Python Variable Assignment on One line

Question

Why does this:

def make_fib():
    cur, next = 0, 1
    def fib():
        nonlocal cur, next
        result = cur
        cur, next = next, cur + next
        return result
    return fib

Work differently than:

def make_fib():
    cur, next = 0, 1
    def fib():
        nonlocal cur, next
        result = cur
        cur = next
        next = cur + next
        return result
    return fib

I see how the second one messes up because at cur = next and next = cur + next because essentially it will become next = next + next but why does the first one run differently?

@cricket_007 Yes that would work but how does the first one differ from the bottom one though. Shouldn't cur, next = next, cur + next essentially be the bottom one? — gymguy42
– gymguy42, Commented Oct 27, 2019 at 6:56
the right-hand side is evaluated first, then applied. So essentially, both assignments happen "at the same time" — njzk2
– njzk2, Commented Oct 27, 2019 at 6:58
@njzk2 Ok so they happen at the same time. What do you mean by the right hand side is evaluated first. Can you use my example? — gymguy42
– gymguy42, Commented Oct 27, 2019 at 7:00

njzk2 · Accepted Answer · 2019-10-27 07:00:11Z

6

cur, next = next, cur + next

is the same operation as:

# right-hand side operations
tmp_1 = next
tmp_2 = cur + next

# assignment
cur = tmp_1
next = tmp_2

Because the right-hand side is fully evaluated, and then the values are assigned to the left-hand side

answered Oct 27, 2019 at 7:00

njzk2

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2 Comments

gymguy42 Over a year ago

ty for your explanation

jirassimok Over a year ago

Here is the documentation reference for that: "An assignment statement evaluates the expression list (… this can be … a comma-separated list, … yielding a tuple)." There is also a caveat for cases like i, x[i] = 1, 2.

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