[Python-ideas] %-formatting with Decimals

[Oscar Benjamin]

On 12 March 2014 00:45, Andrew Barnert <abarnert at yahoo.com> wrote:
>
> On Mar 11, 2014, at 15:49, Oscar Benjamin <oscar.j.benjamin at gmail.com> wrote:
>
>> On 11 March 2014 21:58, Ethan Furman <ethan at stoneleaf.us> wrote:
>>> On 03/11/2014 02:51 PM, random832 at fastmail.us wrote:
>>>>
>>>> The bug to which I was referring involves not calling __format__ on
>>>> whatever object is passed in as str.format does - Decimal not being
>>>> float is no impediment to str.format and Decimal.__format__ accepting
>>>> the same format strings that float.__format__ does.
>>>
>>> That is not a bug.  str.__mod__ is not documented as calling an object's
>>> __format__ method, and indeed it does not.
>>
>> Instead it calls float on the object:
>>
>>>>> class A:
>> ...   def __float__(self):
>> ...     print('__float__ called')
>> ...
>>>>> '%f' % A()
>> __float__ called
>> Traceback (most recent call last):
>>  File "<stdin>", line 1, in <module>
>> TypeError: nb_float should return float object
>>
>> There's no mention of that anywhere in the docs though:
>> http://docs.python.org/3.4/library/stdtypes.html#printf-style-string-formatting
>
> I wouldn't expect it to mention that it calls the __float__ method (much less the nb_float C slot), because it doesn't.
>
> It converts to float in the normal way, by calling the float constructor or it's C equivalent. And the docs for float say pretty clearly that "For a general Python object" (that is, not a string or a float), "float(x) delegates to x.__float__()".
>
> Maybe section 4.7.2 could be more explicit about the fact that the conversion type specifies a conversion to the appropriate type (int, float, or str)?

Yes that's what I meant. The exact words used there to describe the
'f' format code are 'Floating point decimal format'. There's no
mention that it will first need to coerce to a binary float before
converting that to decimal. (Perhaps those words predate the decimal
module).

>> There's also no mention of this in the decimal docs.
>
> Why should there be? Neither printf-style string formatting nor the float constructor have anything to do with decimals.

Because it's a relevant issue to consider when working with Decimals.
In simple cases it will work but in other cases it won't.


Oscar