Error Occurs: Replace a character in a String

[Jimbo]

Hello, I am getting an error in my python script when I try to change
a character in a string. [b]But I dont know why or what to do to fix
it?[/b]

I have commented in my code where the error occurs

[code]
def format_file(filename):

    HTML_file   = open(filename,'r')
    HTML_source = HTML_file.read()
    HTML_file.close()

    x = 0

    # Format all ID's & classes correctly
    temp_buf        = HTML_source.lower()
    class_occurence = temp_buf.count('class')
    ID_occurence    = temp_buf.count('id')

    for n in range(class_occurence):
        hit = temp_buf.find('class')
        if not hit==-1:
            temp_buf[hit] = '~' # Error: 'str' object does not support
item assignment
            x = hit+5

            # delete any spaces until we reach a letter or number
            while x<temp_buf[x]:
                if temp_buf[x] == ' ':
                    tempbuf[x]     = ''
                    HTML_source[x] = ''
                elif temp_buf[x] == '=':
                    pass
                #elif temp_buf[x] == "'" or temp_buf[x] == '"'
isalpha(temp_buf[x])
                else:
                    break
                x += 1
[/code]