What's going on with a Python assignment puzzle

August 21, 2015

Via @chneukirchen, I ran across this tweet:

Armin just came up with this puzzle, how well do you know obscure Python details? What's a after this statement?:
(a, b) = a[b] = {}, 5

This is best run interactively for maximum head-scratching. I had to run it in an interpreter myself and then think for a while, because there are several interesting Python things going on here.

Let's start by removing the middle assignment. That gives us:

(a, b) = {}, 5

This is Python's multiple variable assignment ('x, y = 10, 20') written to make the sequence nature of the variable names explicit (hence why the Python tutorial calls this 'sequence unpacking'). Writing the list of variables as an explicit tuple (or list) is optional but is something even I've done sometimes, although I think writing it this way has fallen out of favour. Thus it's equivalent to:

t = ({}, 5)
(a, b) = t

The next trick is that (somewhat to my surprise) when you're assigning a tuple to several variables at once (as 'x = y = 10') and doing sequence unpacking for one of those assignments, Python doesn't require you to do sequence unpacking for every assignment. The following is valid:

(a, b) = x = t

Here a and b become the individual elements of the tuple t while x is the whole tuple. I suppose this is a useful trick to remember if you sometimes want both the tuple and its elements for different purposes.

The next trick happening is that Python explicitly handles repeated variable assignment (sometimes called 'chained assignment' or 'serial assignment') in left to right order. So first the leftmost set of assignments are handled, and second the next leftmost, and so on. Here we only have two sets of assignments, so the entire statement is equivalent to the much more verbose form:

t = ({}, 5)
(a, b) = t
a[b] = t

(When you do this outside of a function, the first (leftmost) assignment also creates a and b as names, which means that the second (right) assignment then has them available to use and doesn't get a 'name is not defined' error.)

The final 'trick' is due to what variables mean in Python, which creates the recursion in a[b]'s value. The tuple t that winds up assigned to a[b] contains a reference to the dictionary that a becomes another reference to, which means that the tuple contains a dictionary that contains the tuple again and it's recursion all the way down.

(When you combine Python's name binding behavior with serial assignment like this, you can wind up with fun bugs.)

Written on 21 August 2015.
« Using abstract namespace Unix domain sockets and SO_PEERCRED in Python
What surprised me about the Python assignment puzzle »

Page tools: View Source, Add Comment.
Login: Password:
Atom Syndication: Recent Comments.

Last modified: Fri Aug 21 01:57:27 2015
This dinky wiki is brought to you by the Insane Hackers Guild, Python sub-branch.