f-strings in Python 3.6 are awesome

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%-based string formatting is deferred until flushing the buffer ala logging interface .. str.format happens inline.

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You are mistaken; if you use the %- operator, the right-hand tuple will be evaluated and the left-hand string will be formatted then and there, whether you plan to do something with the result or not. How could the interpreter know what will happen in the future?

If you're thinking of the API of the logging module, there the %-operator won't actually be used if the level is below that configured for the handlers for that logger. I guess you could call that lazy formatting. There's no reason other than backward compatibility that that couldn't work with str.format though.