Python運算符優先級

Question

我應該如何解釋Python中的這句話（根據運算符優先級）？

c = not a == 7 and b == 7 

爲c = not (a == 7 and b == 7)或c = (not a) == 7 and b == 7？

感謝

Answer 1

使用dis模塊：

>>> import dis 
>>> def func(): 
...  c = not a == 7 and b == 7 
...  
>>> dis.dis(func) 
    2   0 LOAD_GLOBAL    0 (a) 
       3 LOAD_CONST    1 (7) 
       6 COMPARE_OP    2 (==) 
       9 UNARY_NOT   
      10 JUMP_IF_FALSE_OR_POP 22 
      13 LOAD_GLOBAL    1 (b) 
      16 LOAD_CONST    1 (7) 
      19 COMPARE_OP    2 (==) 
     >> 22 STORE_FAST    0 (c) 
      25 LOAD_CONST    0 (None) 
      28 RETURN_VALUE 

所以，它看起來像：

c = (not(a == 7)) and (b == 7) 

Answer 2

按照documentation的順序，從最低優先級（最低結合），以最高優先（最具約束力）：

1. and
2. not
3. ==

所以表達式not a == 7 and b == 7將這樣的評價：

((not (a == 7)) and (b == 7)) 
^ ^ ^ ^
second first third first 

換句話說，評估樹看起來就像這樣：

 and 
    / \ 
    not == 
    | /\ 
    == b 7 
/\ 
    a 7 

最後完成的事情是將表達式的值賦值給c。