Reading and writing to a file creates null characters

[MRAB]

On 12/01/2012 22:26, Denhua wrote:
> Hi,
>
> I've got a file which I'd like to read, modify and write.
>
> # file contents
> a
> b
> c
> d
>
> My script reads the file contents into a list and rotates the list and
> writes it back to the same file.
> Problem is that the output contains null characters. I don't know
> where they are coming from.
>
> #!/usr/bin/env python
>
> def rotate(l):
>          return l[1:] + [l[0]]
>
>
> f = open("/tmp/.rrd", 'r+')
>
> lines = [ line.strip() for line in f.readlines() ]
>
>
> newlist = rotate(lines)
>
> print newlist
> f.truncate(0)
>
> f.write("\n".join(newlist))
> f.close()
>
> # output
>
> [root at Inferno html]# python rotate.py
> ['b', 'c', 'd', 'a']
> [root at Inferno html]# python rotate.py
> ['c', 'd', 'a', '\x00\x00\x00\x00\x00\x00\x00\x00b']
> [root at Inferno html]#
>
>
> What's going on? Thanks for your help,
 >
I think this is the relevant part of the documentation:

"""The current file position is not changed. Note that if a specified 
size exceeds the file’s current size, the result is platform-dependent: 
possibilities include that the file may remain unchanged, increase to 
the specified size as if zero-filled, or increase to the specified size 
with undefined new content.
"""

In other words, you also need to reset the file pointer to the start of
the file.