Python 2:

def is_a_in_x(A, X):
  for i in xrange(len(X) - len(A) + 1):
    if A == X[i:i+len(A)]: return True
  return False

Python 3:

def is_a_in_x(A, X):
  for i in range(len(X) - len(A) + 1):
    if A == X[i:i+len(A)]: return True
  return False