fileinput – Process lines from input streams

Purpose:Create command-line filter programs to process lines from input streams.
Available In:1.5.2 and later

The fileinput module is a framework for creating command line programs for processing text files in a filter-ish manner.

Converting M3U files to RSS

For example, the m3utorss app I recently wrote for my friend Patrick to convert some of his demo recordings into a podcastable format.

The inputs to the program are one or more m3u file listing the mp3 files to be distributed. The output is a single blob of XML that looks like an RSS feed (output is written to stdout, for simplicity). To process the input, I need to iterate over the list of filenames and:

  • Open each file.
  • Read each line of the file.
  • Figure out if the line refers to an mp3 file.
  • If it does, extract the information from the mp3 file needed for the RSS feed.
  • Print the output.

I could have written all of that file handling out by hand. It isn’t that complicated, and with some testing I’m sure I could even get the error handling right. But with the fileinput module, I don’t need to worry about that. I just write something like:

for line in fileinput.input(sys.argv[1:]):
    mp3filename = line.strip()
    if not mp3filename or mp3filename.startswith('#'):
        continue
    item = SubElement(rss, 'item')
    title = SubElement(item, 'title')
    title.text = mp3filename
    encl = SubElement(item, 'enclosure', {'type':'audio/mpeg', 'url':mp3filename})

The fileinput.input() function takes as argument a list of filenames to examine. If the list is empty, the module reads data from standard input. The function returns an iterator which returns individual lines from the text files being processed. So, all I have to do is loop over each line, skipping blanks and comments, to find the references to mp3 files.

Here’s the complete program:

import fileinput
import sys
import time
from xml.etree.ElementTree import Element, SubElement, tostring
from xml.dom import minidom

# Establish the RSS and channel nodes
rss = Element('rss', {'xmlns:dc':"http://purl.org/dc/elements/1.1/",
                      'version':'2.0',
                      })
channel = SubElement(rss, 'channel')
title = SubElement(channel, 'title')
title.text = 'Sample podcast feed'
desc = SubElement(channel, 'description')
desc.text = 'Generated for PyMOTW'
pubdate = SubElement(channel, 'pubDate')
pubdate.text = time.asctime()
gen = SubElement(channel, 'generator')
gen.text = 'http://www.doughellmann.com/PyMOTW/'

for line in fileinput.input(sys.argv[1:]):
    mp3filename = line.strip()
    if not mp3filename or mp3filename.startswith('#'):
        continue
    item = SubElement(rss, 'item')
    title = SubElement(item, 'title')
    title.text = mp3filename
    encl = SubElement(item, 'enclosure', {'type':'audio/mpeg', 'url':mp3filename})
        
rough_string = tostring(rss)
reparsed = minidom.parseString(rough_string)
print reparsed.toprettyxml(indent="  ")

and its output:

$ python fileinput_example.py sample_data.m3u

<?xml version="1.0" ?>
<rss version="2.0" xmlns:dc="http://purl.org/dc/elements/1.1/">
  <channel>
    <title>
      Sample podcast feed
    </title>
    <description>
      Generated for PyMOTW
    </description>
    <pubDate>
      Thu Feb 21 06:35:49 2013
    </pubDate>
    <generator>
      http://www.doughellmann.com/PyMOTW/
    </generator>
  </channel>
  <item>
    <title>
      episode-one.mp3
    </title>
    <enclosure type="audio/mpeg" url="episode-one.mp3"/>
  </item>
  <item>
    <title>
      episode-two.mp3
    </title>
    <enclosure type="audio/mpeg" url="episode-two.mp3"/>
  </item>
</rss>

Progress Meta-data

In the previous example, I did not care what file or line number we are processing in the input. For other tools (grep-like searching, for example) you might. The fileinput module includes functions for accessing that information (filename(), filelineno(), lineno(), etc.).

import fileinput
import re
import sys

pattern = re.compile(sys.argv[1])

for line in fileinput.input(sys.argv[2:]):
    if pattern.search(line):
        if fileinput.isstdin():
            fmt = '{lineno}:{line}'
        else:
            fmt = '{filename:<20}:{lineno}:{line}'
        print fmt.format(filename=fileinput.filename(),
                         lineno=fileinput.filelineno(),
                         line=line.rstrip())

We can use this basic pattern matching loop to find the occurances of “fileinput” in the source for the examples.

$ python fileinput_grep.py fileinput *.py

fileinput_change_subnet.py:10:import fileinput
fileinput_change_subnet.py:17:for line in fileinput.input(files, inplace=True):
fileinput_change_subnet_noisy.py:10:import fileinput
fileinput_change_subnet_noisy.py:18:for line in fileinput.input(files, inplace=True):
fileinput_change_subnet_noisy.py:19:    if fileinput.isfirstline():
fileinput_change_subnet_noisy.py:20:        sys.stderr.write('Started processing %s\n' % fileinput.filename())
fileinput_example.py:6:"""Example for fileinput module.
fileinput_example.py:10:import fileinput
fileinput_example.py:30:for line in fileinput.input(sys.argv[1:]):
fileinput_grep.py   :10:import fileinput
fileinput_grep.py   :16:for line in fileinput.input(sys.argv[2:]):
fileinput_grep.py   :18:        if fileinput.isstdin():
fileinput_grep.py   :22:        print fmt.format(filename=fileinput.filename(),
fileinput_grep.py   :23:                         lineno=fileinput.filelineno(),

We can also pass input to it through stdin.

$ cat *.py | python fileinput_grep.py fileinput

10:import fileinput
17:for line in fileinput.input(files, inplace=True):
29:import fileinput
37:for line in fileinput.input(files, inplace=True):
38:    if fileinput.isfirstline():
39:        sys.stderr.write('Started processing %s\n' % fileinput.filename())
51:"""Example for fileinput module.
55:import fileinput
75:for line in fileinput.input(sys.argv[1:]):
96:import fileinput
102:for line in fileinput.input(sys.argv[2:]):
104:        if fileinput.isstdin():
108:        print fmt.format(filename=fileinput.filename(),
109:                         lineno=fileinput.filelineno(),

In-place Filtering

Another common file processing operation is to modify the contents. For example, a Unix hosts file might need to be updated if a subnet range changes.

##
# Host Database
#
# localhost is used to configure the loopback interface
# when the system is booting.  Do not change this entry.
##
127.0.0.1       localhost
255.255.255.255 broadcasthost
::1             localhost 
fe80::1%lo0     localhost
172.16.177.128  hubert hubert.hellfly.net
172.16.177.132  cubert cubert.hellfly.net
172.16.177.136  zoidberg zoidberg.hellfly.net

The safe way to make the change automatically is to create a new file based on the input and then replace the original with the edited copy. fileinput supports this automatically using the inplace option.

import fileinput
import sys

from_base = sys.argv[1]
to_base = sys.argv[2]
files = sys.argv[3:]

for line in fileinput.input(files, inplace=True):
    line = line.rstrip().replace(from_base, to_base)
    print line
$ python fileinput_change_subnet.py 172.16.177 172.16.178 etc_hosts.txt

Although the script uses print, no output is produced to stdout because fileinput maps stdout to the file being overwritten.

##
# Host Database
#
# localhost is used to configure the loopback interface
# when the system is booting.  Do not change this entry.
##
127.0.0.1       localhost
255.255.255.255 broadcasthost
::1             localhost
fe80::1%lo0     localhost
172.16.178.128  hubert hubert.hellfly.net
172.16.178.132  cubert cubert.hellfly.net
172.16.178.136  zoidberg zoidberg.hellfly.net

Before processing begins, a backup file is created using the original name plus .bak. The backup file is removed when the input is closed.

import fileinput
import glob
import sys

from_base = sys.argv[1]
to_base = sys.argv[2]
files = sys.argv[3:]

for line in fileinput.input(files, inplace=True):
    if fileinput.isfirstline():
        sys.stderr.write('Started processing %s\n' % fileinput.filename())
        sys.stderr.write('Directory contains: %s\n' % glob.glob('etc_hosts.txt*'))
    line = line.rstrip().replace(from_base, to_base)
    print line

sys.stderr.write('Finished processing\n')
sys.stderr.write('Directory contains: %s\n' % glob.glob('etc_hosts.txt*'))
$ python fileinput_change_subnet_noisy.py 172.16.177 172.16.178 etc_host\
s.txt

Started processing etc_hosts.txt
Directory contains: ['etc_hosts.txt', 'etc_hosts.txt.bak']
Finished processing
Directory contains: ['etc_hosts.txt']

See also

fileinput
The standard library documentation for this module.
Patrick Bryant
Atlanta-based singer/song-writer.
m3utorss
Script to convert m3u files listing MP3s to an RSS file suitable for use as a podcast feed.
Creating XML Documents
More details of using ElementTree to produce XML.
File Access
Other modules for working with files.
Text Processing Tools
Other modules for working with text.
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