how to define a list in Cython

Question

I want to transform the below python code in Cython:

x_array = []

x_array.append(x_new)

I tried the following Cython codes but it gives error:

cdef np.ndarray[double, dim=1] x_array

x_array.append(x_new)

The error shows:

Cannot coerce list to type [double, dim=1]

Answer 1

Your options are:

1. cdef list x_array. This lets Cython know that the type of x_array is actually a list. You may get a small speed-up from this.

2. Make x_array a numpy array instead. If all the elements in the list are the same simple, numeric type then this is probably a better option. Be aware that appending to numpy arrays is likely to be pretty slow, so you should calculate the size in advance.

   cdef np.array[double, dim=1] x_array = np.zeros((some_precomputed_size,))
   # or
   cdef double[:] x_array = np.zeros((some_precomputed_size,))
   

   Note that this will only give you a speed-up for some types of operations (mostly accessing individual elements in Cython)

3. If you're set on using Python lists you can sometimes get a speed-up by accessing them through the Python C API in Cython. This answer provides an example of where that worked well. This works best when you know a size in advance and so you can pre-allocate the array (i.e. don't append!) and also avoid some Cython reference counting. It's very easy to go wrong and make reference counting errors with this method, so proceed carefully.