On Sun, Jun 14, 2009 at 3:42 PM, MRAB<pyt...@mrabarnett.plus.com> wrote: > Seo Sanghyeon wrote: >> >> Exception for setting attributes of built-in type differs between >> CPython and IronPython. This is not purely theoretical, as >> zope.interface tries to set Implements declaration as __implemented__ >> attribute of built-in type object, and excepts TypeError. >> >> Python 2.6.1 >>>>> >>>>> object.flag = True >> >> TypeError: can't set attributes of built-in/extension type 'object' >> >> IronPython 2.6 >>>>> >>>>> object.flag = True >> >> AttributeError: 'object' object has no attribute 'flag' >> >> I was surprised that CPython raises TypeError. Library Reference seems >> to mention it here: >> >> exception AttributeError >> Raised when an attribute reference or assignment fails. (When an >> object does not support attribute references or attribute assignments >> at all, TypeError is raised.) >> http://docs.python.org/library/exceptions.html >> >> What does it mean that "an object does not support attribute >> references or attribute assignments at all"? >> > Here's my guess: > > Some objects have a fixed (and possibly empty) set of attributes. > Attempting to add a new attribute or get a non-existent attribute raises > an AttributeError. > > Other objects, such as types (are there any that aren't types?), don't > have attributes at all. Attempting to add or get an attribute raises a > TypeError.
This particular error comes (grep tells me :-) from type_setattro() in Objects/typeobject.c. It makes a specific check whether the type object whose attribute you want to set is a "built-in type" (this is done by checking the HEAPTYPE flag, which is never set for built-in types and always for user-defined types). For built-in types it disallows setting attributes. This is a pure policy issue: it prevents different 3rd party modules to make incompatible modifications of built-in types. In general, CPython isn't always consistent in raising AttributeError and TypeError when it comes to such policy issues: there are various places that raise TypeError in typeobject.c (and probably elsewhere) that simply forbid setting a specific attribute (another example is __name__). Given how poorly Python exceptions are specified in general, I don't want to hold IronPython to this standard (nor CPython :-). The reason this breaks zope.interfaces in IronPython is probably just that zope.interfaces was primarily written/tested against CPython. Going back to the phrase quoted from the reference manual, it's probably referring to the possibility that a type's tp_getattro slot is NULL; but even so it's wrong: PyObject_GetAttr() raises AttributeError in this case. Uselessly y'rs, -- --Guido van Rossum (home page: http://www.python.org/~guido/) _______________________________________________ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
