In [1]: a=-5

In [2]: b=-5

In [3]: a is b
Out[3]: True

In [4]: a=256

In [5]: b=256

In [6]: a is b
Out[6]: True

In [7]: a=1000

In [8]: b=1000

In [9]: a is b
Out[9]: False
intern机制处理空格一个单词的复用机会大，所以创建一次，有空格创建多次，但是字符串长度大于20，就不是创建一次了。
In [13]: a="abc"

In [14]: b="abc"

In [15]: a is b
Out[15]: True

In [16]: a="helloworld"

In [17]: b="helloworld"

In [18]: a is b
Out[18]: True

In [19]: a="hello world"

In [20]: b="hello world"

In [21]: a is b
Out[21]: False
 

s1 = "abcd"
s2 = "abcd"
print(s1 is s2)

s1 = "a" * 20
s2 = "a" * 20
print(s1 is s2)

s1 = "a" * 21
s2 = "a" * 21
print(s1 is s2)

s1 = "ab" * 10
s2 = "ab" * 10
print(s1 is s2)

s1 = "ab" * 11
s2 = "ab" * 11
print(s1 is s2)
# True
# True
# False
# True
# False

C1.b is C2.b
In [22]: a=1000

In [23]: b=1000

In [24]: a is b
Out[24]: False

In [25]: a=-1888

In [26]: b=-1888

In [27]: a is b
Out[27]: False

In [28]: 
c1 = 1000
d1 = 1000
print(c1 is d1) # True

class C1(object):
  a = 100
  b = 100
  c = 1000
  d = 1000


class C2(object):
  a = 100
  b = 1000


print(C1.a is C1.b) # True
print(C1.a is C2.a) # True
print(C1.c is C1.d) # True
print(C1.b is C2.b) # False