zip as iterator and bad/good practices

[Steven D'Aprano]

On Sat, 13 Jun 2015 13:32:59 +0800, jimages wrote:

> I am a newbie. I also have been confused when I read the tutorial. It
> recommends make a copy before looping. Then I try.
> #--------------------------
> Test = [1, 2]
> For i in Test:
>     Test.append(i)
> #--------------------------

You don't make a copy of Test here. You could try this instead:

Test = [1, 2]
copy_test = Test[:]  # [:] makes a slice copy of the whole list
for i in copy_test:  # iterate over the copy
    Test.append(i)  # and append to the original

print(Test)


But an easier way is:

Test = [1, 2]
Test.extend(Test)
print(Test)


> But when i execute. The script does not end. I know there must something
> wrong. So I launch debugger and deserve the list after each loop. And I
> see:
> Loop 1: [ 1, 2, 1]
> Loop 2: [ 1, 2, 1, 2]
> Loop 3: [ 1, 2, 1, 2, 1]
> Loop 4: [ 1, 2, 1, 2, 1, 2]
> ......
> So you can see that loop will *never* end. So I think you regard the 'i'
> as a pointer.

i is not a pointer. It is just a variable that gets a value from the 
list, the same as:

    # first time through the loop
    i = Test[0]
    # second time through the loop
    i = Test[1]  # the second item


The for loop statement:

    for item in seq: ...

understands sequences, lists, and other iterables, not "item". item is 
just an ordinary variable, nothing special about it. The for statement 
takes the items in seq, one at a time, and assigns them to the variable 
"item". In English:

    for each item in seq ...

or to put it another way:

    get the first item of seq
    assign it to "item"
    process the block
    get the second item of seq
    assign it to "item"
    process the block
    get the third item of seq
    assign it to "item"
    process the block
    ...

and so on, until seq runs out of items. But if you keep appending items 
to the end, it will never run out.


> Change code to this
> #--------------------------
> Test = [1, 2]
> For i in Test[:] :
>     Test.append(i)
> #--------------------------


Yes, this will work.


-- 
Steve