CODEWITHSUNDEEP
pythonsyntax
jason smith
jason smith

Reputation: 43

What is the use of commas in an assignment?

I was reading another page on fibonacci sequence:

How do I print a fibonacci sequence to the nth number in Python?

and I was wondering if anyone can break down what is happening and how this specific line works. I dont fully understand how "cur" and "i" is changing and such. 

cur, old, i = cur+old, cur, i+1

it was part of the fibonacci function

def fib(n):
    cur = 1
    old = 1
    i = 1
    while (i < n):
        cur, old, i = cur+old, cur, i+1
    return cur

Upvotes: 2

Views: 128

Answers (2)

user2555451
user2555451

Reputation:

The line you gave is equivalent to this:

cur, old, i = (cur+old, cur, i+1)

which is using a technique known as unpacking*.

Below is a demonstration:

>>> x, y, z = (1, 2, 3)  # Parenthesis are optional here
>>> x
1
>>> y
2
>>> z
3
>>>

In a longer form, your line is equivalent to this:

tmp = cur
cur = cur+old
old = tmp
i = i+1

which can be simplified to:

tmp = cur
cur += old
old = tmp
i += 1

*Note: Actually, it has quite a few names. In addition to unpacking, a very common one is multiple assignment. @user2864740 also mentioned two more names in his comment.

Upvotes: 3

Dolda2000
Dolda2000

Reputation: 25855

More generally speaking, a commatized list of l-values used as an l-value in Python unpacks an iterable from the right-hand side into the parts of the left-hand side.

In your case, this means that the right-hand side creates a three-tuple of the values cur+old, cur and i+1, which is then unpacked into cur, old and i, respectively, so that it is the same as saying:

old = cur
cur = cur + old
i = i + 1

However, it is more useful, since cur isn't clobbered before old has been assigned to. It can also be used much more generally -- the left-hand side can consist of any l-values and the right-hand side can be any iterable, and you can do things like these:

a, b, c = range(3)

or

d = [0] * 10
d[3], d[7] = 1, 2

or

e, f, g = d[2:5]

In Python3, you can also used asterisk expressions to unpack "the rest" of an iterable; for instance, like this

h, *i, j = range(5)
# h will be 0, j will be 4, and i will be [1, 2, 3]

That doesn't work in Python2, however.

For the details, this is covered in section 7.2 of the language reference.

Upvotes: 3

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