Why am I getting this error in the very basic Python script? What does the error mean?

Error:

Traceback (most recent call last):
  File "cab.py", line 16, in <module>
    if cab in line:
TypeError: 'in <string>' requires string as left operand, not int

Script:

import re
import sys

#loco = sys.argv[1]
cab = 6176
fileZ = open('cabs.txt')
fileZ = list(set(fileZ))

for line in fileZ:
     if cab in line: 
        IPaddr = (line.strip().split())
        print(IPaddr[4])

You simply need to make cab a string:

cab = '6176'

As the error message states, you cannot do <int> in <string>:

>>> 1 in '123'
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'in <string>' requires string as left operand, not int
>>> 

because integers and strings are two totally different things and Python does not embrace implicit type conversion ("Explicit is better than implicit.").

In fact, Python only allows you to use the in operator with a right operand of type string if the left operand is also of type string:

>>> '1' in '123'  # Works!
True
>>>
>>> [] in '123'
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'in <string>' requires string as left operand, not list
>>>
>>> 1.0 in '123'
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'in <string>' requires string as left operand, not float
>>>
>>> {} in '123'
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'in <string>' requires string as left operand, not dict
>>>