Please explain a python zip and unpacking solution

问题:

This question already has an answer here:

• Why does x,y = zip(*zip(a,b)) work in Python? 7 answers

A Python 3 learner here:

The question had the following accepted answer:

rr,tt = zip(*[(i*10, i*12) for i in xrange(4)])

which returns two tuples. I'd be grateful if someone could break down the answer and explain what it is doing with Python 3 in mind ( I know the range() returns an iterator in Python 3). I understand list comprehensions but I'm confused about unpacking (I thought you could only used a starred expression as part of an assignment target).

I'm equally confused by the code below. I understand the outcome and zipping (or think I do) but again the asterisk expression has got me beat.

x2, y2 = zip(*zip(x, y))

from this:

>>> x = [1, 2, 3]
>>> y = [4, 5, 6]
>>> zipped = zip(x, y)
>>> list(zipped)
[(1, 4), (2, 5), (3, 6)]
>>> x2, y2 = zip(*zip(x, y))
>>> x == list(x2) and y == list(y2)
True

回答1:

*expression applies argument unpacking to the function call. It takes the expression part, which has to resolve to a sequence, and makes each element in that sequence a separate parameter to the function.

So, zip(x, y) returns a sequence, and each element in that sequence is made an argument to the outer zip() function.

For zip(*[(i*10, i*12) for i in xrange(4)]) it is perhaps a little clearer; there are several elements here:

• [(i*10, i*12) for i in xrange(4)] (should be range() in python 3) creates a list with 4 tuples, [(0, 0), (10, 12), (20, 24), (30, 36)]
• The zip(*...) part then takes each of those 4 tuples and passes those as arguments to the zip() function.
• zip() takes each tuple and pairs their elements; two elements per tuple means the result is 2 lists of 4 values each.

Because the zip() function return two sequences, they can be assigned to the two variables.

I would have found the following a far more readable version of the same expression:

rr, tt = tuple(range(0, 40, 10)), tuple(range(0, 48, 12))