详解Python 函数参数的拆解

> a, b = [1, 2] # a, b = (1, 2) 也是一样的效果
> print(a, b)
1 2
> a, b = {'x': 1, 'y':2}
> print(a, b)
x y
> a, b = {'x': 1, 'y':2}.keys()
> print(a, b)
x y
> a, b = {'x': 1, 'y':2}.values()
> print(a, b)
1 2
> a, b = (x * x for x in range(2))
> print(a, b)
0 1

> [1, 2, *range(3), *[4, 5], *(6, 7)] # * 号能把集合打散，flatten(unwrap)
[1, 2, 0, 1, 2, 4, 5, 6, 7]
> {'x': 1, **{'y': 2, 'z': 3}}   # ** 把字典打散, flatten(unwrap) 操作
{'x': 1, 'y': 2, 'z': 3}

def print_vector(x, y, z):
 print('<%s, %s, %s>' % (x, y, z))

> list_vec = [2, 1, 3]
> print_vector(*list_vec)
<2, 1, 3>
> print_vector(*(2, 1, 3))
<2, 1, 3>
> dict_vec = {'y': 2, 'z': 1, 'x': 3}
> print_vector(*dict_vec) # 相当于 print_vector(*dict_vec.keys())
<y, z, x>
> print_vector(**dict_vec) # 相当于 print_vector(dict_vec['x'], dict_vec['y'], dict_vec['z']
<3, 2, 1>
> genexpr = (x * x for x in range(3))
> print_vector(*genexpr)
<0, 1, 4>
> print_vector(*dict_vec.values()) # 即 print_vector(*list(dict_vec.values()))
<2, 1, 3>

> print_vector(**{'y': 2, 'z': 1, 'x': 3})
<3, 2, 1>
> print_vector(**{'y': 2, 'z': 1, 'a': 3})  #元素个数是3 个，但出现 x, y, z 之外的 key
Traceback (most recent call last):
 File "<pyshell#39>", line 1, in <module>
 print_vector(**{'y': 2, 'z': 1, 'a': 3})
TypeError: print_vector() got an unexpected keyword argument 'a'
> print_vector(**{'y': 2, 'z': 1, 'x': 3, 'a': 4}) # 包含有 x, y, z, 但有四个元素，key 'a' 不能识别
Traceback (most recent call last):
 File "<pyshell#40>", line 1, in <module>
 print_vector(**{'y': 2, 'z': 1, 'x': 3, 'a': 4})
TypeError: print_vector() got an unexpected keyword argument 'a'
> print_vector(**{'y': 2, 'z': 1})   # 缺少 key 'x' 对应的元素
Traceback (most recent call last):
 File "<pyshell#41>", line 1, in <module>
 print_vector(**{'y': 2, 'z': 1})
TypeError: print_vector() missing 1 required positional argument: 'x'

> print_vector([2, 1, 3])
Traceback (most recent call last):
 File "<pyshell#44>", line 1, in <module>
 print_vector([2, 1, 3])
TypeError: print_vector() missing 2 required positional arguments: 'y' and 'z'

> print_vector(*[2, 1])    # 拆成了 x=2, y=1, 然后 z 呢？
Traceback (most recent call last):
 File "<pyshell#47>", line 1, in <module>
 print_vector(*[2, 1])
TypeError: print_vector() missing 1 required positional argument: 'z'
> print_vector(*[2, 1, 3, 4])  # 虽然拆出了 x=2, y=1, z=3, 但也别想强塞第四个元素给该函数(只定义的三个参数)
Traceback (most recent call last):
 File "<pyshell#48>", line 1, in <module>
 print_vector(*[2, 1, 3, 4])
TypeError: print_vector() takes 3 positional arguments but 4 were given

> a, b = [1]
Traceback (most recent call last):
 File "<pyshell#54>", line 1, in <module>
 a, b = [1]
ValueError: not enough values to unpack (expected 2, got 1)
> a, b = [1, 2, 3]
Traceback (most recent call last):
 File "<pyshell#55>", line 1, in <module>
 a, b = [1, 2, 3]
ValueError: too many values to unpack (expected 2)

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

line = '2020-06-19 22:14:00  2688 abc.json'
date, time, size, name = line.split() # 获得所有字段值
_, time, _, name = line.split()   # 只对 time 和 name 有兴趣
date, *_ = line.split()     # 只对第一个 date 有兴趣
*_, name = line.split()     # 只对最后的 name 有兴趣
date, *_, name = line.split()   # 对两边的 date, name 有兴趣