...one good reason to prefer [format()](https://docs.python.org/2/library/stdtypes.html#str.format).
```python
"Counters: initial={0.counter_initial}, halted={0.counter_halted}, "
"errors={0.counter_error}, finished={0.counter_finished}\n"
"Extra.data: {0.extra_data}".format(self)
```