[Python] Nasty in-place operators trick... 😏

Question

I've been experimenting with a dataset lately and I've used algebraic in-place operators. I've been receiving incorrect output and I couldn't quite find the error. Suddenly it hit me... but let's start from the beginning:

We all know that: x *= 2 is equivalent to: x = x * 2

For example:
x = y = 4
x*=2
y = y * 2
print(x, y)
>>>
8 8

But the equivalence ends here, since:
x = y = 4
x*=2 + 1
y = y * 2 + 1
print(x, y)
>>>
12 9

See here:
https://code.sololearn.com/c6U6nrEn3DR0/

Consider yourself warned! 😉

Answer 1

That's quite expected result for every programmer but not for mathematician, because priority of *= operation (any other of this type) is low and:
variable operator= expression
is equivalent to
variable = variable operator (expression)
But I upvoted it anyway to spread this information. :-)
BTW, you can add such task in Python game challenges, that will be a good one!

Answer 2

Nevfy, Petya Thanks for the replies, never had encountered it before, so that's why I was a bit astounded :)
All clear now ;)

Answer 3

All ok, because precedence of *= lower than precedence of *, so, and all calculation from right to left

Answer 4

Wow, that’s... not what math says. I wonder how a calculator would handle this? I did the math in my head, weird.

Edit: the x*=2+1 is more like x(2+1), or 4(3), huh, guess it’s time to confuse some math teachers.

Answer 5

it follows the BODMAS rule

Answer 6

Yeah, it works on differently global list x, if you put x+＝a, x＝x+a in a function and input list x in func(x) where list a contains several members.