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

>>>开发者_StackOverflow社区; 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?

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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.