How to programmatically count the number of files in an archive using python

In the program I maintain it is done as in:

# count the files in the archive
length = 0
command = ur'"%s" l -slt "%s"' % (u'path/to/7z.exe', srcFile)
ins, err = Popen(command, stdout=PIPE, stdin=PIPE,
                 startupinfo=startupinfo).communicate()
ins = StringIO.StringIO(ins)
for line in ins: length += 1
ins.close()

1. Is it really the only way ? I can't seem to find any other command but it seems a bit odd that I can't just ask for the number of files

2. What about error checking ? Would it be enough to modify this to:

   proc = Popen(command, stdout=PIPE, stdin=PIPE,
                startupinfo=startupinfo)
   out = proc.stdout
   # ... count
   returncode = proc.wait()
   if returncode:
       raise Exception(u'Failed reading number of files from ' + srcFile)
   

   or should I actually parse the output of Popen ?

EDIT: interested in 7z, rar, zip archives (that are supported by 7z.exe) - but 7z and zip would be enough for starters