cython - Mixing cdef and regular python attributes in cdef class

Question

i am learning Cython and now experimenting with it. I tried the basic cdef class sample program and it works perfectly.

Now what i want to do is have a mix of cdef and non cdef mix of attributes in the cdef class type, something like this

cdef class Context:
    cdef public int byteIndex, bitIndex

    def __init__(self, msg = "", msg_len = 0):
        self.msg = msg 
        self.msg_len = msg_len 
        self.byteIndex = 0
        self.bitIndex = 7

but as soon as i instantiate the object i get error

!! AttributeError: 'c_asnbase.Context' object has no attribute 'msg'

Does this mean once you define a python class with cdef all self.* attributes have to be cdef defined?

Answer 1

Does this mean once you define a python class with cdef all self.* attributes have to be cdef defined?

Yes. This is stated pretty explicitly in the documentation:

Attributes in cdef classes behave differently from attributes in regular classes:

• All attributes must be pre-declared at compile-time
• ...

You can quite happily store a string by defining the attribute to be of type object:

cdef public object msg

---

Internally, the reason for this is that the cdef class does not have a dictionary, which saves space and makes attribute access faster, but it does mean that it cannot have arbitrary attributes added at runtime. This is reasonably similar to using __slots__ in a normal Python class.