Classes that doesn't define the __format__ method for custom PEP 3101
formatting inherits it from parents.
Originally the object.__format__ method was designed as [1]:
def __format__(self, format_spec):
return format(str(self), format_spec)
An instance is converted to string and resulting string is formatted
according to format specifier.
Later this design was reconsidered [2], and now object.__format__ is
equivalent to:
def __format__(self, format_spec):
assert format_spec == ''
return format(str(self), '')
Non-empty format specifier is rejected.
But why call format() on resulting string? Why not return resulting
string as is? object.__format__ could be simpler (not just
implementation, but for understanding):
def __format__(self, format_spec):
assert format_spec == ''
return str(self)
This can change the behaviour in corner case. str(self) can return not
exact string, but string subclass with overloaded __format__. But I
think we can ignore such subtle difference.
[1] https://www.python.org/dev/peps/pep-3101/
[2] http://bugs.python.org/issue7994