For this, we will only need the string module.

Since the Atbash cipher works by using the opposite value of a character in the alphabet, we can create a maketrans feature to substitute characters:

import string
input = raw_input("Please enter the value you would like to Atbash Cipher: ")
transform = string.maketrans(
"ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz",
"ZYXWVUTSRQPONMLKJIHGFEDCBAzyxwvutsrqponmlkjihgfedcba")
final = string.translate(input, transform)
print final

After importing the correct module, we request the input from the user for the value they would like encipher into the Atbash cipher:

import string
input = raw_input("Please enter the value you would like to Atbash Ciper: ")

Next, we create the maketrans feature to be used. We do this by listing the first set of characters that we would like to be substituted and then listing another set of characters that we will use to replace the previous ones:

transform = string.maketrans(
"ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz",
"ZYXWVUTSRQPONMLKJIHGFEDCBAzyxwvutsrqponmlkjihgfedcba")

Finally, we just need to give a value to the transformation, apply it, and print the value out to get the end result:

final = string.translate(input, transform)
print final

Here is an example of the script in action:

Please enter the value you would like to Atbash Cipher: testing
gvhgrmt