How to access the typed-memory view element of a class declared in cython? - cython

Question

I am a beginner and I am sure this question is too simple. I am trying to test memory views in cython to get to know them much better.In my code I pass each memory view element (like [1,2]) as the cy class element move.
cdef class cy:
cdef public long[:] move
def __init__(self, move):
self.move = move
lst = []
for i in range(100):
lst.append([i, i+1])
cdef long[:, :] memview = np.asarray(lst)
b0 = cy(memview[0])
print(b0.move)
When I print the results. I get this:
<MemoryView of 'ndarray' object> # I expect for sth like [12, 13]
I need cy class prints out a list. How can I fix it?
there is another problem which occurs to me when I use this code:
cdef class parent:
cdef public:
list children
list moves
def __init__(self):
self.children = []
def add_children(self, moves):
cdef int i = 0
cdef int N = len(moves)
for i in range(N):
self.children.append(cy(moves[i]))
cdef int[:, :] moves = np.asarray(lst, dtype=np.int32)
obj = parent()
for move in moves:
obj.add_children(move)
After running this code I always get this error:
TypeError: a bytes-like object is required, not 'int'.
What causes this error and how can I fix this one?

Answer 1

Your first issue is just that a memoryview doesn't have a useful __str__ function for print to use. You can either convert it to an object that does print nicely
print(list(b0.moves))
print(np.asarray(b0.moves))
Or you can iterate through it yourself:
for i in range(b0.moves.shape[0]):
print(b0.moves[i], end=' ') # need to have Cython set to use Python 3 syntax for this line
print()
Your second problem is harder to solve since you don't tell us what line the error comes from. I think it's the constructor of cy which expects a memoryview but you pass an integer to. (I get a slightly different error message though).