Simple list.append() question

[Bernhard Herzog]

m.faassen at vet.uu.nl (Martijn Faassen) writes:

> Generally it's a good idea to avoid * on sequences, unless that sequence is
> immutable and contains immutable things; i.e. strings.

It seems to me that the important criterion is whether the elements are
immutable. The mutability of the sequence itself doesn't really matter
here.

> Is there any useful way to use * on lists that I missed?

Something like [None] * 1000 is very useful IMO.

> Usually we want the copy semantics here, not the reference semantics.

If the elements are mutable. But then the question is how deeply do you
copy them?

> Perhaps it's a good idea to completely forbid * on lists in p3k?
> Tuples too for all I care. Then again I may be missing important uses,
> so enlighten me.

AFAICT, I use * with lists most often to create a new list of n
identical immutable elements, most often None or '' or somesuch and I
find that really useful.

What seems to cause the most problems is that newbies try to use it to
create empty multi-dimensional arrays and that fails. I think they try
it because they see it more as a declaration similar to C's
'int[10][20]' than an expression. It would be a good idea IMO to find a
good syntax for this case and ISTM that list comprehensions might be
useful here:

  multi_list = [[] for i in range(10)]

or whatever the precise syntax was. Hmm, the unused variable in there is
a bit ugly though.


-- 
Bernhard Herzog   | Sketch, a drawing program for Unix
herzog at online.de  | http://sketch.sourceforge.net/