Very good idea, I support this and will use it from now on (even though I don't like the (thing,) Python syntax very much), as it's the only way to say that the iterable should only have one element.
[thing] = stuff
works too. I think I like that even better.
There doesn't appear to be any difference in speed between the two. Assigning the result to one variable (totally different to this) is about 6% faster, so this syntax is what I'll use, thank you!
On your exact machine with your exact version of Python, today.
Please don't let microbenchmarks dictate what code you write. If you need a 6% performance gain in a microbenchmark, you've chosen the wrong language to use. Python is about readability and maintainability, not syntax hacks to make some benchmark slightly faster.
thing, = [stuff]
though some people think that it's less legible since it has a completely different meaning without the comma.
EDIT: you can even play with the whitespace and pretend that ",=" is a list->scalar conversion operator:
x ,= [1]