Python: check whether a network interface is up

The interface can be configured with an IP address and not be up so the accepted answer is wrong. You actually need to check /sys/class/net/<interface>/flags. If the content is in the variable flags, flags & 0x1 is whether the interface is up or not.

Depending on the application, the /sys/class/net/<interface>/operstate might be what you really want, but technically the interface could be up and the operstate down, e.g. when no cable is connected.

All of this is Linux-specific of course.

Answer using psutil:

import psutil
import socket

def check_interface(interface):
    interface_addrs = psutil.net_if_addrs().get(interface) or []
    return socket.AF_INET in [snicaddr.family for snicaddr in interface_addrs]

With pyroute2.IPRoute:

from pyroute2 import IPRoute
ip = IPRoute()
state = ip.get_links(ip.link_lookup(ifname='em1'))[0].get_attr('IFLA_OPERSTATE')
ip.close()

With pyroute2.IPDB:

from pyroute2 import IPDB
ip = IPDB()
state = ip.interfaces.em1.operstate
ip.release()

As suggested by @Gabriel Samfira, I used netifaces. The following function returns True when an IP address is associated to a given interface.

def is_interface_up(interface):
    addr = netifaces.ifaddresses(interface)
    return netifaces.AF_INET in addr

The documentation is here