Beginning Python - From Novice to Professional

CHAPTER 10 ■ BATTERIES INCLUDED 247
Making a template system. A template is a file you can put specific values into to get a finished text of some kind.
For example, you may have a mail template requiring only the insertion of a recipient name. Python already has an
advanced template mechanism: string formatting. However, with regular expressions you can make the system
even more advanced. Let’s say you want to replace all occurrences of '[something]' (the “fields”) with the result
of evaluating something as an expression in Python. Thus, the string
'The sum of 7 and 9 is [7 + 9].'
should be translated to
'The sum of 7 and 9 is 16.'
Also, you want to be able to perform assignments in these fields, so that the string
'[name="Mr. Gumby"]Hello, [name]'
should be translated to
'Hello, Mr. Gumby'
This may sound like a complex task, but let’s review the available tools:
• You can use a regular expression to match the fields and extract their contents.
• You can evaluate the expression strings with eval, supplying the dictionary containing the scope. You
do this in a try/except statement; if a SyntaxError is raised, you probably have a statement (such
as an assignment) on your hands and should use exec instead.
• You can execute the assignment strings (and other statements) with exec, storing the template’s scope
in a dictionary.
• You can use re.sub to substitute the result of the evaluation into the string being processed.
Suddenly it doesn’t look so intimidating, does it?
■Tip If a task seems daunting, it almost always helps to break it down into smaller pieces. Also, take stock
of the tools at your disposal for ideas on how to solve your problem.
See Listing 10-11 for a sample implementation.
Listing 10-11. A Template System
# templates.py
import fileinput, re
# Matches fields enclosed in square brackets:
field_pat = re.compile(r'\[(.+?)\]')