How to check the value of variable in python?

I need to check the value of all 3 variables(username, password, secret). If a value is NA then I need to execute a print statement or else some condition needs to be executed.

Example 1:

username = person1
password = pass1
secret   = NA

Example 2:

username = person1
password = pass1
secret   = secret1

Example

#value = Check all 3 variables for NA value
if value is NA:
   print("NA Block")
else:
    print("Value Block")
    #Calling some function

How to check the value of those variables all at once?

Thanks in advance.