a.out

$ python test.py
  4           0 LOAD_CONST               3 ('aaaaaaaaaaaaaaaaaaaa')
              3 STORE_FAST               0 (a)

  5           6 LOAD_CONST               4 ('aaaaaaaaaaaaaaaaaaaa')
              9 STORE_FAST               1 (b)

  7          12 LOAD_FAST                0 (a)
             15 LOAD_FAST                1 (b)
             18 COMPARE_OP               8 (is)
             21 RETURN_VALUE        
********************************************************************************
 10           0 LOAD_CONST               1 ('a')
              3 LOAD_CONST               2 (21)
              6 BINARY_MULTIPLY     
              7 STORE_FAST               0 (a)

 11          10 LOAD_CONST               1 ('a')
             13 LOAD_CONST               2 (21)
             16 BINARY_MULTIPLY     
             17 STORE_FAST               1 (b)

 13          20 LOAD_FAST                0 (a)
             23 LOAD_FAST                1 (b)
             26 COMPARE_OP               8 (is)
             29 RETURN_VALUE        

test.py

from dis import dis

def test1():
	a = 'a' * 20
	b = 'a' * 20

	return a is b

def test2():
	a = 'a' * 21
	b = 'a' * 21

	return a is b

if __name__ == '__main__':
	dis(test1)
	dis(test2)