finding and replacing elements in a list

Question

I have to search through a list and replace all occurrences of one element with another. So far my attempts in code are getting me nowhere, what is the best way to do this?<

Answer 1

On long lists and rare occurrences its about 3x faster using list.index() - compared to single step iteration methods presented in the other answers.

def list_replace(lst, old=1, new=10):
    """replace list elements (inplace)"""
    i = -1
    try:
        while 1:
            i = lst.index(old, i + 1)
            lst[i] = new
    except ValueError:
        pass