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

This commit is contained in:
Georg Brandl 2012-03-17 16:58:05 +01:00
parent f2123d2db5
commit 12c3cd7c1f
1 changed files with 5 additions and 5 deletions
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Doc/faq/design.rst
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@ -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