Antti Haapala added the comment:
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.
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<http://bugs.python.org/issue27078>
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