The line 

filename = zipfile.ZipFile(zip_file_name, "a") 

throws an exception if the given filename is not 
present already.

Shouldn't it create a file (in case one is not there) 
since it is "append" mode ??

It's throwing an OSError Exception stating "No such 
file/directory".

Normal convention has been that when opening a file 
in append mode, if the file is not present a new file 
is created.

Why is this non-standard?