Reading a file and then writing something back

[Kevin T. Ryan]

Remy Blank wrote:

> Kevin T. Ryan wrote:
>> I'm not sure, but I'm wondering if this is a bug, or maybe (more
>> likely) I'm misunderstanding something...see below:
>> 
>> 
>>>>>f = open('testfile', 'w')
>>>>>f.write('kevin\n')
>>>>>f.write('dan\n')
>>>>>f.write('pat\n')
>>>>>f.close()
>>>>>f = open('testfile', 'r+')
>>>>>f.readline()
>> 
>> 'kevin\n'
>> 
>>>>>f.readline()
>> 
>> 'dan\n'
>> 
>>>>>f.write('chris\n')
>> 
>> Traceback (most recent call last):
>>   File "<stdin>", line 1, in ?
>> IOError: (0, 'Error')
> 
> This is just a guess, I don't know the inner workings of files in
> Python, but here we go:
> 
> I think that readline() doesn't read one character at a time from the
> file, until it finds a newline, but reads a whole block of characters,
> looks for the first newline and returns that string (for efficiency
> reasons). Due to this buffering, the file pointer position is undefined
> after a readline(), and so a write() afterwards doesn't make sense.
> Python tries to help you not to fall into this trap.
> 
>> I've figured out that I can do an open('testfile', 'r+') and then seek
>> and write something (without an error), but it just seems odd that I
>> would get an IOError for what I was trying to do.
> 
> When you do a seek(), the file pointer position is clearly defined, so a
> write() makes sense.
> 
> A tentative solution could be:
> 
> pos = f.tell()
> s = f.readline()         # Reads 'dan\n'
> f.seek(pos + len(s))
> f.write('chris\n')
> 
> However, I'm not sure you want to do that, as the string written will
> just overwrite the previous content, and will probably not be aligned
> with the next newline in the file. Except if you don't care about the
> data following your write.
> 
> Hope this helps.
> -- Remy
> 
> 
> Remove underscore and anti-spam suffix in reply address for a timely
> response.
Thanks all for the suggestions.  I'm guessing that what Remy stated seems to
be about right.  Jeff:  I AM using windows (or at least, was today while i
was writing that script)...if Remy was wrong, then it still might be a bug
though - I tried to do the f.flush(), but the error still occurred.

Byron - thanks for the advice.  For my simple example, you're totally
correct, but I was thinking along the lines of a much bigger file w/ tons
of records - and therefore didn't want to slurp everything in to memory in
case the file got to be too big.  Maybe I'm wrong though - I don't know how
much a "normal" computer could hold in memory (maybe 100's of 1,000's of
lines?).

Oh well, thanks again :)