python中赋值语句的特点和形式

>>> a = 'abc'          # 普通形式
>>> b = 123
>>> c = [1, 2, 3]

>>> (a, b) = ('1', '2')   # 元组赋值，括号可省略
>>> [a, b] = ['1', '2']   # 列表赋值
>>> a, b, c, d = 'spam'   # 字符串序列赋值
# python中交换两个变量的值可以不用穿件第三个临时变量
>>> x, y = 1, 2
>>> x, y = y, x
>>> x, y
(2, 1)
# 当等号左侧与右侧变量长度不一致时，考虑用分片解决
>>> num = '123'
>>> a, b = num[:1], num[1:]
>>> a, b
('1', '23')
>>> num = '123'
>>> a, b = list(num[:1]) + [num[1:]]
>>> a, b
('1', '23')
# 嵌套赋值
>>> ((a, b), c) = ('12','3')
>>> a, b, c
('1', '2', '3')

>>> num = [1, 2, 3, 4]
>>> a, b = num   # 变量个数不同会报错
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: too many values to unpack (expected 2)  
>>> a, *b = num   # 扩展序列解包
>>> a, b
(1, [2, 3, 4])
>>> first, *mid, last = num  # 扩展序列解包的*可以出现在任何位置
>>> first, mid, last
(1, [2, 3], 4)

>>> a = [1, 2, 3, 4]
>>> b, c, d, *e = a
>>> (b, c, d, e)
(1, 2, 3, [4])

>>> a = [1, 2, 3, 4]
>>> b, c, d, e, *f = a
>>> (b, c, d, e, f)
(1, 2, 3, 4, [])

>>> a = [1, 2, 3, 4]
>>> *b, c, *d = a
  File "<stdin>", line 1
SyntaxError: two starred expressions in assignment

>>> a = [1, 2, 3, 4]
>>> *b = a
  File "<stdin>", line 1
SyntaxError: starred assignment target must be in a list or tuple
>>> *b, = a
>>> b
[1, 2, 3, 4]

>>> a = b = c = 0
>>> (a, b, c)
(0, 0, 0)

>>> a = b = 1
>>> (a, b)
(1, 1)
>>> b = 2
>>> (a, b)
(1, 2)
# a的值没有因为b的改变而改变
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>>> a = b = [1, 2]
>>> (a, b)
([1, 2], [1, 2])
>>> b[1] = 3
>>> (a, b)
([1, 3], [1, 3])
# a的值随着b值得改变而改变，即使没有主动给a赋值，这是因为a，b同时引用了一个可变对象，改变该对象则会通知改变a和b的值