Python运算子优先

Question

我应该如何在Python中解释这句话（根据运算符的优先级）？

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

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

谢谢

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  

因此，它看起来像：

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

Answer 2

根据文档 ，顺序是从最低优先级（最低绑定）到最高优先级（最大绑定）：

1. and
2. not
3. ==

因此，表达式not a == 7 and b == 7计算方式如下：

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

换句话说，评估树将如下所示：

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

最后要做的就是将表达式的值赋给c 。