Unpacking? strange commas in python

Question

I was looking at some code that returns the second largest element of a list and came across a weird use of commas. Hopefully someone can explain it to me:

it is the

m1, m2 = x, m1

part of the following code:

def second_largest(numbers):
    m1, m2 = None, None

    for x in numbers:
        if x >= m1:
            m1, m2 = x, m1
        elif x > m2:
           m2 = x

    return m2

What is getting assigned what in this if statement?

x is getting assigned to m1 and m1 to m2, in unison. — voithos
– voithos, Commented Jul 23, 2013 at 18:26
This is poor coding for the exact reason why it wasn't entirely clear to you what was happening here at first glance. Although it's a line saver, it's less apparent what you're trying to do and makes your code harder to read. — scohe001
– scohe001, Commented Jul 23, 2013 at 18:29
I'd say what makes it hard to read is the variable names m1 and m2. The multiple assignment is standard Python. — user2357112
– user2357112, Commented Jul 23, 2013 at 18:39
Multiple assignments are standard in python, I agree, but not like that, more like x, y, z = some_tuple. If you're setting two variables to two other variables I was taught it's improper to try to squish it all into one line — scohe001
– scohe001, Commented Jul 23, 2013 at 19:01

Blair · Accepted Answer · 2013-07-23 18:26:16Z

3

Essentially, the tuple (m1,m2) is recieving the values in the tuple (x,m1). After the statement m1 will have the old value of x and m2 will have the old value of m1. Example:

>>> x = 2
>>> y = 3
>>> z = 4
>>> x,y = y,z
>>> x
3
>>> y
4

The tuple (x,m1) is created before any assignments are made. As a result, this syntax is often used for swapping two variables. For example, x,y = y,x will swap the values in x and y.

answered Jul 23, 2013 at 18:26

Blair

6,7031 gold badge38 silver badges42 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

jh314 · Accepted Answer · 2013-07-23 18:26:21Z

2

This code: m1, m2 = x, m1 means storing the value of x to m1, and value of m1 to m2.

answered Jul 23, 2013 at 18:26

jh314

27.9k16 gold badges66 silver badges83 bronze badges

Collectives™ on Stack Overflow

Unpacking? strange commas in python

2 Answers 2

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related