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Mastering Object-oriented Python by Steven Lott

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The __format__() method

The __format__() method is used by string.format() as well as the format() built-in function. Both of these interfaces are used to get presentable string versions of a given object.

The following are the two ways in which arguments will be presented to __format__():

  • someobject.__format__(""): This happens when the application does format(someobject) or something equivalent to "{0}".format(someobject). In these cases, a zero-length string specification was provided. This should produce a default format.
  • someobject.__format__(specification): This happens when the application does format(someobject, specification) or something equivalent to "{0:specification}".format(someobject).

Note that something equivalent to "{0!r}".format() ...

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