The reason I prefer named formatting is it's easier to refactor and prepares us for Python 3 (queue laughter). You could do this:
````
'Preparing webpay JWT for inapp {a}: {j}'.format(a=inapp, j=jwt_)
````
then refactor it easily to:

````
'Preparing {f} webpay JWT for inapp {a}: {j}'.format(a=inapp, j=jwt_, f=foo)
````
It's more readable and less fragile than substitution.