When is {!s} meaningful in format()?

Question

>>> import math
>>> print "{} {!s} {!r}".format(math.pi, math.pi, math.pi)
3.14159265359 3.14159265359 3.141592653589793

When is the !s inside {} meaningful?

Or in other words, when is calling str() on a format() argument meaningful?

Answer 1

Whenever the type to be printed implements __format__ in an an unsuitable way for your purpose. In this case, you can use !s or !r (and I think there is even !a) to specify an alternate formatter.

You certainly won't need it for printing floating-point numbers, but probably for custom types.