human
Solve the following python programming problem given the following code:
def download_url(source_url, target_path, print_status = False) : 
    '
    Download the file at |source_url| and write it to |target_path|.
    ' 
    in_file = urllib2.urlopen (source_url) 
    total_bytes = in_file.info ().getheader ('Content-Length') 
    if total_bytes : 
        total_bytes = int (total_bytes) 
num_bytes = 0 
    out_file = open (target_path, 'wb') 
    def status_str() : 
        if total_bytes : 
            return ('Downloaded %s/%s (%d%%)' % (formatting.size_str (num_bytes), formatting.size_str (total_bytes), ((100.0 * num_bytes) / total_bytes))) 
else : 
            return ('Downloaded %s' % formatting.size_str (num_bytes)) 
while True : 
        s = in_file.read (BUFFER_SIZE) 
        if (not s) : 
            break 
out_file.write (s) 
        num_bytes += len (num_bytes) 
        if print_status : 
            ((print >> sys.stderr), ('\r' + status_str ())) 
            sys.stderr.flush () 
if print_status : 
        ((print >> sys.stderr), (('\r' + status_str ()) + ' [done]')) 

    
    

    Fix the buggy line: num_bytes += len (num_bytes)
assistant
#Here is Python code to fix the buggy line: num_bytes += len (num_bytes)
.

num_bytes += len (s)