Delete first line from file

[Tor Erik Sønvisen]

Hi

How can I read the first line of a file and then delete this line, so that
line 2 is line 1 on next read?

regards

[Peter Nuttall]

Re: Delete first line from file

On Tue, Mar 01, 2005 at 01:27:27PM +0100, Tor Erik S?nvisen wrote:[color=blue]
> Hi
>
> How can I read the first line of a file and then delete this line, so that
> line 2 is line 1 on next read?
>
> regards
>
>[/color]

I think you can do something like:

n=false
f=file.open("") #stuff here
g=[]
for line in f.readlines():
if n: g.append(line)
n=true

#write g to file

if you are on a unix box, then using the standard untils might be a
better idea.

Pete

[Pieter Claerhout]

Re: Delete first line from file

what about the following?

f = open( 'file.txt', 'r' )
lines = f.readlines()
f.close()

f = open( 'file.txt'.'w' )
f.write( '\n'.join( lines[1:] ) )
f.close()

cheers,


pieter

On Tue, 1 Mar 2005 12:42:00 +0000, Peter Nuttall
<p.s.nuttall@du rham.ac.uk> wrote:[color=blue]
> On Tue, Mar 01, 2005 at 01:27:27PM +0100, Tor Erik S?nvisen wrote:[color=green]
> > Hi
> >
> > How can I read the first line of a file and then delete this line, so that
> > line 2 is line 1 on next read?
> >
> > regards
> >
> >[/color]
>
> I think you can do something like:
>
> n=false
> f=file.open("") #stuff here
> g=[]
> for line in f.readlines():
> if n: g.append(line)
> n=true
>
> #write g to file
>
> if you are on a unix box, then using the standard untils might be a
> better idea.
>
> Pete
>
> --
> http://mail.python.org/mailman/listinfo/python-list
>[/color]


--
pieter claerhout . pieter@yellowdu ck.be . http://www.yellowduck.be/

[Jeff Sandys]

Re: Delete first line from file

You describe the standard behavior, unless you close the file,
is that what you want to do; open file, read line 1, close file,
then open file, read line 2, close file? The other suggestions
here destory content, do you want that?
[color=blue][color=green][color=darkred]
>>> f = open("D:/Pydev/test.txt")
>>> f.readline()[/color][/color][/color]
'line one\n'[color=blue][color=green][color=darkred]
>>> f.readline()[/color][/color][/color]
'line two\n'[color=blue][color=green][color=darkred]
>>> f.readline()[/color][/color][/color]
'line three\n'


"Tor Erik Sønvisen" wrote:[color=blue]
>
> Hi
>
> How can I read the first line of a file and then delete this line, so that
> line 2 is line 1 on next read?
>
> regards[/color]

[Jeff Sandys]

Re: Delete first line from file

Here is a non-destructive way keeping track of the
current file position when closing the file and
re-open the file where you left off. You can
always use seek(0) to go back to the beginning.

# ---------------------- start of myfile class
class myfile(file):
myfiles = {}
def __init__(self, fname, *args):
file.__init__(s elf, fname, *args)
if self.name in myfile.myfiles:
pos = myfile.myfiles[fname]
else:
pos = 0
return self.seek(pos)
def close(self):
myfile.myfiles[self.name] = self.tell()
file.close(self )
# ------------------------ end of myfile class

Below is an example with a simple four line file.

PythonWin 2.3.3 (#51, Dec 18 2003, 20:22:39) [MSC v.1200 32 bit (Intel)]
on win32.
Portions Copyright 1994-2001 Mark Hammond (mhammond@skipp inet.com.au) -
see 'Help/About PythonWin' for further copyright information.[color=blue][color=green][color=darkred]
>>> f = open("C:/Pydev/test.txt")
>>> f.readlines()[/color][/color][/color]
['line one\n', 'line two\n', 'line three\n', 'last line\n'][color=blue][color=green][color=darkred]
>>> ### short four line file
>>> f.close()
>>> from myfile import myfile
>>> f = myfile("C:/Pydev/test.txt")
>>> f.readline()[/color][/color][/color]
'line one\n'[color=blue][color=green][color=darkred]
>>> f.readline()[/color][/color][/color]
'line two\n'[color=blue][color=green][color=darkred]
>>> f.close()
>>> ### test, is the file really closed?
>>> f.readline()[/color][/color][/color]
Traceback (most recent call last):
File "<interacti ve input>", line 1, in ?
ValueError: I/O operation on closed file[color=blue][color=green][color=darkred]
>>> f = myfile("C:/Pydev/test.txt")
>>> f.readline()[/color][/color][/color]
'line three\n'[color=blue][color=green][color=darkred]
>>> ### reopened file starts where it left off
>>> f.readline()[/color][/color][/color]
'last line\n'[color=blue][color=green][color=darkred]
>>> f.close()
>>> f = myfile("C:/Pydev/test.txt")
>>> f.seek(0)
>>> ### return to the beginning of the file
>>> f.readline()[/color][/color][/color]
'line one\n'[color=blue][color=green][color=darkred]
>>>[/color][/color][/color]

This turned out really cool,
thanks for the good question,
Jeff Sandys


"Tor Erik Sønvisen" wrote:[color=blue]
>
> Hi
>
> How can I read the first line of a file and then delete this line, so that
> line 2 is line 1 on next read?
>
> regards[/color]