[SQL-CVS] r1504 - SQLObject/docs

[<sub...@co...>]

Author: phd
Date: 2006-01-13 02:28:39 -0700 (Fri, 13 Jan 2006)
New Revision: 1504

Modified:
   SQLObject/docs/FAQ.txt
Log:
Added FAQ about __len__; copied Andrew Bennetts' answer verbatim from
http://sourceforge.net/mailarchive/forum.php?thread_id=9455066&forum_id=30269.


Modified: SQLObject/docs/FAQ.txt
===================================================================
--- SQLObject/docs/FAQ.txt	2006-01-13 09:19:24 UTC (rev 1503)
+++ SQLObject/docs/FAQ.txt	2006-01-13 09:28:39 UTC (rev 1504)
@@ -4,6 +4,25 @@
 
 .. contents::
 
+Why there is no __len__?
+------------------------
+
+There are reasons why there is no __len__ method, though many people think
+having those make them feel more integrated into Python.
+
+One is that len(foo) is expected to be fast, but issuing a COUNT query can
+be slow.  Worse, often this causes the database to do essentially redundant
+work when the actual query is performed (generally taking the len of a
+sequence is followed by accessing items from that sequence).
+
+Another is that list(foo) implicitly tries to do a len first, as an
+optimization (because len is expected to be cheap -- see previous point).
+Worse, it swallows *all* exceptions that occur during that call to __len__,
+so if it fails (e.g. there's a typo somewhere in the query), the original
+cause is silently discarded, and instead you're left with mysterious errors
+like "current transaction is aborted, commands ignored until end of
+transaction block" for no apparent reason.
+
 How can I do a LEFT JOIN?
 -------------------------