simultaneous reading and writing a textfile

[Peter Otten]

Mark Wilkinson wrote:

> Why this:
> 
>> lines = iter(infile.readline, "")
>> # copy one line at a time
>> for line in lines:
> 
> Rather than this?
> 
>> for line in infile:

file.next() uses an internal cache. The data in that cache would be lost
when the rest of the file is copied using file.read() with

for block in iter(lambda: infile.read(BLOCKSIZE), ""):
    outfile.write(block)

Demonstration:

>>> open("tmp.txt", "w").write("alpha\nbeta\ngamma\n")
>>> infile = open("tmp.txt")
>>> infile.next()
'alpha\n'
>>> infile.read()
'' # where is 'beta\ngamma\n'?

I admit I didn't profile whether the suggested approach is an overall win
over line-wise copying of the whole file or manually keeping track of the
position in the file followed by a seek().

Peter