Allow use of `format` to be Python 2.6 compatible

This pull request adds indices to the interpolated variables in strings.

The string format method in Python 2.6 doesn't like the `{}` style of variable interpolation - it requires an index or name in there.

Running this:
`python -c '"{}".format(1)'`

Gives this error:
```python
Traceback (most recent call last):
  File "<string>", line 1, in <module>
ValueError: zero length field name in format
```