write to log file

[Tom]

Hi,
I have a log file and of course I want to add the new information to the 
end of that log file. Unfortunately I always delete the old information 
and only append the current info.

Right now I do it like this:
I call a module which consists of a class and a function. It actually 
doesn't matter, but I just included it for the sake of completeness.
The module looks like this:

class log_C:
   def errorlog(self, filename, Data, d):
       LogFile = file(filename, 'w')
       ErrorInfo = Data[d+1]
       LogFile.write(ErrorInfo)
       LogFile.close()

How can I add the new info without deleting the old info?
I tried the modes 'a' and 'a+' but they both didn't work. Actually the 
data was totally unreadable! It was kind of messed up. I don't know why.

Thanks for your help.
Regards, Tom