Why does str not have a __radd__ method?
wxjmfauth at gmail.com
wxjmfauth at gmail.com
Thu Aug 14 02:24:36 EDT 2014
Le mercredi 13 août 2014 20:36:55 UTC+2, Jason Swails a écrit :
> On Wed, Aug 13, 2014 at 1:55 PM, Steven D'Aprano <steve+comp.... at pearwood.info> wrote:
>
>
> Ethan Furman wrote:
>
>
>
> > On 08/13/2014 09:00 AM, Steven D'Aprano wrote:
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> >>
>
>
> >> What is the rationale for str not having __radd__ method?
>
> >
>
>
> > At a guess I would say because string only knows how to add itself to
>
> > other strings, so __add__ is sufficient.
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>
>
> # Python 2.7
>
> py> "Hello" + u"World"
>
> u'HelloWorld'
>
> py> unicode.__radd__
>
>
> Traceback (most recent call last):
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> File "<stdin>", line 1, in <module>
>
> AttributeError: type object 'unicode' has no attribute '__radd__'
>
>
>
> This happens because the str.__add__ function calls string_concat under the hood (see Objects/stringobject.c) -- there's a unicode check on the other operand that results in the result of PyUnicode_Concat being returned instead of the concatenated str type. This doesn't require that unicode define __radd__.
>
>
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> When the left-hand operand is Unicode, PyUnicode_Concat is called directly (which is why the exception message is different for u'this' + 1 and 'this' + 1):
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>
>
>
> >>> 'this' + 1
> Traceback (most recent call last):
>
> File "<stdin>", line 1, in <module>
> TypeError: cannot concatenate 'str' and 'int' objects
>
> >>> u'this' + 1
> Traceback (most recent call last):
> File "<stdin>", line 1, in <module>
>
> TypeError: coercing to Unicode: need string or buffer, int found
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>
>
>
> All the best,
> Jason
--------
This is more funny.
>>> print u'a' + 'a'
aa
>>> print 'a' + u'a'
aa
>>> print u'a' + 'é'
Traceback (most recent call last):
File "<pyshell#4>", line 1, in <module>
print u'a' + 'é'
UnicodeDecodeError: 'ascii' codec can't decode byte 0xe9 in position 0: ordinal not in range(128)
>>>
Python? The single language, which tried (and still tries
to make the ascii world compatible with the unicode
world.
In a somwhow symmetrical way, but a little bit
differently, the situation in Python 3 is even worse.
jmf
PS See my comment about the Euro sign in a previous
thread.
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