try/except block is both efficient and expensive?

[tshepang]

BPO	14306
Nosy	@birkenfeld, @terryjreedy, @ezio-melotti

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2012-03-17.15:58:16.670>
created_at = <Date 2012-03-14.21:41:18.517>
labels = ['docs']
title = 'try/except block is both efficient and expensive?'
updated_at = <Date 2012-03-17.15:58:55.261>
user = 'https://bugs.python.org/tshepang'

bugs.python.org fields:

activity = <Date 2012-03-17.15:58:55.261>
actor = 'python-dev'
assignee = 'docs@python'
closed = True
closed_date = <Date 2012-03-17.15:58:16.670>
closer = 'python-dev'
components = ['Documentation']
creation = <Date 2012-03-14.21:41:18.517>
creator = 'tshepang'
dependencies = []
files = []
hgrepos = []
issue_num = 14306
keywords = ['patch']
message_count = 6.0
messages = ['155798', '155802', '155803', '156074', '156164', '156165']
nosy_count = 6.0
nosy_names = ['georg.brandl', 'terry.reedy', 'ezio.melotti', 'docs@python', 'tshepang', 'python-dev']
pr_nums = []
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue14306'
versions = []

[tshepang]

In Doc/faq/design.rst, we got this text:

"A try/except block is extremely efficient. Actually catching an exception is expensive."

The 2 sentences appear contradictory.

[ezio-melotti]

What it's trying to say is that in case the operation in the "try" block succeeds there's almost no overhead, so it's very efficient. On the other hand, raising and then catching the error is expensive.

[birkenfeld]

It could be reworded, yes.

[terryjreedy]

I think it would be sufficient to add ' if no exceptions are raised' to the first sentence. The example that follows in the entry clarifies the implications of "cheap to try, expensive to catch".

[python-dev]

New changeset 9a7dcfbcf726 by Georg Brandl in branch '3.2':
Closes bpo-14306: clarify expensiveness of try-except and update code snippet
http://hg.python.org/cpython/rev/9a7dcfbcf726

[python-dev]

New changeset 7a93f6ee2ebc by Georg Brandl in branch '2.7':
Closes bpo-14306: clarify expensiveness of try-except and update code snippet
http://hg.python.org/cpython/rev/7a93f6ee2ebc