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Date: Thu, 30 Apr 2015 04:31:26 -0400
From: Dave Angel <davea@davea.name>
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To: python-list@python.org
Subject: Re: seek operation in python
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On 04/30/2015 04:06 AM, Cecil Westerhof wrote:
> Op Thursday 30 Apr 2015 09:33 CEST schreef Chris Angelico:
>
>> On Thu, Apr 30, 2015 at 4:27 PM, Cecil Westerhof <Cecil@decebal.nl> wrote:
>>>> with open("input.cpp") as f:
>>>> lines = f.readlines()
>>>> print(lines[7])
>>>
>>> Is the following not better:
>>> print(open('input.cpp', 'r').readlines()[7])
>>>
>>> Time is the same (about 25 seconds for 100.000 calls), but I find
>>> this more clear.
>>
>> The significant difference is that the 'with' block guarantees to
>> close the file promptly. With CPython it probably won't make a lot
>> of difference, and in a tiny script it won't do much either, but if
>> you do this on Jython or IronPython or MicroPython or some other
>> implementation, it may well make a gigantic difference - your loop
>> might actually fail because the file's still open.
>
> I thought that in this case the file was also closed. But if that is
> not the case I should think about this when I switch to another
> version as CPython.
>
> I wrote a module where I have:
>      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()
>
> But this can be used by others also and they could be using Jython or
> another implementation. So should I rewrite this and other functions?
> Or would it be OK because the open is in a function?
>

No, it's not going to close the file just because the open is in a 
function.  The "with" construct was designed to help solve exactly this 
problem.  Please use it.



-- 
DaveA