python水平怎么证明_Python中的“右求值”是如何证明的？

使用Python 3：>>> a = []

>>> f = print(

a.append(1), a[:],

a.append(2), a[:],

a.append(3), a[:]

)

None [1] None [1, 2] None [1, 2, 3]

存档：

^{pr2}$

奇怪的是(一开始)，这段代码产生了：None [1, 2, 3] None [1, 2, 3] None [1, 2, 3]

然而，dis(f)使这一点更加清楚：>>> dis(f)

1 0 LOAD_NAME 0 (print) #Loads the value of 'print' into memory. Precisely, the value is pushed to the TOS (Top of Stack)

--> 3 LOAD_NAME 1 (a) #Loads the value of object 'a'

6 LOAD_ATTR 2 (append) #Loads the append attribute (in this case method)

9 LOAD_CONST 0 (1) #Loads the constant 1

12 CALL_FUNCTION 1 #a.append(1) is called

15 LOAD_NAME 1 (a) #for print(...,a,...)

18 LOAD_NAME 1 (a) #for the next a.append()

21 LOAD_ATTR 2 (append)

24 LOAD_CONST 1 (2)

27 CALL_FUNCTION 1 #a.append(2)

30 LOAD_NAME 1 (a)

33 LOAD_NAME 1 (a)

36 LOAD_ATTR 2 (append)

39 LOAD_CONST 2 (3)

42 CALL_FUNCTION 1 #a.append(3)

45 LOAD_NAME 1 (a) #loads a to be used thrice by print

48 CALL_FUNCTION 6 #calls print

51 PRINT_EXPR #prints TOS and clears it

52 LOAD_CONST 3 (None) #Loads None

55 RETURN_VALUE #Returns None

dis(f)的输出是我们期望的-L-to-R求值。本质上，这种“差异”是最后一次评估print()的结果。到那时，a的值已更改为[1, 2, 3]，同一最终对象将被打印三次。在

如果我们用a替换a[:]，我们就得到了预期的结果。在