Tim Chase wrote:
If you want to literally remove None objects from a list....(or
mutable sequence)
def deNone(alist):
n=len(alist)
i=j=0
while i < n:
if alist[i] is not None:
alist[j] = alist[i]
j += 1
i += 1
alist[j:i] = []
blist=[None,1,None,2,None,3,None,None,4,None]
deNone(blist)
print(blist)
# prints [1, 2, 3, 4]
...wouldn't a cleaner way of doing this just be
>>> blist=[None,1,None,2,None,3,None,None,4,None]
No, making a filtered copy that is then copied back before being deleted
is algorithmically much messier. My code does the minimum work
necessary and is algorithmically cleaner.
>>> alist = blist
>>> blist[:] = [x for x in blist if x is not None]
>>> blist
[1, 2, 3, 4]
>>> alist
[1, 2, 3, 4]
By using the slice assignment, it leaves the blist referring to the same
list-object (as shown by the "alist" bit), and modifying it in place.
This reads a lot more cleanly in my estimation.
If the data-set is large, in 2.5+, you can just use a generator:
blist[:] = (x for x in blist if x is not None)
Given that this works, that the setup for slice assignment does not mess
up the co-routine iteration over the same loop, this is pretty good. It
amounts to deNone with the i-j loop separated into an i loop and a j
loop in source and consumer co-routines. One could argue that that is
even better, even if written out as
def deNone2(alist):
src = (item for item in alist if item is not None)
j=0
try:
while True:
alist[j] = next(src)
j += 1
except StopIteration:
pass
alist[j:] = []
tjr
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