python - is & ==

[junxnone]

Reference

• Python判断is与==
• python中判断为None
• Python 小坑之字符串驻留

Brief

name	Description
is	身份判断符，判断两个对象是否为同一对象
==	等值判断，判断两个对象是否内容相等

Example

• 所有长度为0和1的字符串都会被驻留
• 字符串在编译时被实现的会被驻留（如'str'会被驻留，但是 ''.join(['s', 't', 'r']) 不会）
• 字符串中只包含ASCII下的字母、数字和下划线时会被驻留. 所以'str!'由于包含!不会被驻留

In [1]: a = 'Test'
In [2]: b = 'Test'
In [3]: a is b
Out[3]: True
In [4]: a is 'Test'
Out[4]: True
In [5]: c = 'Test!'
In [6]: d = 'Test!'
In [7]: c is d
Out[7]: False
In [8]: c is 'Test!'
Out[8]: False
In [9]: print(id(a))
140400778717760
In [10]: print(id(b))
140400778717760
In [11]: print(id(c))
140400777527680
In [12]: print(id(d))
140400777530536
In [13]: c == d
Out[13]: True
In [14]: e = '!'
In [15]: f = '!'
In [16]: e is f
Out[16]: True

• 特殊字符不驻留

In [17]: print(id('Test!'))
140400777708800
In [18]: print(id('Test!'))
140400777710368
In [19]: print(id('Test!'))
140400776986552
In [20]: print(id('Test!'))
140400776969328

• None是一个特殊的常量，不同的 None 的id是一样的。

In [8]: c = None
In [9]: print(id(c))
10306432
In [10]: print(id(None))
10306432
In [11]: c is None
Out[11]: True

• 对于整数对象，如果其值处于[-5,256]的闭区间内，则值相同的对象是同一个对象，否则为不同对象

In [26]: print(id(1))
10914496
In [27]: print(id(1))
10914496
In [28]: print(id(1))
10914496
In [29]: print(id(257))
140400778124592
In [30]: print(id(257))
140400779126896
In [31]: print(id(257))
140400778124592
In [32]: print(id(257))
140400779127536
In [33]: print(id(257))
140400778125104