如何理解Python中复杂的解包表达式？

理解复杂的拆包表达式

考虑以下表达式：

a, b = 1, 2                          # 简单序列赋值
a, b = ['green', 'blue']             # 列表赋值
a, b = 'XY'                          # 字符串赋值
a, b = range(1,5,2)                  # 任何可迭代对象都可以

                                     # 嵌套序列赋值

(a,b), c = "XY", "Z"                 # a = 'X', b = 'Y', c = 'Z' 

(a,b), c = "XYZ"                     # 错误 -- 太多值需要拆包
(a,b), c = "XY"                      # 错误 -- 需要拆包的数值太少

(a,b), c, = [1,2],'this'             # a = '1', b = '2', c = 'this'
(a,b), (c,) = [1,2],'this'           # 错误 -- 太多值需要拆包


                                     # 扩展序列拆包

a, *b = 1,2,3,4,5                    # a = 1, b = [2,3,4,5]
*a, b = 1,2,3,4,5                    # a = [1,2,3,4], b = 5
a, *b, c = 1,2,3,4,5                 # a = 1, b = [2,3,4], c = 5

a, *b = 'X'                          # a = 'X', b = []
*a, b = 'X'                          # a = [], b = 'X'
a, *b, c = "XY"                      # a = 'X', b = [], c = 'Y'
a, *b, c = "X...Y"                   # a = 'X', b = ['.','.','.'], c = 'Y'

a, b, *c = 1,2,3                     # a = 1, b = 2, c = [3]
a, b, c, *d = 1,2,3                  # a = 1, b = 2, c = 3, d = []

a, *b, c, *d = 1,2,3,4,5             # 错误 -- 赋值中出现了两个星号表达式

(a,b), c = [1,2],'this'              # a = '1', b = '2', c = 'this'
(a,b), *c = [1,2],'this'             # a = '1', b = '2', c = ['this']

(a,b), c, *d = [1,2],'this'          # a = '1', b = '2', c = 'this', d = []
(a,b), *c, d = [1,2],'this'          # a = '1', b = '2', c = [], d = 'this'

(a,b), (c, *d) = [1,2],'this'        # a = '1', b = '2', c = 't', d = ['h', 'i', 's']

*a = 1                               # 错误 -- 目标必须在一个列表或元组中
*a = (1,2)                           # 错误 -- 目标必须在一个列表或元组中
*a, = (1,2)                          # a = [1,2]
*a, = 1                              # 错误 -- 'int' 对象不可迭代
*a, = [1]                            # a = [1]
*a = [1]                             # 错误 -- 目标必须在一个列表或元组中
*a, = (1,)                           # a = [1]
*a, = (1)                            # 错误 -- 'int' 对象不可迭代

*a, b = [1]                          # a = [], b = 1
*a, b = (1,)                         # a = [], b = 1

(a,b),c = 1,2,3                      # 错误 -- 太多值需要拆包
(a,b), *c = 1,2,3                    # 错误 - 'int' 对象不可迭代
(a,b), *c = 'XY', 2, 3               # a = 'X', b = 'Y', c = [2,3]


                                     # 扩展序列拆包 -- 嵌套

(a,b),c = 1,2,3                      # 错误 -- 太多值需要拆包
*(a,b), c = 1,2,3                    # a = 1, b = 2, c = 3

*(a,b) = 1,2                         # 错误 -- 目标必须在一个列表或元组中
*(a,b), = 1,2                        # a = 1, b = 2

*(a,b) = 'XY'                        # 错误 -- 目标必须在一个列表或元组中
*(a,b), = 'XY'                       # a = 'X', b = 'Y'

*(a, b) = 'this'                     # 错误 -- 目标必须在一个列表或元组中
*(a, b), = 'this'                    # 错误 -- 拆包的值太多了
*(a, *b), = 'this'                   # a = 't', b = ['h', 'i', 's']

*(a, *b), c = 'this'                 # a = 't', b = ['h', 'i'], c = 's'

*(a,*b), = 1,2,3,3,4,5,6,7           # a = 1, b = [2, 3, 3, 4, 5, 6, 7]

*(a,*b), *c = 1,2,3,3,4,5,6,7        # 错误 -- 赋值中出现了两个星号表达式
*(a,*b), (*c,) = 1,2,3,3,4,5,6,7     # 错误 -- 'int' 对象不可迭代
*(a,*b), c = 1,2,3,3,4,5,6,7         # a = 1, b = [2, 3, 3, 4, 5, 6], c = 7
*(a,*b), (*c,) = 1,2,3,4,5,'XY'      # a = 1, b = [2, 3, 4, 5], c = ['X', 'Y']

*(a,*b), c, d = 1,2,3,3,4,5,6,7      # a = 1, b = [2, 3, 3, 4, 5], c = 6, d = 7
*(a,*b), (c, d) = 1,2,3,3,4,5,6,7    # 错误 -- 'int' 对象不可迭代
*(a,*b), (*c, d) = 1,2,3,3,4,5,6,7   # 错误 -- 'int' 对象不可迭代
*(a,*b), *(c, d) = 1,2,3,3,4,5,6,7   # 错误 -- 赋值中出现了两个星号表达式


*(a,b), c = 'XY', 3                  # 错误 -- 需要拆包的数值太少
*(*a,b), c = 'XY', 3                 # a = [], b = 'XY', c = 3
(a,b), c = 'XY', 3                   # a = 'X', b = 'Y', c = 3

*(a,b), c = 'XY', 3, 4               # a = 'XY', b = 3, c = 4
*(*a,b), c = 'XY', 3, 4              # a = ['XY'], b = 3, c = 4
(a,b), c = 'XY', 3, 4                # 错误 -- 拆包的值太多了

手动推导出这些表达式的结果

掌握一些基本规则后，对其进行概括并不难。我将尽我最大的努力通过一些示例进行解释。由于你提到了“手动”计算这些表达式，我会建议一些简单的替换规则。基本上，你会发现理解所有可迭代对象以相同方式格式化的表达式会更容易一些。

仅为拆包目的，以下替换在右值（即可迭代对象）中有效：

'XY' -> ('X', 'Y')
['X', 'Y'] -> ('X', 'Y')

如果你发现某个值未被拆包，那么你需要撤销该替换。（有关进一步的解释，请参见下文。）

此外，当你看到“裸露的”逗号时，想象最顶层有一个元组。在左右两侧（即可迭代对象和右可迭代对象）执行此操作：

'X', 'Y' -> ('X', 'Y')
a, b -> (a, b)

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