Linux Format forums

Re: Opening and processing files in Python

nelz — Tue Oct 04, 2005 2:01 pm

Author: nelz

Posted: Tue Oct 04, 2005 2:01 pm

Because the code doesn't look like line noise

Re: Opening and processing files in Python

PhilC — Tue Oct 04, 2005 1:51 pm

Author: PhilC

Posted: Tue Oct 04, 2005 1:51 pm

Thanks overflow,
That was a very insightful answer and a great example to demonstrate.
(Still wondering why people prefer to use this to perl though)

Re: Opening and processing files in Python

overflow — Wed Aug 24, 2005 1:00 pm

Author: overflow

Posted: Wed Aug 24, 2005 1:00 pm

Python doesn't have the equivalent of the Perl <> operator but it's easy enough to reproduce it. Here's a simple cat program written in Python which supports multiple filenames on the command line and '-' for stdin.

Code:

#! /usr/bin/python
import sys
def fileargs():
for filename in sys.argv[1:]:
   if filename == '-':
      for line in sys.stdin:
         yield line
   else:
      for line in file(filename):
         yield line

for line in fileargs():
print line

The fileargs function is an iterator (or generator, I can't remember the difference) and passes back records from each input file in turn. It switches to stdin if '-' is specified.

I hope that helps.

Re: Opening and processing files in Python

overflow — Wed Aug 24, 2005 12:40 pm

Author: overflow

Posted: Wed Aug 24, 2005 12:40 pm

It is even easier than the example posted above.

Code:

for line in file('foo.bar'):
print line

file (which is a synonym for open) support iterators and it yields a class which simply passes records back to you one at a time.

The example posted above would read the whole file into memory in the readlines() function which might be a performance problem for large files.

RE: Opening and processing files in Python

PhilC — Thu Aug 18, 2005 1:34 pm

Author: PhilC

Posted: Thu Aug 18, 2005 1:34 pm

Thanks for getting me started nelz, I'll be using that a lot in the future.

I did a little bit more digging though since the perl diamond operator can do a little bit more than this code.
i.e. it will read each file given as an argument to the program and process each file in turn. Much more useful than that though is that it can read input from stdin or a pipe.

Just in case anyone else is looking for this feature in Python, I had another look around python.org and found this code: (http://www.python.org/doc/2.2.3/lib/module-fileinput.html)

import fileinput
for line in fileinput.input():
dosomethingwith (line)

RE: Opening and processing files in Python

nelz — Wed Aug 17, 2005 6:35 pm

Author: nelz

Posted: Wed Aug 17, 2005 6:35 pm

Code:

lines = open('foo/bar').readlines()
for line in lines:
dosomethingwith(line)

Opening and processing files in Python

PhilC — Wed Aug 17, 2005 4:16 pm

Author: PhilC

Posted: Wed Aug 17, 2005 4:16 pm

Hi,

I'm trying to make a start in Python programming. I consider myself competant with Perl, and I want to do some of the same things which are easy in Perl, in Python.

In particular, I want to be open a file and process each line at a time.
In Perl I just do:
while (<>) {
doSomethingWith ($_);
}

I've had a browse around python.org, but can't find any good references to opening and processing files.

Can anyone tell me how to do the above Perl code in Python? I really would appreciate it, thanks.