o_18ufhmfmq19t513t3lgmn5l1qa8a.pdf

248 CHAPTER 10 ■ BATTERIES INCLUDED
# We'll collect variables in this:
scope = {}
# This is used in re.sub:
def replacement(match):
code = match.group(1)
try:
# If the field can be evaluated, return it:
return str(eval(code, scope))
except SyntaxError:
# Otherwise, execute the assignment in the same scope...
exec code in scope
# ...and return an empty string:
return ''
# Get all the text as a single string:
# (There are other ways of doing this; see Chapter 11)
lines = []
for line in fileinput.input():
lines.append(line)
text = ''.join(lines)
# Substitute all the occurrences of the field pattern:
print field_pat.sub(replacement, text)
Simply put, this program does the following:
1. Defines a pattern for matching fields
2. Creates a dictionary to act as a scope for the template
3. Defines a replacement function that does the following:
a. Grabs group 1 from the match and puts it in code.
b. Tries to evaluate code with the scope dictionary as namespace, converts the result to a string, and
returns it. If this succeeds, the field was an expression and everything is fine. Otherwise (i.e., a
SyntaxError is raised), go to Step 3c.
c. Executes the field in the same namespace (the scope dictionary) used for evaluating expressions,
and then returns an empty string (because the assignment doesn’t evaluate to anything).
4. Uses fileinput to read in all available lines, puts them in a list, and joins them into one big string
5. Replaces all occurrences of field_pat using the replacement function in re.sub, and prints the result