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24 Apr, 2024: Testing a new version of the Overboard here. If you have an issue post about it to rocksolid.nodes.help (I know. Everyone on Usenet has issues)

Re: EAFP

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From: __pete...@web.de (Peter Otten)
Newsgroups: comp.lang.python
Subject: Re: EAFP
Date: Mon, 16 May 2022 12:05:14 +0200
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by: Peter Otten - Mon, 16 May 2022 10:05 UTC

On 13/05/2022 18:37, bryangan41 wrote:
> Is the following LBYL:foo = 123if foo < 200: do()If so, how to change to EAFP?Thanks!Sent from Samsung tablet.

The distinction between look-before-you-leap and
easier-to-ask-forgiveness-than-permission is weaker than yo might expect.

When you write

filename = ...
if exists(filename):
with open(filename) as instream:
# do stuff
else:
# fallback

there are two checks for the file's existence, one explicit, and one
implicitly inside open() -- and worse, the first, explicit, check is
unreliable because between exists() and open() there is a small delay
that may be sufficient to create or delete the file. Therefore the
recommended (EAFP) version of the above is

filename = ...
try:
with open(filename) as instrem:
# do stuff
except FileNotFoundError
# fallback

or just

with open(filename) as instream:
# do stuff

if there is no meaningful fallback.

Regarding your example code, whether you can write an EAFP version for

if foo < 200:
do()

depends on the contents of do(). If do() fails in a well-defined way,
let's say by raising a FooAbove199 exception you can write just

do()

or, if you need a fallback

try:
do()
except FooAbove199:
# fallback

Note that if you change do() from

do():
# do stuff

def do():
if foo < 200:
# do stuff
else:
raise FooAbove199(f"Invalid {foo=!r}")

the

do()

invocation becomes EAFP even though you are actually performing the same
test as before.

24 Apr, 2024: Testing a new version of the Overboard here. If you have an issue post about it to rocksolid.nodes.help (I know. Everyone on Usenet has issues)

devel / comp.lang.python / Re: EAFP