Python运算符优先级

Question

我应该如何解释Python中的这句话（根据运算符优先级）？

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

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

感谢

Answer 1

使用dis模块：

>>> 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，它看起来像：

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

Answer 2

按照documentation的顺序，从最低优先级（最低结合），以最高优先（最具约束力）：

1. and
2. not
3. ==

所以表达式not a == 7 and b == 7将这样的评价：

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

换句话说，评估树看起来就像这样：

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

最后完成的事情是将表达式的值赋值给c。