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The fileinput module

This module allows you to loop over the contents of one or more text files.

Example: Using the fileinput module to loop over a text file
# File: fileinput-example-1.py

import fileinput
import sys

for line in fileinput.input("samples/sample.txt"):
    sys.stdout.write("-> ")
    sys.stdout.write(line)

-> We will perhaps eventually be writing only small
-> modules which are identified by name as they are
-> used to build larger ones, so that devices like
-> indentation, rather than delimiters, might become
-> feasible for expressing local structure in the
-> source language.
->      -- Donald E. Knuth, December 1974

You can pass in a single filename, or a list of filenames. If you leave out the filename argument, fileinput gets the names from sys.argv.

The module also allows you to get metainformation about the current line. This includes isfirstline, filename, and lineno:

Example: Using the fileinput module to process multiple files
# File: fileinput-example-2.py

import fileinput
import glob
import string, sys

for line in fileinput.input(glob.glob("samples/*.txt")):
    if fileinput.isfirstline(): # first in a file?
        sys.stderr.write("-- reading %s --\n" % fileinput.filename())
    sys.stdout.write(str(fileinput.lineno()) + " " + string.upper(line))

-- reading samples\sample.txt --
1 WE WILL PERHAPS EVENTUALLY BE WRITING ONLY SMALL
2 MODULES WHICH ARE IDENTIFIED BY NAME AS THEY ARE
3 USED TO BUILD LARGER ONES, SO THAT DEVICES LIKE
4 INDENTATION, RATHER THAN DELIMITERS, MIGHT BECOME
5 FEASIBLE FOR EXPRESSING LOCAL STRUCTURE IN THE
6 SOURCE LANGUAGE.
7    -- DONALD E. KNUTH, DECEMBER 1974

Processing text files in place is also easy. Just call the input function with the inplace keyword argument set to 1, and the module takes care of the rest.

Example: Using the fileinput module to convert CRLF to LF
# File: fileinput-example-3.py

import fileinput, sys

for line in fileinput.input(inplace=1):
    # convert Windows/DOS text files to Unix files
    if line[-2:] == "\r\n":
        line = line[:-2] + "\n"
    sys.stdout.write(line)