On Sun, 27 Oct 2013 20:43:07 -0700, Victor Hooi wrote:
> Hi,
>
> I'd like to double-check something regarding using try-except for
> controlling flow.
>
> I have a script that needs to lookup things in a SQLite database.
>
> If the SQLite database file doesn't exist, I'd like to create an empty
> database, and then setup the schema.
>
> Is it acceptable to use try-except in order to achieve this? E.g.:
>
> try:
> # Try to open up the SQLite file, and lookup the required
> entries
> except OSError:
> # Open an empty SQLite file, and create the schema
Yes, that's the right way to do it.
> My thinking is that it is (easier to ask forgiveness than permission),
> but I just wanted to check if there is a better way of achieving this?
>
> I'd also be doing the same thing for checking if a file is gzipped or
> not - we try to open it as a gzip, then as an ordinary text file, and if
> that also fails, raise a parsing error.
Correct.
The problem with checking in advance is that there is a race condition
between checking and the using the file:
if database exists: # at this moment, the file is guaranteed to exist
# but a moment later, guarantee is no longer valid
open database
In a multitasking operating system, some other process may have deleted
the database. Or changed its name, removed your access privileges, even
replaced it with a different file. Apart from hard-to-diagnose bugs, this
is also the source of some security vulnerabilities:
https://www.owasp.org/index.php/Race_Conditions
Scroll down and read the section on "Time of check, time of use race
condition".
So using a try...except block is precisely the right solution.
--
Steven
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