What is this piece of Python code doing?

Question

This following is a snippet of Python code I found that solves a mathematical problem. What exactly is it doing? I wasn't too sure what to Google for.

x, y = x + 3 * y, 4 * x + 1 * y

Is this a special Python syntax?

Answer 1

x, y = x + 3 * y, 4 * x + 1 * y

is the equivalent of:

x = x + 3 * y
y = 4 * x + 1 * y

EXCEPT that it uses the original values for x and y in both calculations - because the new values for x and y aren't assigned until both calculations are complete.

The generic form is:

x,y = a,b

where a and b are expressions the values of which get assigned to x and y respectively. You can actually assign any tuple (set of comma-separated values) to any tuple of variables of the same size - for instance,

x,y,z = a,b,c

would also work, but

w,x,y,z = a,b,c

would not because the number of values in the right-hand tuple doesn't match the number of variables in the left-hand tuple.

Answer 2

It's an assignment to a tuple, also called sequence unpacking. Probably it's clearer when you add parenthesis around the tuples:

(x, y) = (x + 3 * y, 4 * x + 1 * y)

The value x + 3 * y is assigned to x and the value 4 * x + 1 * y is assigned to y.

It is equivalent to this:

x_new = x + 3 * y
y_new = 4 * x + 1 * y
x = x_new
y = y_new

Answer 3

I also recently saw this referred to as "simultaneous assignment", which seems to capture the spirit of several of the answers.