python: How to Store output of an OS call to a variable?

>>> import os
>>> os.system("ping google.com")
PING google.com (216.58.222.78) 56(84) bytes of data.
64 bytes from rio01s15-in-f14.1e100.net (216.58.222.78): icmp_seq=1 ttl=56 time=111 ms
64 bytes from rio01s15-in-f14.1e100.net (216.58.222.78): icmp_seq=2 ttl=56 time=111 ms

--- google.com ping statistics ---
2 packets transmitted, 2 received, 0% packet loss, time 1000ms
rtt min/avg/max/mdev = 111.795/111.829/111.863/0.034 ms
0
>>> a
0

How can I store the output in variable a?