The ol' [[]] * 500 bug...

[kj]

....just bit me in the "fuzzy posterior". The best I can come up with
is the hideous

lol = [[] for _ in xrange(500)]

Is there something better? What did one do before comprehensions
were available? I suppose in that case one would have to go all
the way with

lol = [None] * 500
for i in xrange(len(lol)):
lol = []

Yikes. 10 miles uphill, both ways...

kynn

 

[Jon Clements]

...just bit me in the "fuzzy posterior".  The best I can come up with
is the hideous

  lol = [[] for _ in xrange(500)]

Is there something better?  

That's generally the accepted way of creating a LOL.

What did one do before comprehensions
were available?  I suppose in that case one would have to go all
the way with

  lol = [None] * 500
  for i in xrange(len(lol)):
      lol = []

Yikes.  10 miles uphill, both ways...

lol = map(lambda L: [], xrange(5))
[id(i) for i in lol]

[167614956, 167605004, 167738060, 167737996, 167613036]

Jon.

 

[kj]

lol =3D map(lambda L: [], xrange(5))
[id(i) for i in lol]

[167614956, 167605004, 167738060, 167737996, 167613036]

Oh yeah, map! I'd forgotten all about it. Thanks!

kynn

 

[Diez B. Roggisch]

...just bit me in the "fuzzy posterior". The best I can come up with
is the hideous

lol = [[] for _ in xrange(500)]

If you call that hideous, I suggest you perform the same exercise in
Java or C++ - and then come back to python and relax....


Diez

 

[Paul Rubin]

lol = [None] * 500
for i in xrange(len(lol)):
lol = []

lol = map(list, [()] * 500)

 

[Vlastimil Brom]

  lol = [None] * 500
  for i in xrange(len(lol)):
      lol = []

lol = map(list, [()] * 500)

Could someone explain what the deal is with this thread? Thanks.
[[]]*500

Try

lst=[[]]*500
lst[7].append(2)
lst

to see...

vbr

 

[Ulrich Eckhardt]

lol = [[] for _ in xrange(500)]

If you call that hideous, I suggest you perform the same exercise in
Java or C++ - and then come back to python and relax....

I might be missing something that's not explicitly mentioned here, but I'd
say that all non-associative containers in C++ have a resize() method, some
even taking an initial size in their constructor.

That said, [[]]*500 is IMHO more readable.

Uli

 

[Paul Rubin]

That said, [[]]*500 is IMHO more readable.

But the issue in the thread is that it does the wrong thing.

 

[Steven D'Aprano]

...just bit me in the "fuzzy posterior".

It's not a bug. Just because it doesn't behave as you would like it to
behave doesn't mean it isn't behaving as designed.

The best I can come up with is the hideous

lol = [[] for _ in xrange(500)]

That's not hideous.

Is there something better? What did one do before comprehensions were
available? I suppose in that case one would have to go all the way with

lol = [None] * 500
for i in xrange(len(lol)):
lol = []

Yikes. 10 miles uphill, both ways...

What's wrong with that?


lol = []
for _ in xrange(500): lol.append([])


is a simple alternative too, although the list comp is better.