Using an even easier and safer way to update dhcpd configuration

main.py

@@ -113,15 +113,9 @@ def switch_status(turn_on):
 
 
 def update_dhcp_conf(turn_on):
-    right_interface = False
-    for line in fileinput.input(USB_DHCPD_FILENAME, inplace=True):
-        if line.startswith('interface'):
-            right_interface = line.strip().split(' ')[-1] == AIR_INTERFACE_NAME
-
-        if right_interface and 'metric' in line:
-            print("\tmetric {}\n".format(METRIC_ON if turn_on else METRIC_OFF), end='')
-        else:
-            print(line, end='')
+    with open(USB_DHCPD_FILENAME, 'w') as dhcpd_file:
+        dhcpd_file.write("interface {}\n".format(AIR_INTERFACE_NAME))
+        dhcpd_file.write("\tmetric {}\n".format(METRIC_ON if turn_on else METRIC_OFF))
 
 
 def update_usb_modem(turn_on):