[Python-Dev] Rationale for NamedTemporaryFile revisited [SEC=UNCLASSIFIED]

[Steve Holden]

Ole.Nielsen at ga.gov.au wrote:
> Thank you very much for the quick reply.
> 
> I believe we have to close the file in order be able to read it in - in this
> case to feed a unittest. I actually tried to read it in before closing it,
> but (as I suspected) the data wasn't available.
>  
Did you try seeking back to the beginning before attempting your read?

> We use unit testing for pretty much all functions and classes in our system.
> Some functions read files and do something with them. We therefore need a
> standardised and robust way to generate temporary files that exist on the
> filesystem long enough for the function being tested to read them.
> 
Well you can hardly believe this requirement is so novel that its 
absence hasn't been missed in Python, then.

> We are happy to write a wrapper ourselves that will do just that - but before
> doing so we had a look at the standard functionality. Hence the question
> about NamedTemporaryFile: What is the purpose of creating a name if it
> disappears after the file object is closed? I agree with Dustin that it would
> make sense for the file to live on the disk for as long as the File object is
> available to the script, closed or not. But I don't think that is the way
> NamedTemporaryFile currently works as illustrated in the test script in my
> first posting.
> 
> Any help or comments, please?
> 
The mistake you made was to change the mode from the default "w+b". The 
following code works on both Windows and Linux:

from tempfile import NamedTemporaryFile
fid = NamedTemporaryFile(mode='w+b',
                          suffix='.tmp',
                          dir='.')

fid.write('My temp file')
fid.seek(0)
data = fid.read()
print data
fid.close()


By the way, this is the sort of topic normally reserved for 
comp.lang.python, whose denizens would have straightened you out in no 
time flat.

regards
  Steve
> Cheers
> Ole Nielsen, Geoscience Australia
> 
> 
> -----Original Message-----
> From: gvanrossum at gmail.com [mailto:gvanrossum at gmail.com] On Behalf Of Guido
> van Rossum
> Sent: Friday, 18 January 2008 3:45
> To: Nielsen Ole
> Cc: python-dev at python.org
> Subject: Re: [Python-Dev] Rationale for NamedTemporaryFile revisited
> [SEC=UNCLASSIFIED]
> 
> Don't close it until you're done with it. Isn't that obvious?
> 
> On Jan 17, 2008 8:30 PM,  <Ole.Nielsen at ga.gov.au> wrote:
>> I found this posting, and those following it, as I too am baffled that
>> NamedTemporaryFile doesn't let you read the generated file - even within
> the
>> same script.
>> For unit testing it is very commonplace to generate a test file on the fly
>> and then use it as input the function being tested. NamedTemporaryFile
> would
>> be the perfect candidate for this except that the file seems to disappear
>> immediately after it has been closed. This is contrary to Dustin's comment
>> which states that the lifetime extends until the object is garbage
> collected.
>> The following script illustrates this isn't the case:
>>
>> from tempfile import NamedTemporaryFile
>> fid = NamedTemporaryFile(mode='w',
>>                          suffix='.tmp',
>>                          dir='.')
>>
>> fid.write('My temp file')
>> fid.close()
>>
>> # Read it (fid hasn't been garbage collected yet)
>> print fid.name
>>
>> # But the file itself has gone
>> fid2 = open(fid.name)
>> print fid2.readlines()
>>
>>
>> Can someone please help point me to the best way of creating temporary
> files
>> in Python that can be read in immediately after their creation.
>>
>> Thanks
>> Ole Nielsen, Geoscience Australia
>>
>>
>>
>> On Sun, Mar 18, 2007 at 11:49:55AM +1200, Greg Ewing wrote:
>>> I've just discovered the hard way that NamedTemporaryFile
>>> automatically deletes the file when you close it. That
>>> doesn't seem very useful to me, since surely the reason
>>> you're using NamedTemporaryFile instead of TemporaryFile
>>> is that you want to do something else with it afterwards?
>>> What's the rationale for this behaviour?
>> For both TemporaryFile and NamedTemporaryFile, the rationale is that
>> "temporary" extends until the object is garbage collected.
>> TemporaryFile is deleted immediately (to prevent other applications from
>> modifying your temporary file).  NamedTemporaryFile is inteded for use
>> when you need access to the file by filename during the lifetime of the
>> file.  In either case, when the object goes, the file should too.
>>
>> Dustin
>>
>>
>>
>>
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> 
> 


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