[Patches] [ python-Patches-968063 ] Add fileinput.islastline()

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Wed Mar 14 02:25:05 CET 2007


Patches item #968063, was opened at 2004-06-07 11:45
Message generated for change (Comment added) made by sonderblade
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Category: Library (Lib)
Group: Python 2.3
Status: Open
Resolution: None
Priority: 5
Private: No
Submitted By: Relm Arrowny (relm)
Assigned to: Nobody/Anonymous (nobody)
Summary: Add fileinput.islastline()

Initial Comment:
You can test for the first line of a file with 
fileinput.isfirstline(), but there is no corresponding 
fileinput.islastline() to test for the last line of a file.

Note that there is already an unapplied patch for this at 
request ID 776100, but it is faulty in that it only tests 
for end of buffer and not end of file.

I was not sure how to append this patch to the existing 
request, so have started a new one.  Hope this is okay.

This patch also includes documentation and test 
updates.

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Comment By: Björn Lindqvist (sonderblade)
Date: 2007-03-14 02:25

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I have applied this patch and it works as specified. Although it does not
apply cleanly and it contains an error; self._lastline = None should
probably be in the __init__() method. Please reconsider this patch and see
Relms attached wc program. It is definitely sensible and useful for
fileinput to have a islastline method. In fact, every program that uses
fileinput and wants to make a summary for each read file. 

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Comment By: Ilya Sandler (isandler)
Date: 2004-08-05 20:17

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An alternative to islastline() would be to add a single-line
look-ahead capability:

nextline()  which would return a structure containing
  next line, and its file name
(or have 2 functions
nextline()
and nextlinefile()
)

This would cover most of the OP's needs but would also
provide extra useful functionality....E.g. when processing
files which have a 

header line
[data line]*
structure

its very convenient to know in advance when section ends
while processing the last line of section not when
enountering the header line of the next section...

Another use case would be processing of files with line groups:

group1line1
group1line2
group1line3
group2line1
group2line2

(grouping could be for example on  the value of a certian
field in the line)

again it would be very useful to know when a group ends
while still processing this group


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Comment By: Dave Harrison (dharriso)
Date: 2004-07-04 13:24

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I would say that I'm -0.5 on this feature.  After looking
over the current code for fileinput.py in the cvs tree, it
looks like the current code implements an iterator model. 
Since fileinput.py uses the next() call to move along its
input, and raises StopIteration if there are no more lines
to be returned, this function would have to be polled to
find out if the final line had been reached - where a
try/except block that caught StopIteration would seem more
intuitive to me (Inkeeping with the current code).

However it might be considered that the nextfile() function
be changed to raise StopIteration after the last file had
been completed to maintain consistency between the lines and
files aspects of the class.

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Comment By: Relm Arrowny (relm)
Date: 2004-06-15 20:42

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The OP's use case was too simplified to convey
the usefulness of fileinput.islastline().  It does not
just save a couple of lines.  I am attaching a "wc" or
"word count" program to demonstrate.  Note the following
problems with the current work-around:

1. Instead of doing end-of-file processing at the end of
   a file, we must do it at the beginning of the *next*
   file.

2. Useful functions such as fileinput.filelineno() and
   fileinput.filename() now refer to the *wrong* file,
   so now we need to manually keep track of these values
   ourselves.

3. Perhaps a minor point, but this work-around only works
   at the *beginning* of the loop instead of the end.  It
   feels somewhat awkward.

4. End-of-file processing must occur both in the loop and
   once outside.  A function avoids code duplication, but
   to refer to variables in the loop, they must be passed
   into and out of the function (6 in, 2 out for the "wc"
   example... maybe keyword arguments would have helped).

5. Most important is readablility.  It's not obvious that
   `fileinput.isfirstline() and fileinput.lineno() != 1`
   means we are the end of the (previous!) file.  Using
   `fileinput.islastline()` would be much clearer.

And not that it is a reason, but Perl and Ruby solve
this with an `eof` function and `eof?` method,
respectively.


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Comment By: Raymond Hettinger (rhettinger)
Date: 2004-06-15 14:05

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I'm -0 on complicating this venerable interface.   Also, I
find it unnatural for the inside of a loop to be able to
ascertain whether it is in its final iteration.  The closest
approximation is the for-else clause which is vary rarely
used in normal python programming.  While the OP was able to
sketch a use case, it involved only saving a couple of lines
over what can be done with the existing API.


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