Coercing a sliced C array to Python should create a list

[scoder]

Cython already copies a C array into a Python list automatically on object coercion. It should do the same for sliced C arrays, e.g.

def test():
    cdef int i
    cdef int a[100]
    for i in range(100):
        a[i] = i**2
    return a[:20]  # rejected by type analysis

Currently results in
Cannot convert 'int *' to Python object.

[robertwb]

Or maybe a tuple to avoid assumptions that mutating one would change the other. IMHO, an explicit list comprehension or list(c_array) might not be too bad to require to make the fact that it's a copy explicit.

…

On Mar 17, 2018 8:47 AM, "scoder" ***@***.***> wrote: Cython already copies a C array into a Python list automatically on object coercion. It should do the same for sliced C arrays, e.g. def test(): cdef int i cdef int a[100] for i in range(100): a[i] = i**2 return a[:20] # rejected by type analysis Currently results in Cannot convert 'int *' to Python object. — You are receiving this because you are subscribed to this thread. Reply to this email directly, view it on GitHub <#2159>, or mute the thread <https://github.com/notifications/unsubscribe-auth/AAdqgbISOH4fV9I1sWv2ojDfFXjpK1wvks5tfTAQgaJpZM4Su2fU> .

[scoder]

Too late ;)
We already support this for plain arrays with known size, which coerce into a Python list. And I think a list makes more sense than a tuple because a Python list is the normal representation of a sequence of homogeneous objects. Tuples are more for heterogeneous objects.

[gabrieldemarmiesse]

I'll note here for whoever wants to fix this issue the places in the documentation that will need to be changed accordingly:

cython/docs/examples/tutorial/memory_allocation/malloc.pyx

Line 20 in cd3a007

return [x for x in my_array[:number]]

cython/docs/examples/tutorial/cython_tutorial/primes.pyx

Line 22 in cd3a007

result_as_list = [prime for prime in p[:len_p]]

https://github.com/cython/cython/blob/master/docs/src/tutorial/cython_tutorial.rst#primes

https://github.com/cython/cython/blob/master/docs/src/userguide/language_basics.rst#automatic-type-conversions