[Pyrex] extension type attributes

[Mark Ellis]

On Fri, 2009-05-29 at 19:29 +0200, Lenard Lindstrom wrote:
> horace wrote:
> > hi,
> >
> > i have a question about extension type attributes.
> >
> > cdef class MyComplex:
> >     """A mutable complex number type (incomplete)"""
> >     cdef complex *c
> >
> > do all attributes have to be cdef? if i want to have a python object 
> > as an attribute i have to use
> >
> >     cdef object mypythonobject
> >
> > and then initialize it in __init__()? or did i misunderstand 
> > something? i didn't find the manual very clear about this.
> >
> > (i use cython in case this makes any difference.)
> >
> Hi,
> 
> The familiar object attributes of Python class instances - class 
> Something - are stored in a per object dictionary keyed by attribute 
> name. To see this dictionary just do obj.__dict__ for some object 
> created from a Python class. Extension types, at least Pyrex/Cython 
> extension types, don't have that dictionary. And I don't see any way 
> directly enable it. Extension type "attributes" are really just C 
> structure fields, so not directly accessible from Python. There are the 
> public and readonly qualifiers that automatically create properties for 
> a cdef attribute though. So is you what an object attribute accessible 
> form Python do something like this:
> 
> cdef class MyClass:
>     cdef public object x
> 
> Attribute x can now be read and altered from Python. Initialize it 
> either from __cinit__ or __init__. It will default to None.
> 
> If you don't know what the valid attributes will be before hand, then 
> the easiest way to add object attributes is to subclass the extension 
> type with a normal class:
> 
> cdef class _MyClass:
>     .....
> 
> class MyClass(_MyClass):
>     ....
> 
> Now the MyClass class has a per object dictionary. Use __init__ to set 
> initial values. Make sure the class __init__ function and extension type 
> __cinit__ have compatible arguments or one of them will complain.
> 
> 

Or you could implement __setattr__ and __getattr__ methods for your
class, in which case attributes can be whatever you want. These methods
could then default to accessing a dict object stored as part of the C
class struct, to act as 'normal' python object attributes.

Mark

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