python - Cython class AttributeError

Question

I have started to experiment with Cython and ran into the following problem. Consider the following class representing a vertex in the 3D space:

#Vertex.pyx
cdef class Vertex(object):
    cdef double x, y, z
    def __init__(self, double x, double y, double z):        
        self.x = x
        self.y = y
        self.z = z

Now I try to create an object from the Python console:

import Vertex as vt
v1 = vt.Vertex(0.0, 1.0, 0.0) 

which works fine. However, when I'm trying to access the class attributes, I'm getting an AttributeError:

print v1.x
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-6-83d928d774b7> in <module>()
----> 1 print v1.x

AttributeError: 'Vertex.Vertex' object has no attribute 'x'

Any ideas why this could happen?

Answer 1

By default cdef attributes are only accessible from within Cython. If you make it a public attribute with cdef public in front of the attribute name then Cython will generate suitable properties to be able to access it from Python.

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Some extra notes about related problems:

If you're getting the same error from within Cython then you have probably forgotten to tell Cython the type of your instance variable (e.g. v1) - Cython itself can happily access cdef attributes but it only knows about them if it knows the type. If it's just a variable then cdef that variable. If you're trying to use the return value from a function, or index a list, or similar, then you can use a cast: <Vectex>(func()).x. Only do this if you're certain of the type.

You can get similar errors with cdef functions, which are again only visible within Cython. In this case cpdef makes the function visible from to both Cython and Python. However, cpdef functions are in some ways the worst of all worlds (they have all the limitations of cdef functions and all the limitations of def functions) - you're usually better choosing an Cython-only (cdef) or Python (def) interface.