When to use try and except?

[Cameron Laird]

In article <00c7058c-2b26-42bf-8f5b-2bebe31b2130 at k7g2000hsd.googlegroups.com>,
Carl Banks  <pavlovevidence at gmail.com> wrote:
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>> Basically, there's a general principle (EAFP: Easier to ask
>> forgiveness than permission) in Python to just "try" something and
>> then catch the exception if something goes wrong. This is in contrast
>> to e.g. C where you're supposed to "Look before you leap" (LBYL) and
>> check for possible error conditions before performing the operation.
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>I wouldn't say that the possibility of EAFP in Python makes it
>obsolute to use LBYL.  (Error checking seems to be too broad a subject
>to apply the One Obvious Way maxim to.)  C isn't always LBYL anyway;
>sometimes it's DFTCFE "Don't forget to check for errors".
>
>I tend to use EAFP to check if something "wrong" happened (a missing
>file, invalid input, etc.), and LBYL for expected conditions that can
>occur with valid input, even when that condition could be tested with
>a try...except.  For instance, I'd write something like this:
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I'll reinforce what Carl tells:  a wise developer knows how to
work in different styles in different circumstances.  In a retail
C program, for example, it might be wisest to emphasize ahead-of-
time diagnostics such as, "the configuration file ... is {missing,
inaccessible,locked, ...}", while a system-level library making
what apparently is the same call needs to check return values
carefully, to guard against the possibility that the system's
state has changed while executing consecutive statements.