Let exception fire or return None

Cecil Westerhof Cecil at decebal.nl
Thu Apr 30 13:26:01 CEST 2015

Op Thursday 30 Apr 2015 11:30 CEST schreef Peter Otten:

> 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 

I already did that. In another thread I got to know my assumptions
where wrong.

> (2) you may want to take measures to limit memory usage, e. g. 
> assert index >= 0

I put that in, but as first statement.

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

In my case it is not important. (The biggest file I use has between
100 and 200 lines), but I publish it, so I should do my best to make
it so lean as possible.

To quote a famous comedian: ‘Learning all the time’.

Cecil Westerhof
Senior Software Engineer
LinkedIn: http://www.linkedin.com/in/cecilwesterhof

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