s1 = 'janes@！#*ewq'
s2 = 'janes@！#*ewq'
print(s1 is s2)	 # True 

a1 = 'janes45613256132!@#$%#^%@$%' * 1
b1 = 'janes45613256132!@#$%#^%@$%' * 1
print(a1 is b1) # True

s1 = 'hah_' * 6
s2 = 'hah_' * 6
print(s1 is s2) # True

# cmd, -5~256 的小整數(shù)雖然不在同一代碼塊中, 但是它們適用小數(shù)據(jù)池機制
>>>a = 245
>>>b = 245
>>>a is b # True

# 長度為0與1的字符串一定會被駐留;
# 字符串駐留發(fā)生在程序編譯時;
# 被駐留的字符串必須由 ASCll字母, 數(shù)字以及下劃線組成;
>>>s1 = '@'
>>>s2 = '@'
>>>s1 is s2 # True

>>>s1 = ''
>>>s2 = ''
>>>s1 is s2 # True

>>>s1 = 'a_b_c'
>>>s2 = 'a_b_c'
>>>s1 is s2 # True

>>>s1 = 'a b_c'
>>>s2 = 'a b_c'
>>>s1 is s2 # False

>>>s1 = 'a_b_c' * 1
>>>s2 = 'a_b_c' * 1
>>>s1 is s2 # True

>>>s1 = 'abd_d23' * 3
>>>s2 = 'abd_d23' * 3
>>>s1 is s2 # True

>>>a, b = "some_thing!", "some_thing!"
>>>a is b # False

>>>a, b = "some_thing", "some_thing"
>>>a is b # True

a1 = 1000
b1 = 1000
a1 is b1 # 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.c is C2.b)  # False

# 3.7.5, ipython7.18.1
a = -5
b = -5
a is b # True

a = -6
b = -6
a is b # False

a = 256
b = 256
a is b # True

a = 257
b = 257
a is b # Flase

# cmd 終端
a = 1000
b = 1000
a is b # False
--------------------
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 ?? 難道 cmd 中也有大整數(shù)池 ?? 類加載的時候是在一塊內(nèi)存中,同值同地址 ?? 
print(C1.c is C2.b)  # False

# pycharm 等編輯器中
a = 1000
b = 1000
a is b # 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.c is C2.b)  # False

# cmd 中浮點數(shù)沒有被緩存
a = 1.0
b = 1.0
a is b # False

# cmd 中并非全部的字符串都會采用intern機制; 僅 包括下劃線、數(shù)字、字母的字符串才會被 intern--類標識符
s1="hello"
s2="hello"
s1 is s2 # True

# 如果有空格，默認不啟用intern機制
s1="hell o"
s2="hell o"
s1 is s2 # False

s1 = "hell!*o"
s2 = "hell!*o"
print(s1 is s2) # False

# 如果一個字符串長度超過20個字符，不啟動intern機制 -- 看網(wǎng)上很多都是這么寫的, 不超過二十個就為真，但是我在自己 3.7/8.5 版本上試了一下，發(fā)現(xiàn)好像沒有限制，不知道是 Python 更新了，還是什么問題……
s1 = "a" * 20
s2 = "a" * 20
s1 is s2 # True

s1 = "a" * 21
s2 = "a" * 21
s1 is s2 # True

s1 = "ab" * 10
s2 = "ab" * 10
s1 is s2 # True

s1 = "ab" * 11
s2 = "ab" * 11
s1 is s2 # True

# 'kz' + 'c' 編譯時已經(jīng)變成 'kzc'，而 s1 + 'c' 中 s1 是變量, 會在運行時進行拼接,所以沒有被intern?
'kz' + 'c' is 'kzc' # True

s1 = 'kz'
s2 = 'kzc'
s1+'c' is 'kzc' # False

# pycharm 等編輯器中，只要是同一個字符串，都為 True，并不用是下劃線、數(shù)字、字母的字符串
s1 = "hell o"
s2 = "hell o"
print(s1 is s2) # True

s1 = "hell!*o"
s2 = "hell!*o"
print(s1 is s2) # True

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

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

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

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

'kz' + 'c' is 'kzc' # True

s1 = 'kz'
s2 = 'kzc'
s1+'c' is 'kzc' # False

# 編輯器中，float 也被緩存了
a = 1.0
b = 1.0
a is b