From: Georg Brandl <georg@python.org>
Date: Sat, 17 Mar 2012 15:58:05 +0000 (+0100)
Subject: Closes #14306: clarify expensiveness of try-except and update code snippet
X-Git-Tag: v3.3.0a2~124^2
X-Git-Url: https://granicus.if.org/sourcecode?a=commitdiff_plain;h=12c3cd7c1f090cb055d395a5d54deba955af9d7b;p=python

Closes #14306: clarify expensiveness of try-except and update code snippet
---

diff --git a/Doc/faq/design.rst b/Doc/faq/design.rst
index e45aaaacb6..87cc942f31 100644
--- a/Doc/faq/design.rst
+++ b/Doc/faq/design.rst
@@ -284,8 +284,9 @@ Similar methods exist for bytes and bytearray objects.
 How fast are exceptions?
 ------------------------
 
-A try/except block is extremely efficient.  Actually catching an exception is
-expensive.  In versions of Python prior to 2.0 it was common to use this idiom::
+A try/except block is extremely efficient if no exceptions are raised.  Actually
+catching an exception is expensive.  In versions of Python prior to 2.0 it was
+common to use this idiom::
 
    try:
        value = mydict[key]
@@ -296,11 +297,10 @@ expensive.  In versions of Python prior to 2.0 it was common to use this idiom::
 This only made sense when you expected the dict to have the key almost all the
 time.  If that wasn't the case, you coded it like this::
 
-   if mydict.has_key(key):
+   if key in mydict:
        value = mydict[key]
    else:
-       mydict[key] = getvalue(key)
-       value = mydict[key]
+       value = mydict[key] = getvalue(key)
 
 For this specific case, you could also use ``value = dict.setdefault(key,
 getvalue(key))``, but only if the ``getvalue()`` call is cheap enough because it