Our DynamicArray class, as given in Code Fragment 5.3, does not support use of negative indices with _ _getitem_ _. Update that method to better match the semantics of a Python list.

Code Fragment 5.3

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1 import ctypes # provides low-level arrays 3 class DynamicArray: "A dynamic array class akin to a simplified Python list." 4 def --init.--(self): "Create an empty array." self._n = 0 6. # count actual elements # default array capacity # low-level array self.capacity = 1 self.A = self.make_array(self._capacity) 9. 10 11 def --len -(self): """ Return number of elements stored in the array." 12 13 return self._n 14 15 def --getitem --(self, k): """' Return element at index k." 16 17 if not 0 <= k < self._n: 18 raise IndexError('invalid index') return self._A[k] 19 # retrieve from array 20 21 def append(self, obj): """ Add object to end of the array." if self.n == self._capacity: self._resize(2 * self.capacity) self._A[self._n] = obj self._n += 1 22 23 # not enough room 24 # so double capacity 25 26 27 28 def -resize(self, c): 'Resize internal array to capacity c.' B = self._make_array(c) for k in range(self.n): B[k] = self.A[k] self._A = B # nonpublic utitity 29 30 # new (bigger) array # for each existing value 31 32 33 # use the bigger array 34 self._capacity = c 35 36 def -make.array(self, c): Return new array with capacity c.' return (c * ctypes.py_object)( ) # nonpublic utitity 37 38 # see ctypes documentation 39