Steven D'Aprano wrote:
And how often do you have an list that you are creating where you don't
know what items you have to initialise the list with?
[snip]
You are right to point out that the third case is a Python gotcha: [[]]*n
doesn't behave as expected by the naive or inexperienced Python
programmer. I should have mentioned it, and pointed out that in that case
you do want a list comp [[] for i in range(n)].
But that doesn't mean that the list comp is the general purpose solution.
Consider the obvious use of the idiom:
def func(arg, count):
# Initialise the list.
L = [arg for i in range(count)]
# Do something with it.
process(L, some_function)
def process(L, f):
# Do something with each element.
for item in enumerate(L):
f(item)
Looks good, right? But it isn't, because it will suffer the exact same
surprising behaviour if f modifies the items in place. Using a list comp
doesn't save you if you don't know what the object is.
I've only been using Python for a couple years on a part-time basis, so
I am not aquainted with this obvious use -- could you give a more
concrete example? Also, I do not see what the list comp has to do with
the problem in process() -- the list has already been created at that
point, so how is it the list comp's fault?
~Ethan~
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