Cython unable to use setattr

Question

I have a cdef class in cython and I want to initialize its fields with the setattr builtin function. However, when I do that I got an execution error :

/path/.../cimul.cpython-34m.so in cimul.Simulation.__init__ (cimul.c:5100)()
AttributeError: 'Simulation' object has no attribute 'Re'

My code is as follow:

cdef class Simulation:
    cdef double Re, Pr, Ra, a, dt_security
    cdef int Nz, NFourier, freq_output, freq_critical_Ra, maxiter
    cdef bool verbose

    def __init__(self, *args, **kargs):
        param_list = {'Re': 1, 'Pr': 1, 'Ra': 1, 'a' : 1, 'Nz': 100,
                      'NFourier': 50, 'dt_security': 0.9,
                      'maxiter': 100, 'freq_output': 10,
                      'freq_critical_Ra':50, 'verbose': False}
        # save the default parameters
        for param, value in param_list.items():
            setattr(self, param, value)

Do you have any idea of how I could circumvent this problem?

If you want the attributes to be accessible at Python level, don't you need `cdef public` instead of just `cdef`? That's what it looks like from the tutorial at least, down at the bottom: http://docs.cython.org/src/tutorial/cdef_classes.html — user2357112, Jun 02 '14 at 02:51
I edited my code so that it is more "obvious". The error I get at execution time tells my that the attribute `Re` doesn't exist. However, as my example shows it, it is supposed to be created via `setattr`. The error is happening when I try to access my variable inside the class itself (and not from a python frame). — cphyc, Jun 02 '14 at 03:13
`setattr` is Python-level. It doesn't see the attribute. As far as it knows, your object doesn't support that attribute, and that's the message you get when you try to set an unsupported attribute. — user2357112, Jun 02 '14 at 03:19
Ok. How could I do it then? I know I could give an explicit list of variables as the __init__ arguments, but I'm wondering if I can do it the way I'm doing right now using setattr. — cphyc, Jun 02 '14 at 03:37
If I'm reading the tutorial correctly, `cdef public` the attributes instead of `cdef`ing them. — user2357112, Jun 02 '14 at 03:43
It solved the problem! Thanks. However, it would very nice to have a cython equivalent to setattr to avoid any overhead caused by using cdef public. — cphyc, Jun 02 '14 at 04:01
If you want to set an attribute on the Cython level without the Python overhead, you will first have to share your extension type properly. See [The documentation](http://docs.cython.org/src/userguide/sharing_declarations.html). Once you've done that, you should be able to do something like `thing.Re = 1.234` to set the attribute. — IanH, Jun 02 '14 at 18:12

score 2 · Answer 1 · answered Jun 02 '14 at 20:35

When cdefining (without public) attribute in a class, what you are really doing is defining some field in a C structure. Therefore after the compilation (Cython + C), the name of the attribute are lost, they are only identified by some offset from the beginning of a C struct. A as consequence, they are not accessible from Python.
If you add cdef public, they Cython add some property access function which not only allows access from Python but also keep the association identifier <--> offset in C struct. There is a slight overhead going through those property functions. Note also that those function perform a Python -> C type check / conversion.

Now to answer your question, you need somehow to keep the association ident <--> offset. If you want thing to be fast, the only way is to do it by hand:

self.RE = param_list['RE']   # self.RE is a C struct access
self.Pr = param_list['Pr']
...

Cython unable to use setattr

1 Answers1