https://github.com/python/cpython/blob/829b49cbd2e4b1d573470da79ca844b730120f3d/Python/peephole.c#L223

t.py

from __future__ import print_function

import dis


def f1():
    x = 'a' * 20
    y = 'a' * 20
    return x is y


print(f1())
# True
dis.dis(f1)
# 7           0 LOAD_CONST               3 ('aaaaaaaaaaaaaaaaaaaa')
#             3 STORE_FAST               0 (x)

# 8           6 LOAD_CONST               4 ('aaaaaaaaaaaaaaaaaaaa')
#             9 STORE_FAST               1 (y)

# 9          12 LOAD_FAST                0 (x)
#            15 LOAD_FAST                1 (y)
#            18 COMPARE_OP               8 (is)
#            21 RETURN_VALUE

def f2():
    x = 'a' * 21
    y = 'a' * 21
    return x is y


print(f2())
# False
dis.dis(f2)
# 27           0 LOAD_CONST               1 ('a')
#              3 LOAD_CONST               2 (21)
#              6 BINARY_MULTIPLY
#              7 STORE_FAST               0 (x)

# 28          10 LOAD_CONST               1 ('a')
#             13 LOAD_CONST               2 (21)
#             16 BINARY_MULTIPLY
#             17 STORE_FAST               1 (y)

# 29          20 LOAD_FAST                0 (x)
#             23 LOAD_FAST                1 (y)
#             26 COMPARE_OP               8 (is)
#             29 RETURN_VALUE