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CHAPTER 10 ■ BATTERIES INCLUDED 219
fileinput
You learn a lot about reading from and writing to files in Chapter 11, but here is a sneak preview.
The fileinput module enables you to easily iterate over all the lines in a series of text files. If
you call your script like this (assuming a UNIX command line):
$ python some_script.py file1.txt file2.txt file3.txt
you will be able to iterate over the lines of file1.txt through file3.txt in turn. You can also
iterate over lines supplied to standard input (sys.stdin, remember?), for example, in a UNIX
pipe (using the standard UNIX command cat):
$ cat file.txt | python some_script.py
If you use fileinput, this way of calling your script (with cat in a UNIX pipe) works just as
well as the previous one (supplying the file names as command-line arguments to your script).
The most important functions of the fileinput module are described in Table 10-4.
Table 10-4. Some Important Functions in the fileinput Module
Function
input([files[, inplace[, backup]])
filename()
lineno()
filelineno()
isfirstline()
isstdin()
nextfile()
close()
Description
Facilitates iteration over lines in multiple input
streams
Returns name of current file
Returns current (cumulative) line number
Returns line number within current file
Checks whether current line is first in file
Checks whether last line was from sys.stdin
Closes current file and moves to the next
Closes the sequence
The function fileinput.input is the most important of the functions. It returns an object
that you can iterate over in a for loop. If you don’t want the default behavior (in which fileinput
finds out which files to iterate over), you can supply one or more file names to this function
(as a sequence). You can also set the inplace parameter to a true value (inplace=True) to enable
in-place processing. For each line you access, you’ll have to print out a replacement, which will
be put back into the current input file. The optional backup argument gives a file name extension to
a backup file created from the original file when you do in-place processing.
The function fileinput.filename returns the file name of the file you are currently in (that
is, the file that contains the line you are currently processing).
The function fileinput.lineno returns the number of the current line. This count is
cumulative so that when you are finished with one file and begin processing the next, the line
number is not reset but starts at one more than the last line number in the previous file.