[Pyrex] FAQ contribution

[John J Lee]

Despite the clarity of the docs, I managed to trip over this about four
times, and it took me a while to figure it out each time :-(don't ask me
why I didn't learn the *third* time...).

Worth a FAQ?


Q. Pyrex says my extension type object has no attribute 'rhubarb', but
   I know it does.

A. Have you declared the type at the point where you're using it?

cdef class ExtClass1
    cdef MyCType rhubarb
...


cdef class ExtClass2:
    cdef object mylist

    def __init__(self):
        self.mylist = []

    def append(self, ExtClass1 e):
        self.mylist.append(e)

    def foo(self):
        for e in self.mylist:
            do_something(e.rhubarb)  # WRONG

    def bar(self, e):
        do_something(e.rhubarb)  # WRONG


Pyrex will say (at runtime):

...
AttributeError: 'mymodule.ExtClass1' object has no attribute 'rhubarb'


Pyrex didn't know at compile time that e is an instance of the extension
class ExtClass1 (which is needed to do a direct C lookup).  You can't make
rhubarb a public attribute either (which is needed to do a dynamic Python
attribute lookup), because it has a plain C type, not an extension type.
Here's how to tell Pyrex the type at compile time:

    def foo(self):
        cdef ExtClass1 e
        for e in self.mylist:
            do_something(e.rhubarb)

    def bar(self, ExtClass1 e):
        do_something(e.rhubarb)


John