Answer to print(9 +-+-+-+ 2) in Python?

[Vajeri]

Body

why does print(9 +-+-+-+ 2) give an answer of 7 in python?

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[CopperEagle]

Hey @Vajeri , funky question :)

In the +-+-+-+ string, the leftmost is treated as binary operator (taking two arguments) and the others modify the sign of 2.

Starting with a simpler example:

a = 9 +- 2
a == 9 + (-2) == 7

Then, getting the result of the expression you provide is like solving one of these algebra exercises from school:

a = 9 +-+-+-+ 2
  = 9 + (- (+ (- (+ (- (+ 2))))))
  = 9 (- (+ (- (+ (- 2)))))
  = 9 (- (+ (- (- 2))))
  = 9 (- (+ 2))
  = 9 - 2 == 7

If this answers your question, please consider marking it as the answer :)

[spenserblack]

Let's break it down a bit:

print(9 +- 7)

The spacing makes it confusing, but it's 9 plus negative 7 (-7).

print(9 + -7)

Now the interesting thing is that +n and -n are unary operators. It's the same as calling n.__pos__() and n.__neg__(). So 9 + -7 is the same as

9 + (2).__neg__()

When the -2 is lead by an extra +, you're calling the positive unary operator on the result of -2, so you get

9 + (2).__neg__().__pos__()

And all the other -+ are just extending that chain.

Also, even addition is a function call.

(9).__add__((2).__neg__().__pos__())

So, you can write what you have as this to make it a bit more readable:

left = 9
right = -+-+-+ 2
print(left + right)  # 7

Here's an experiment:

class MyInt(int):
    """
    This class acts like an int, except it will print the function that gets called.
    """
    def __add__(self, other):
        print('add!')
        return MyInt(super().__add__(other))

    def __neg__(self):
        print('neg!')
        return MyInt(super().__neg__())

    def __pos__(self):
        print('pos!')
        return MyInt(super().__pos__())

print(MyInt(9) +-+-+-+ MyInt(2))