FWIW, `.format` is more correct going forward. From the python docs for `.format` (https://docs.python.org/2.6/library/stdtypes.html#str.format):
> This method of string formatting is the new standard in Python 3.0, and should be preferred to the % formatting described in String Formatting Operations in new code.
Because it is the new Right Way, it is likely to get the most attention for making sure it is efficient.

We should get in the habit of using `.format` instead of the `%` operator.