New to Python Trying to understand Syntax

Question

I am new to Python and I am trying to understand this syntax:

a, b = b, a + b

• miku: It's called parallel assignment.
• Gerrat: @miku: Neato - I've used the technique many times, but didn't realize there was a formal term for it.
• morningstar: lol. This is like an interview question. I kind of hope nobody actually writes the above code, but then, I think, probably I would. It would make more sense with real variable names at least.
• wim: @morningstar looks like a perfectly normal line of code to write, in a recursive fibonacci number function
• neur0n-7: This is called multiple assignment... it sets a to b, and b to a+b

Answer 1

We could rewrite this to: (a, b) = (b, a + b)

Considering that a = 3 and b = 6

The operation (b, a + b) returns a tupple (6, 9) and assigns these values to the listed variables (a, b) and assign (a = 6, b = 9).

So the final values are a = 6 and b = 9.

Answer 2

This is syntactic sugar for a python feature called 'unpacking'. It effectively means this:

(a, b) = (b, a + b) # This is also valid syntax

The tuple (b, a + b) is created, locking in the values. Then, the values are piece-wise assigned to the identifiers in the tuple (a, b). Since the values are locked in before assignment starts, each takes on the expected value. This idea is derived from pattern matching in Haskell.

Answer 3

This is called sequence unpacking. The right-hand side is packed into a tuple (because of the comma). Python packs then evaluates the right side, then unpacks those values into the left hand side.

See Tuples and Sequences.

Answer 4

a is given the value of b and b is given the value of a+b

A more technical explanation can be found here

Answer 5

Python has the ability to transfer multiple values at once. That means "set a to b, and b to the sum of a and b".

There is a more comprehensive explanation here.

• user447356: The link is dead, did you mean different page maybe?
• cwallenpoole: @ShadowWizard Nope. I meant that when I wrote it. Unfortunately, the link has since moved, but you can still find it with the wayback machine: web.archive.org/web/20100419171004/http://diveintopython.org/…

Answer 6

It is an assignment with a tuple-decomposition (or sequence unpacking) and might also be called a multiple assignment statement.

The tuple on the RIGHT is evaluated and then the result is assigned slot-for-slot to the variables on the LEFT. (The left side is not really a tuple even though the syntax makes it look like one.)

So,

a = 1
b = 2
a, b = b, a + b
// a, b = 2, 1 + 2 
// a, b = 2, 3
// a = 2
// b = 3

Happy coding.

Answer 7

it does two initializations in one line of code:

1. a = b
2. b = a + b

it's a alternative to use it as a swap() function in python by the way.