From 12c3cd7c1f090cb055d395a5d54deba955af9d7b Mon Sep 17 00:00:00 2001 From: Georg Brandl Date: Sat, 17 Mar 2012 16:58:05 +0100 Subject: [PATCH] Closes #14306: clarify expensiveness of try-except and update code snippet --- Doc/faq/design.rst | 10 +++++----- 1 file changed, 5 insertions(+), 5 deletions(-) diff --git a/Doc/faq/design.rst b/Doc/faq/design.rst index e45aaaacb65..87cc942f319 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