On 01/16/2014 11:47 PM, Steven D'Aprano wrote:
On Thu, Jan 16, 2014 at 08:23:13AM -0800, Ethan Furman wrote:
As I understand it, str.format will call the object's __format__. So, for
example, if I say:
u'the value is: %d' % myNum(17)
then it will be myNum.__format__ that gets called, not int.__format__;
I seem to have missed something, because I am completely confused... Why
are you talking about str.format and then show an example using % instead?
Sorry, PEP 46x fatigue. :/
It should have been
u'the value is {:d}'.format(myNum(17))
and yes I meant the str type.
%d calls __str__, not __format__. This is in Python 3.3:
py> class MyNum(int):
... def __str__(self):
... print("Calling MyNum.__str__")
... return super().__str__()
... def __format__(self):
... print("Calling MyNum.__format__")
... return super().__format__()
...
py> n = MyNum(17)
py> u"%d" % n
Calling MyNum.__str__
'17'
And that's a bug we fixed in 3.4:
Python 3.4.0b1 (default:172a6bfdd91b+, Jan 5 2014, 06:39:32)
[GCC 4.7.3] on linux
Type "help", "copyright", "credits" or "license" for more information.
--> class myNum(int):
... def __int__(self):
... return 7
... def __index__(self):
... return 11
... def __float__(self):
... return 13.81727
... def __str__(self):
... print('__str__')
... return '1'
... def __repr__(self):
... print('__repr__')
... return '2'
...
--> '%d' % myNum()
'0'
--> '%f' % myNum()
'13.817270'
After all, consider:
'%d' % True
'1'
'%s' % True
'True'
So, in fact, on subclasses __str__ should *not* be called to get the integer representation. First we do a conversion
to make sure we have an int (or float, or ...), and then we call __str__ on our tried and trusted genuine core type.
The *worst* solution would be to completely ignore MyNum.__str__.
That's a nasty violation of the Principle Of Least Surprise, and will
lead to confusion ("why isn't my class' __str__ method being called?")
Because you asked for a numeric representation, not a string representation [1].
--
~Ethan~
[1] for all the gory details, see:
http://bugs.python.org/issue18780
http://bugs.python.org/issue18738
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