Why does x,y = zip(*zip(a,b)) work in Python?
OK I love Python's zip() function. Use it all the time, it's brilliant. Every now and again I want to do the opposite of zip() , think "I used to know how to do that", then google python unzip, then remember that one uses this magical * to unzip a zipped list of tuples. Like this:
x = [1,2,3]
y = [4,5,6]
zipped = zip(x,y)
unzipped_x, unzipped_y = zip(*zipped)
unzipped_x
Out[30]: (1, 2, 3)
unzipped_y
Out[31]: (4, 5, 6)
What on earth is going on? What is that magical asterisk doing? Where else can it be applied and what other amazing awesome things in Python are so mysterious and hard to google?
Python中的星号记录在Python教程中的Unpacking Argument Lists下。
The asterisk performs apply (as it's known in Lisp and Scheme). Basically, it takes your list, and calls the function with that list's contents as arguments.
It's also useful for multiple args:
def foo(*args):
print args
foo(1, 2, 3) # (1, 2, 3)
# also legal
t = (1, 2, 3)
foo(*t) # (1, 2, 3)
And, you can use double asterisk for keyword arguments and dictionaries:
def foo(**kwargs):
print kwargs
foo(a=1, b=2) # {'a': 1, 'b': 2}
# also legal
d = {"a": 1, "b": 2}
foo(**d) # {'a': 1, 'b': 2}
And of course, you can combine these:
def foo(*args, **kwargs):
print args, kwargs
foo(1, 2, a=3, b=4) # (1, 2) {'a': 3, 'b': 4}
Pretty neat and useful stuff.
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