Re: [Python-ideas] %-formatting with Decimals

12 Mar 2014


      On 12 March 2014 00:45, Andrew Barnert  wrote:
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On Mar 11, 2014, at 15:49, Oscar Benjamin  wrote:
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On 11 March 2014 21:58, Ethan Furman  wrote:
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On 03/11/2014 02:51 PM, random832@fastmail.us wrote:
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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:
...
...
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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-formatt...
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