string.format() with optional placeholders

Here is one option:

from collections import defaultdict

my_csv = '{d[first]},{d[middle]},{d[last]}'
print( my_csv.format( d=defaultdict(str, first='John', last='Doe') ) )

"It does{cond} contain the the thing.".format(cond="" if condition else " not")

Thought I'd add this because it's been a feature since the question was asked, the question still pops up early in google results, and this method is built directly into the python syntax (no imports or custom classes required). It's a simple shortcut conditional statement. They're intuitive to read (when kept simple) and it's often helpful that they short-circuit.