What is this Python magic?

Question

If you do this {k:v for k,v in zip(*[iter(x)]*2)} where x is a list of whatever, you'll get a dictionary with all the odd elements as keys and even ones as their values. woah!

>>> x = [1, "cat", "hat", 35,2.5, True]
>>> d = {k:v for k,v in zip(*[iter(x)]*2)}
>>> d
{1: "cat", "hat": 35, 2.5: True}

I have a basic understanding of how dictionary comprehensions work, how zip works, how * extracts arguments, how [iter(x)]*2 concatenates two copies of the list, and so I was expecting a one-to-one correspondence like {1: 1, "cat": "cat" ...}.

What's going on here?

Answer 1

This is an interesting little piece of code for sure! The main thing it utilizes that you might not expect is that objects are, in effect, passed by reference (they're actually passed by assignment, but hey). iter() constructs an object, so "copying" it (using multiplication on a list, in this case) doesn't create a new one, but rather adds another reference to the same one. That means you have a list where l[0] is an iterator, and l[1] is the same iterator - accessing them both accesses the very same object.

Every time the next element of the iterator is accessed, it continues where it last left off. Since elements are accessed alternately between the first and second elements of the tuples that zip() creates, the single iterator's state is advanced across both elements in the tuple.

After that, the dictionary comprehension simply consumes these pair tuples as they expand to k, v - as they would in any other dictionary comprehension.