求問Python的一個列表賦值問題

Question

為什麼

s = [1, 2, 3, 4, 5, 6]
i = 0
i = s[i] = 3
結果是: [1, 2, 3, 3, 5, 6] 而非[3, 2, 3, 4, 5, 6]

Answer 1

可以參考下寫過的一篇文章: Python: 鍊式賦值的坑

Answer 2

根據 Assignment statements 可知:

a = b = c = d = ... = E

等同於

a = E
b = E
c = E
d = E
...

所以: i=s[i]=3等同於:

i = 3
s[i] = 3

Python 中的賦值是一個語句,並不是一個操作子,所以表達式(a=b)會產生語法錯誤,賦值語句沒有值.

Answer 3

參考 https://stackoverflow.com/que...

等同於

s = [1, 2, 3, 4, 5, 6]
i = 0
temp_value = 3
i = temp_value
s[i] = temp_value

首先是i變成了3，然後才賦值s[i]

Answer 4

看結果反推，是i=3 在 s[i] = 3之前執行了。

你就分開兩句寫不行嗎？

Answer 5

可使用 PythonTutor.com
i = s[i] = 3 那一行基本上就是先後執行i=3 及s[i]=3

Answer 6

i = s[i] = 3 等價於 i = 3; s[i] = 3

用dis模組來解析執行過程：

>>> def f():
    s = [1, 2, 3, 4, 5, 6]
    i = 0
    i = s[i] = 3

    
>>> import dis
>>> dis.dis(f)
  2           0 LOAD_CONST               1 (1)
              3 LOAD_CONST               2 (2)
              6 LOAD_CONST               3 (3)
              9 LOAD_CONST               4 (4)
             12 LOAD_CONST               5 (5)
             15 LOAD_CONST               6 (6)
             18 BUILD_LIST               6
             21 STORE_FAST               0 (s) # s = [1, 2, 3, 4, 5, 6]

  3          24 LOAD_CONST               7 (0)
             27 STORE_FAST               1 (i) # i = 0

  4          30 LOAD_CONST               3 (3) # 常量3 入栈
             33 DUP_TOP                        # 复制栈顶，也就是 常量3
             34 STORE_FAST               1 (i) # i = 3
             37 LOAD_FAST                0 (s)
             40 LOAD_FAST                1 (i)
             43 STORE_SUBSCR                   # s[i] = 3
             44 LOAD_CONST               0 (None) # 返回 None
             47 RETURN_VALUE

分開寫的範例

>>> def f2():
    s = [1, 2, 3, 4, 5, 6]
    i = 0
    i = 3
    s[i] = 3

    
>>> dis.dis(f2)
  2           0 LOAD_CONST               1 (1)
              3 LOAD_CONST               2 (2)
              6 LOAD_CONST               3 (3)
              9 LOAD_CONST               4 (4)
             12 LOAD_CONST               5 (5)
             15 LOAD_CONST               6 (6)
             18 BUILD_LIST               6
             21 STORE_FAST               0 (s) # s = [1, 2, 3, 4, 5, 6]

  3          24 LOAD_CONST               7 (0)
             27 STORE_FAST               1 (i) # i = 0

  4          30 LOAD_CONST               3 (3)
             33 STORE_FAST               1 (i) # i = 3

  5          36 LOAD_CONST               3 (3)
             39 LOAD_FAST                0 (s)
             42 LOAD_FAST                1 (i)
             45 STORE_SUBSCR                   # s[i] = 3
             46 LOAD_CONST               0 (None)
             49 RETURN_VALUE
>>>