[Python-Dev] PEP 461 Final?

Steven D'Aprano steve at pearwood.info
Sat Jan 18 02:27:53 CET 2014


On Fri, Jan 17, 2014 at 08:49:21AM -0800, Ethan Furman wrote:

> Overriding Principles
> =====================
> 
> In order to avoid the problems of auto-conversion and Unicode exceptions 
> that
> could plague Py2 code, all object checking will be done by duck-typing, not 
> by
> values contained in a Unicode representation [3]_.

I don't understand this paragraph. What does "values contained in a 
Unicode representation" mean?


[...]
> %s is restricted in what it will accept::
> 
>   - input type supports Py_buffer?
>     use it to collect the necessary bytes

Can you give some examples of what types support Py_buffer? Presumably 
bytes. Anything else?



>   - input type is something else?
>     use its __bytes__ method; if there isn't one, raise a TypeError

I think you should explicitly state that this is a new special method, 
and state which built-in types will grow a __bytes__ method (if any).


> Numeric Format Codes
> --------------------
> 
> To properly handle int and float subclasses, int(), index(), and float()
> will be called on the objects intended for (d, i, u), (b, o, x, X), and
> (e, E, f, F, g, G).


-1 on this idea.

This is a rather large violation of the principle of least surprise, and 
radically different from the behaviour of Python 3 str. In Python 3, 
'%d' interpolation calls the __str__ method, so if you subclass, you can 
get the behaviour you want:

py> class HexInt(int):
...     def __str__(self):
...             return hex(self)
...
py> "%d" % HexInt(23)
'0x17'


which is exactly what we should expect from a subclass.

You're suggesting that bytes should ignore any custom display 
implemented by subclasses, and implicitly coerce them to the superclass 
int. What is the justification for this? You don't define or even 
describe what you consider "properly handle".



> Unsupported codes
> -----------------
> 
> %r (which calls __repr__), and %a (which calls ascii() on __repr__) are not
> supported.

+1 on not supporting b'%r' (i.e. I agree with the PEP).

Why not support b'%a'? That seems to be a strange thing to prohibit.



Everythng else, well done and thank you.


-- 
Steven


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