To find ip of nic eth0:

import os

ip = os.popen('"ip -4 -br addr show eth0 | grep -E -o '([0-9]{1,3}\.){3}[0-9]{1,3}'"').read().strip()
print(ip)

#Or using variable to define network interface card
nic = "eth0"
ip = os.popen('"ip -4 -br addr show " + nic + " | grep -E -o '([0-9]{1,3}\.){3}[0-9]{1,3}'"').read().strip()
print(ip)

Instead of using external tools like awk one can use a regular expression.

1. Extend regular expression (-E)

2. Print only the matched parts (-o)

([0-9]{1,3}\.){3}[0-9]{1,3}'"') -> Breaks into two parts

1. ([0-9]{1,3}\.){3}

   1. [0-9] -> Digits 0 to 9.

   2. {1,3} -> Minimum 1 digit, maximum 3 digits.

   3. \. Adds a '.', the '\' works as a escape character.

   4. = {3} Repeats three times.

2. [0-9]{1,3} Same as part 1, no '.', no repeat.