[Cython] CEP 507/513

[Stefan Behnel]

Hi,

Robert Bradshaw wrote:
> The original proposal was that some Python objects (e.g. list, tuple,  
> dict, str) would be known by the compiler and optimized accordingly  
> (e.g. using the appropriate macros for indexing). One thing that is  
> radical about this proposal (and why a pxd file wouldn't suffice) is  
> that something declared "cdef list" would have to be *exactly* a  
> list, not a subclass of list. In this case I think it is worth it  
> because there are enormous gains to be made and 99% of Python lists  
> are actually just lists. These would be exactly like C types which  
> have no hierarchy.

I think this is the right way to do it. Due to the usual naming convention of
having extension types start with an upper-case letter, these declarations
even look like C types - and the only reason I could imagine why you would
declare a variable "cdef list" is to avoid having to write

    PyList_Append(l, x)
    print PyList_GetItem(l, i)

instead of

    l.append(x)
    print l[i]

You wouldn't gain *anything* by declaring a variable "cdef list" that holds a
subtype of list.

BTW, at least PyList_Append() seems to work just fine for subtypes:

------------------------------------
cdef extern from "Python.h":
    ctypedef class __builtin__.list [object PyListObject]:
        pass

    cdef int PyList_Append(object l, object obj) except -1

cdef class csublist(list):
    def append(l, x):
        print "APPEND", x

l = csublist()
PyList_Append(l, 1)
print l

import __builtin__
class pysublist(__builtin__.list):
    def append(l, x):
        print "APPEND", x

l = pysublist()
PyList_Append(l, 1)
print l
------------------------------------

The "APPEND"s are not printed (as expected), so the output is:

------------------------------------
[1]
[1]
------------------------------------

Stefan