On 10/25/2016 5:37 AM, Serhiy Storchaka wrote:
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
I don't feel strongly about this, one way or the other. As you say, it
would take an unusual case to see this behavior:
>>> class S(str):
... def __format__(self, fmt):
... return 'aha!'
...
>>> class O:
... def __str__(self):
... return S()
...
>>> format(O(), '')
'aha!'
>>>
But on the other hand, the existing behavior is well specified and has
been around since object.__format__ was added. I'm not sure it needs
changing. What's the harm in leaving it?
Eric.
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