为什么结果不同？

Question

我举了一个例子：

a = 200 + 56
b = 256
c = 200 + 57
d = 257

print('%r == %r: %r' % (a, b, a == b))
print('%r is %r: %r' % (a, b, a is b))
print('%r == %r: %r' % (c, d, c == d))
print('%r is %r: %r' % (c, d, c is d))

# Result:

# 256 == 256: True
# 256 is 256: True
# 257 == 257: True
# 257 is 257: False

请为我解释，为什么257 is 257: False？ （256 is 256: True）