Drowning in a teacup?

[Steven D'Aprano]

On Sat, 2 Apr 2016 07:27 am, Fillmore wrote:

> 
> notorious pass by reference vs pass by value biting me in the backside
> here. Proceeding in order.

Python is NEITHER pass by reference nor pass by value.

Please read this:

http://import-that.dreamwidth.org/1130.html

before asking any additional questions.


> def bringOrderStringToFront(mylist, key):
> 
>      for i in range(len(mylist)):
>          if(mylist[i].startswith(key)):
>              mylist = [mylist[i]] + mylist[:i] + mylist[i+1:]

[mylist[i]] + mylist[:i] + mylist[i+1:]

creates a new list. It doesn't modify the existing list.

mylist = ...

performs an assignment to the LOCAL VARIABLE mylist, setting it to the new
list just created. The original list is unchanged.

Trick: you can use slicing to write to the original list:

mylist[:] = [mylist[i]] + mylist[:i] + mylist[i+1:]


Or you can modify the list directly in place:

    for i in range(len(mylist)):
        if mylist[i].startswith(key):
            x = mylist[i]
            del mylist[i]
            mylist.insert(0, x)


which is better written as:

    for i, x in enumerate(mylist):
        if x.startswith(key):
            del mylist[i]
            mylist.insert(0, x)

or if you prefer:

    for i, x in enumerate(mylist):
        if x.startswith(key):
            mylist[:] = [x] + mylist[:i] + mylist[i+1:]



-- 
Steven