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Yet the test cases just prove what is so expensive there: name lookups (global name `str`; looking up `join` on a string instance); building a tuple (for function arguments) is expensive as well. Of course `__format__` will be costly as well as it is not a slot-method, needs to build a new string etc. 
However for strings, 'foo'.format() already returns the instance itself, so if you were formatting other strings into strings there are cheap shortcuts available to even overtake 
 a = 'Hello'
 b = 'World'
 '%s %s' % (a, b)
for fast string templates, namely, make FORMAT_VALUE without args return the original if `PyUnicode_CheckExact` and no arguments, don't need to build a tuple to join it.