Beginning Python - From Novice to Professional

266 CHAPTER 11 ■ FILES AND STUFF
object. Note that xreadlines is somewhat “old-fashioned,” and you should instead use file iterators
(explained next) in your own code.
The New Kids on the Block: File Iterators
It’s time for the coolest technique of all. If Python had had this since the beginning, I suspect
that several of the other methods (at least xreadlines) would never have appeared. So what is
this cool technique? In recent versions of Python (from version 2.2), files are iterable, which
means that you can use them directly in for loops to iterate over their lines. See Listing 11-12
for an example. Pretty elegant, isn’t it?
Listing 11-12. Iterating Over a File
f = open(filename)
for line in f:
process(line)
In these iteration examples, I’ve been pretty casual about closing my files. Although I probably
should have closed them, it’s not critical, as long as I don’t write to the file. If you are willing to
let Python take care of the closing (as I have done so far), you could simplify the example even
further, as shown in Listing 11-13. Here I don’t assign the opened file to a variable (like the
variable f I’ve used in the other examples), and therefore I have no way of explicitly closing it.
Listing 11-13. Iterating Over a File Without Storing the File Object in a Variable
for line in open(filename):
process(line)
Note that sys.stdin is iterable, just like other files, so if you want to iterate over all the lines
in standard input, you can use
import sys
for line in sys.stdin:
process(line)
Also, you can do all the things you can do with iterators in general, such as converting
them into lists of strings (by using list(open(filename))), which would simply be equivalent
to using readlines.
Consider the following example:
>>> f = open('somefile.txt', 'w')
>>> print >> f, 'This is the first line'
>>> print >> f, 'This is the second line'
>>> print >> f, 'This is the third line'
>>> f.close()
>>> first, second, third = open('somefile.txt')
>>> first
'This is the first line\n'
>>> second
'This is the second line\n'