Drowning in a teacup?

Fri Apr 1 16:27:13 EDT 2016

notorious pass by reference vs pass by value biting me in the backside 
here. Proceeding in order.
I need to scan a list of strings. If one of the elements matches the 
beginning of a search keyword, that element needs to snap to the front 
of the list.
I achieved that this way:
 for i in range(len(mylist)):
 if(mylist[i].startswith(key)):
 mylist = [mylist[i]] + mylist[:i] + mylist[i+1:]
Since I need this code in multiple places, I placed it inside a function
def bringOrderStringToFront(mylist, key):
 for i in range(len(mylist)):
 if(mylist[i].startswith(key)):
 mylist = [mylist[i]] + mylist[:i] + mylist[i+1:]
and called it this way:
 if orderstring:
 bringOrderStringToFront(Tokens, orderstring)
right?
Nope, wrong! contrary to what I thought I had understood about how 
parameters are passed in Python, the function is acting on a copy(!) and 
my original list is unchanged.
I fixed it this way:
def bringOrderStringToFront(mylist, key):
 for i in range(len(mylist)):
 if(mylist[i].startswith(key)):
 mylist = [mylist[i]] + mylist[:i] + mylist[i+1:]
 return(mylist)
and:
if orderstring:
 Tokens = bringOrderStringToFront(Tokens, orderstring)
but I'm left with a sour taste of not understanding what I was doing 
wrong. Can anyone elaborate? what's the pythonista way to do it right?
Thanks