Greg Ewing wrote:
> Ron Adam wrote:
>> The way to think of 'repr' and 'str' is that of a general "object" format
>> type/specifier. That puts str and repr into the same context as the rest
>> of the format types. This is really a point of view issue and not so much
>> of a semantic one. I think {0:r} and {0:s} are to "object", as {0:d} and
>> {0:e} are to "float" ... just another relationship relative to the value
>> being formatted. So I don't understand the need to treat them differently.
>
> There's no need to treat 's' specially, but 'r' is different,
> at least if we want
>
> "{0:r}".format(x)
>
> to always mean the same thing as
>
> "{0:s}".format(repr(x))
>
> To achieve that without requiring every __format__ method
> to recognise 'r' and handle it itself is going to require
> format() to intercept 'r' before calling the __format__
> method, as far as I can see. It can't be done by str,
> since by the time str.__format__ gets called, the object
> has already been passed through str(), and it's too late
> to call repr() on it.
This doesn't require a different syntax to do.
Lets start at the top... what will the str.format() method look like?
Maybe an approximation might be: (only one possible variation)
class str(object):
...
def format(self, *args, **kwds):
return format(self, *args, **kwds) #calls global function.
...
And then for each format field, it will call the __format__ method of the
matching position or named value.
class object():
...
def __format__(self, value, format_spec):
return value, format_spec
...
It doesn't actually do anything because it's a pre-format hook so that
users can override the default behavior. An overridden __format__ method
can do one of the following...
- handle the format spec on it's own and return a (string, None) [*]
- alter the value and return (new_value, format_spec)
- alter the format_spec and return (value, new_format_spec)
- do logging of some values, and return the (value, format_spec)
unchanged.
- do something entirely different and return ('', None)
[* None could indicate an already formatted value in this case.]
Does this look ok, or would you do it a different way?
If we do it this way, then the 'r' formatter isn't handled any different
than any of the others. The exceptional case is the custom formatters in
__format__ methods not the 'r' case.
Cheers,
Ron
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