On 02/03/2012 07:07 PM, mark florisson wrote:
On 3 February 2012 18:06, mark florisson<markflorisso...@gmail.com> wrote:
On 3 February 2012 17:53, Dag Sverre Seljebotn
<d.s.seljeb...@astro.uio.no> wrote:
On 02/03/2012 12:09 AM, mark florisson wrote:
On 2 February 2012 21:38, Dag Sverre Seljebotn
<d.s.seljeb...@astro.uio.no> wrote:
On 02/02/2012 10:16 PM, mark florisson wrote:
On 2 February 2012 12:19, Dag Sverre Seljebotn
<d.s.seljeb...@astro.uio.no> wrote:
I just realized that
cdef int[:] a = None
raises an exception; even though I'd argue that 'a' is of the
"reference"
kind of type where Cython usually allow None (i.e., "cdef MyClass b =
None"
is allowed even if type(None) is NoneType). Is this a bug or not, and
is
it
possible to do something about it?
Dag Sverre
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Yeah I disabled that quite early. It was supposed to be working but
gave a lot of trouble in cases (segfaults, mainly). At the time I was
trying to get rid of all the segfaults and get the basic functionality
working, so I disabled it. Personally, I have never liked how things
Well, you can segfault quite easily with
cdef MyClass a = None
print a.field
so it doesn't make sense to slices different from cdef classes IMO.
can be None unchecked. I personally prefer to write
cdef foo(obj=None):
cdef int[:] a
if obj is None:
obj = ...
a = obj
Often you forget to write 'not None' when declaring the parameter (and
apparently that it only allowed for 'def' functions).
As such, I never bothered to re-enable it. However, it does support
control flow with uninitialized slices, and will raise an error if it
is uninitialized. Do we want this behaviour (e.g. for consistency)?
When in doubt, go for consistency. So +1 for that reason. I do believe
that
setting stuff to None is rather vital in Python.
What I typically do is more like this:
def f(double[:] input, double[:] out=None):
if out is None:
out = np.empty_like(input)
...
Having to use another variable name is a bit of a pain. (Come on -- do
you
use "a" in real code? What do you actually call "the other obj"? I
sometimes
end up with "out_" and so on, but it creates smelly code quite quickly.)
No, it was just a contrived example.
It's easy to segfault with cdef classes anyway, so decent nonechecking
should be implemented at some point, and then memoryviews would use the
same
mechanisms. Java has decent null-checking...
The problem with none checking is that it has to occur at every point.
Well, using control flow analysis etc. it doesn't really. E.g.,
for i in range(a.shape[0]):
print i
a[i] *= 3
can be unrolled and none-checks inserted as
print 0
if a is None: raise ....
a[0] *= 3
for i in range(1, a.shape[0]):
print i
a[i] *= 3 # no need for none-check
It's very similar to what you'd want to do to pull boundschecking out of the
loop...
Oh, definitely. Both optimizations may not always be possible to do,
though. The optimization (for boundschecking) is easier for prange()
than range(), as you can immediately raise an exception as the
exceptional condition may be issued at any iteration. What do you do
with bounds checking when some accesses are in-bound, and some are
out-of-bound? Do you immediately raise the exception? Are we fine with
aborting (like Fortran compilers do when you ask them for bounds
checking)? And how do you detect that the code doesn't already raise
an exception or break out of the loop itself to prevent the
out-of-bound access? (Unless no exceptions are propagating and no
break/return is used, but exceptions are so very common).
With initialized slices the control flow knows when the slices are
initialized, or when they might not be (and it can raise a
compile-time or runtime error, instead of a segfault if you're lucky).
I'm fine with implementing the behaviour, I just always left it at the
bottom of my todo list.
Wasn't saying you should do it, just checking.
I'm still not sure about this. I think what I'd really like is
a) Stop cdef classes from being None as well
b) Sort-of deprecate cdef in favor of cast/assertion type statements that
help the type inferences:
def f(arr):
if arr is None:
arr = ...
arr = int[:](arr) # equivalent to "cdef int[:] arr = arr", but
# acts as statement, with a specific point
# for the none-check
...
or even:
def f(arr):
if arr is None:
return 'foo'
else:
arr = int[:](arr) # takes effect *here*, does none-check
...
# arr still typed as int[:] here
If we can make this work well enough with control flow analysis I'd never
cdef declare local vars again :-)
Hm, what about the following?
def f(arr):
if arr is None:
return 'foo'
cdef int[:] arr # arr may not be None
The above would work in general, until the declaration is lexically
encountered, the object is typed as object.
This was actually going to be my first proposal :-) That would finally
define how "cdef" inside of if-statements etc. behave too (simply use
control flow analysis and treat it like a statement).
But I like int[:] as a way of making it pure Python syntax compatible as
well. Perhaps the two are orthogonal -- a) make variable declaration a
statement, b) make cython.int[:](x) do, essentially, a cdef declaration,
for Python compatability.
Dag
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