How do I create a fixed-length, mutable array of Python objects in Cython?

I don't know about the best solution, but here's a solution:

from cpython.ref cimport PyObject, Py_XINCREF, Py_XDECREF

DEF SIZE = 32

cdef class TrieNode:
    cdef PyObject *members[SIZE]

    def __cinit__(self):
        cdef object temp_object
        for i in range(SIZE):
            temp_object = int(i)
            # increment its refcount so it's not gc'd.
            # We hold a reference to the object and are responsible for
            # decref-ing it in __dealloc__.
            Py_XINCREF(<PyObject*>temp_object)
            self.members[i] = <PyObject*>temp_object

    def __init__(self):
        # just to show that it works...
        for i in range(SIZE):
            print <object>self.members[i]

    def __dealloc__(self):
        # make sure we decref the members elements.
        for i in range(SIZE):
            Py_XDECREF(self.members[i])
            self.members[i] = NULL

A Cython object is an automatically refcounted PyObject *. You can always roll your own arrays of PyObject *'s as long as you take responsibility for refcounting the little buggers. This can be a major headache for non-trivial cases.

If you only need few fixed sizes of such a structure, I'd look at making classes with uniformly named __slots__, including one size slot to store the size. You'll need to declare a separate class for each size (number of slots). Define a cdecl function to access slots by index. Access performance will probably be not as great as with plain address arithmetics of a C array, but you'll be sure that there's only so many slots and none more.

How about this?

class TrieNode():
   def __init__(self, length = 32):
      self.members = list()
      self.length = length
      for i in range(length):
         self.members.append(None)

   def set(self, idx, item):
      if idx < self.length and idx >= 0:
         self.members[idx] = item
      else:
         print "ERROR: Specified index out of range."
         # Alternately, you could raise an IndexError.

   def unset(self, idx):
      if idx < self.length and idx >= 0:
         self.members[idx] = None
      else:
         raise IndexError("Specified index out of range (0..%d)." % self.length)