Что такое in - Python

if usrname in users:

#-----------------------------------------------
#password_test.py
#   example of if/else, lists, assignments,raw_input,
#   comments and evaluations
#-----------------------------------------------
# Assign the users and passwords
users = ['Fred','John','Steve','Ann','Mary']
passwords = ['access','dog','12345','kids','qwerty']
#-----------------------------------------------
# Get username and password
usrname = raw_input('Enter your username => ')
pwd = raw_input('Enter your password => ')
#-----------------------------------------------
# Check to see if user is in the list
if usrname in users:
    position = users.index(usrname) #Get the position in the list of the users
    if pwd == passwords[position]: #Find the password at position
        print 'Hi there, %s. Access granted.' % usrname
    else:
        print 'Password incorrect. Access denied.'
else:
    print "Sorry...I don't recognize you. Access denied."