python kwargs, 定义和调用函数时的*和**

python - What do * and ** before a variable name mean in a function signature? - Stack Overflow

# * collects all the positional arguments in a tuple.
# ** collects all the keyword arguments in a dictionary.
def fn(*a, **kw): print(a); print(kw)

fn(1, 2, 3, 4, a=2, c=5, b=3)
# (1, 2, 3, 4)
# {'a': 2, 'c': 5, 'b': 3}

# In a function call:
# * unpacks a list or tuple into position arguments.
# ** unpacks a dictionary into keyword arguments.
lis = [1, 2, 3, 4]; dic = {'a': 10, 'b':20}

fn(*lis, **dic) # Same as fn(1, 2, 3, 4, a=10, b=20)
# (1, 2, 3, 4)
# {'a': 10, 'b': 20}

fn(lis, dic)
# ([1, 2, 3, 4], {'a': 10, 'b': 20})
# {}

# list可以修改其中的元素，tuple不可以。list用[和]括起来，tuple用(和)。
# (1)不是tuple, (1,)是个只有1个元素的tuple.