Searching and manipulating lists of tuples

[MTD]

Hello,

I'm wondering if there's a quick way of resolving this problem.

In a program, I have a list of tuples of form (str,int), where int is a
count of how often str occurs

e.g. L = [ ("X",1),("Y",2)] would mean "X" occurs once and "Y" occurs
twice

If I am given a string, I want to search L to see if it occurs already.
If it does, I find the corresponding tuple and increment the integer
part. If not, I append the new element with int = 1.

e.g.

algorithm(L, "X") would produce output L = [("X",2),("Y",2)]
algorithm(L,"Z") would produce L = [("X",1),("Y",2),("Z",1)]

I tried to create an algorithm of the following form:
>>> def algorith(list,str):
... 	flag = True
... 	for l in list:
... 		if l[0] == str:
... 			l[1] += 1
... 			flag = False
... 	if flag:
... 		list.append((str,1))
...


But:

>>> algorith(L,"X")

gives:

Traceback (most recent call last):
  File "<interactive input>", line 1, in ?
  File "<interactive input>", line 5, in algorith
TypeError: object does not support item assignment


So clearly that doesn't work... any ideas?