of the cases instead of flagged as errors.
 
 When to Freeze the Dispatch Dict
 --------------------------------