Beginning Python - From Novice to Professional

CHAPTER 10 ■ BATTERIES INCLUDED 249
■Note It is much more efficient to put the lines into a list and then join them at the end than to do something
like this:
# Don't do this:
text = ''
for line in fileinput.input():
text += line
Although this looks elegant, each assignment has to create a new string, which is the old string with the new
one appended. This leads to a terrible waste of resources and makes your program slow. Don’t do this. If you
want a more elegant way to read in all the text of a file, take a peek at Chapter 11.
So, I have just created a really powerful template system in only 15 lines of code (not counting whitespace and comments).
I hope you’re starting to see how powerful Python becomes when you use the standard libraries. Let’s finish this
example by testing the template system. Try running it on the simple file shown in Listing 10-12.
Listing 10-12. A Simple Template Example
[x = 2]
[y = 3]
The sum of [x] and [y] is [x + y].
You should see this:
The sum of 2 and 3 is 5.
■Note It may not be obvious, but there are three empty lines in the preceding output—two above and one
below the text. Although the first two fields have been replaced by empty strings, the newlines following them
are still there. Also, the print statement adds a newline, which accounts for the empty line at the end.
But wait, it gets better! Because I have used fileinput, I can process several files in turn. That means that I can
use one file to define values for some variables, and then another file as a template where these values are inserted.
For example, I might have one file with definitions as in Listing 10-13, named magnus.txt, and a template file as
in Listing 10-14, named template.txt.