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From: Peter Otten <__peter__@web.de>
Subject: Re: Let exception fire or return None
Date: Thu, 30 Apr 2015 11:30:21 +0200
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Xref: csiph.com comp.lang.python:89617

Cecil Westerhof wrote:

> I have a function to fetch a message from a file:
>     def get_indexed_message(message_filename, index):
>         """
>         Get index message from a file, where 0 gets the first message
>         """
> 
>         return open(expanduser(message_filename),
>         'r').readlines()[index].rstrip()
> 
> What is more the Python way: let the exception fire like this code
> when index is to big, or catching it and returning None?

Fire an exception, but you may also allow the user to provide a default.

> I suppose working zero based is OK.

Not just OK, it's de rigueur. 


You didn't ask for that, but

(1)

with open(...) as f:
    return f.readlines()[index].rstrip()

is preferrable because it closes the file in a controlled way and 

(2) you may want to take measures to limit memory usage, e. g. 

assert index >= 0
try:
    [line] = itertools.islice(f, index, index+1)
except ValueError:
    raise IndexError
return line.rstrip()