can list comprehensions replace map?

[David Isaac]

Newbie question:

I have been generally open to the proposal that list comprehensions
should replace 'map', but I ran into a need for something like
map(None,x,y)
when len(x)>len(y). I cannot it seems use 'zip' because I'll lose
info from x. How do I do this as a list comprehension? (Or,
more generally, what is the best way to do this without 'map'?)

Thanks,
Alan Isaac

[Peter Otten]

Andrew Dalke wrote:

Me:

Could make it one line shorter with

from itertools import chain, izip, repeat
def fillzip(*seqs):
def done_iter(done=[len(seqs)]):
done[0] -= 1
if not done[0]:
return []
return repeat(None)
seqs = [chain(seq, done_iter()) for seq in seqs]
return izip(*seqs)
Peter Otten:

that won't work because done_iter() is now no longer a generator.
In effect you just say

seqs = [chain(seq, repeat(None)) for seq in seqs[:-1]] + [chain(seq[-1],
[])]

It does work - I tested it. The trick is that izip takes iter()
of the terms passed into it. iter([]) -> an empty iterator and
iter(repeat(Non e)) -> the repeat(None) itself.

Seems my description didn't convince you. So here's an example:

from itertools import chain, izip, repeat
def fillzip(*seqs): .... def done_iter(done=[len(seqs)]):
.... done[0] -= 1
.... if not done[0]:
.... return []
.... return repeat(None)
.... seqs = [chain(seq, done_iter()) for seq in seqs]
.... return izip(*seqs)
.... list(fillzip(ra nge(6), range(3))) [(0, 0), (1, 1), (2, 2)]
versus
map(None, range(6), range(3))

[(0, 0), (1, 1), (2, 2), (3, None), (4, None), (5, None)]

Now where's the typo?
'Course then the name should be changed.

My variable names where ill-chosen to begin with.

Peter

[Peter Otten]

Scott David Daniels wrote:

Can I play too?
Not unless you buy the expensive but good-looking c.l.py gaming license
which is only available trough me :)
How about:
import itertools

def fillzip(*seqs):
def Nones(countacti ve=[len(seqs)]):
countactive[0] -= 1
while countactive[0]:
yield None
seqs = [itertools.chain (seq, Nones()) for seq in seqs]
return itertools.izip( *seqs)

You may be introducing a lot of extra tests in the while loop with the
non-constant condition -- which in practice is fairly cheap, though.
I'm willing to take the performance hit for the introduction of sane
variable names alone...

Peter

[Andrew Dalke]

Peter Otten wrote:

Seems my description didn't convince you. So here's an example:

Got it. In my test case the longest element happened to be the last
one, which is why it didn't catch the problem.

Thanks.

Andrew
da***@dalkescie ntific.com