newbie - infinite loop

[Paul Brian]

All,

I have solved the opriginal problem but am left with a serious question.

Before I discovered the keywords in and not in, I tried to count unique
instances of names in a file.
I devcided to create a (neaarly) empty list, compare each name in the file
with that list. If the name was not in, append to list, otherwise leave it
alone.

After the following was run (and crashed) a couple of times I started using
my brain and realised there must be some "in" operator. However the problem
still stands - a for condition seems completely mutable.

So, line 3 sets a condition, 4 compares "q" to "q" and (wrongly for my
needs) adds "q" to the end of the list.
But then it seems that line 3 realises it has another element in its list
and goes back and compares
the newly appended "q" to my lineand so on ad infinitum (I have to kill the
process each time) !

Surely this should not happen.  Surely the line 3 condition "for every
element in this list", should not constantly expand but should be set solid
till the next pass?

What have I done wrong - is it the mutable list v the immutable tuple
problem (but it should work..surely?) Or is this just the way things are -
"live with it and do not try anything like this again?"

thanks in advance
==============
uniqueList = ['q']               # create an nearl empyty list (seems to
ignore line 3 if truly empty)
line = 'q'                          # fake the retrieval of line from file

for listElement in uniqueList:             #3
    if listElement == line:                    #4
        uniqueList.append(line)            #5 at this point it seems to
think that there is now another list
                                                        #element to compare
and so loops for ever...help!

print len(uniqueList)
==============