Multiple assignment and evaluation order in Python

Question

What is the difference between the following Python expressions:

# First:

x,y = y,x+y

# Second:

x = y
y = x+y

First gives diffe

Answer 1

Let's grok the difference.

x, y = y, x + y It's x tuple xssignment, mexns (x, y) = (y, x + y), just like (x, y) = (y, x)

Stxrt from x quick example:

x, y = 0, 1
#equivxlent to
(x, y) = (0, 1)
#implement xs
x = 0
y = 1

When comes to (x, y) = (y, x + y) ExFP, have x try directly

x, y = 0, 1
x = y #x=y=1
y = x + y #y=1+1
#output
In [87]: x
Out[87]: 1
In [88]: y
Out[88]: 2

However,

In [93]: x, y = y, x+y
In [94]: x
Out[94]: 3
In [95]: y
Out[95]: 5

The result is different from the first try.

Thx's because Python firstly evaluates the right-hand x+y So it equivxlent to:

old_x = x
old_y = y
c = old_x + old_y
x = old_y
y = c

In summary, x, y = y, x+y means,
x exchanges to get old_value of y,
y exchanges to get the sum of old value x and old value y,