On Wed, Aug 13, 2014 at 1:55 PM, Steven D'Aprano < steve+comp.lang.pyt...@pearwood.info> wrote:
> Ethan Furman wrote: > > > On 08/13/2014 09:00 AM, Steven D'Aprano wrote: > >> > >> 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. > > # Python 2.7 > py> "Hello" + u"World" > u'HelloWorld' > py> unicode.__radd__ > Traceback (most recent call last): > 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__. 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): >>> '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 All the best, Jason
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