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.