FAQ
Ray was tryin to follow the thread ...

Dan was talking to Tim about something.
(see below signature if your not following
thread and want to know what it's about.)

... and Ray had a question.
[Dan to Tim paraphrased]

My solution, which I'm worried about, was this:

for x in range(n):
temp = [x+y for y in range(3)] #or whatever
v[x] = temp + [] #'assignment by copy'???
Ray's question = That last line "v[x] = temp + []",
that's equivelent to "v.append(temp)", is it not?

TIA
Ray St.Marie
Rastm2 at aol.com

Thread I got this from
"Tim Peters" <tim.one at comcast.net> wrote in message
news:<mailman.1058992140.17079.python-list at python.org>...
[Tim] ...
Bjorn added more appropriate words -- it would be bad design for the
__iadd__ method of a mutable type to return a pre-existing object other than
self.
a = [1, 2]
b = [1]
b += [2]
While "a == b" must be true at this point, it would be a nightmare if "a is
b" could be true at this point. For immutable types it doesn't matter:
a = (1, 2)
b = (1,)
b += (2,)
It so happens that "a is b" is not true at this point under any Python
released so far, but it could be true someday without causing harm.

Sorry for the confusion!
Ok, that makes sense to me. I still think it would be a good idea to
put a section in the Tutorial -- perhaps under "for computer
scientists and those who want to learn more" -- describing how Python
binds names to objects.

In particular, the fact that a list is a list of pointers, not the
objects themselves, seems like an important point to make, even for
relative newcomers.

Here's the thing that tripped me up, and the solution I came up with,
which I am now unsure of:

#original code -- v[] is a list

for x in range(n):
temp = [x+y for y in range(3)] #or whatever
v[x] = temp

so I was quite surprised to find all members of v[] were in fact
pointers to one object, temp[].

My solution, which I'm worried about, was this:

for x in range(n):
temp = [x+y for y in range(3)] #or whatever
v[x] = temp + [] #'assignment by copy'???

So I'm still unclear as to whether an expression like temp+[] is
defined as returning a new list or not. Seems like it might be
undefined, since either way is arguably 'correct'.

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 group python-list categories python posted Jul 23, '03 at 12:59a active Jul 27, '03 at 10:25p posts 24 users 14 website python.org

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