Multiple operators between operands

Question

Can someone explain why the Python interpreter (2.7.3) gives the following:

>>> 5 -+-+-+ 2

3

Is this ever useful, and for what purpose?

Answer 1

You can use dis here to see how the expression was actually evaluated:

In [29]: def func():
   ....:     return 5 -+-+-+ 2
   ....: 

In [30]: import dis

In [31]: dis.dis(func)
  2           0 LOAD_CONST               1 (5)
              3 LOAD_CONST               2 (2)
              6 UNARY_POSITIVE      
              7 UNARY_NEGATIVE      
              8 UNARY_POSITIVE      
              9 UNARY_NEGATIVE      
             10 UNARY_POSITIVE      
             11 BINARY_SUBTRACT     
             12 RETURN_VALUE        

So that expression is equivalent to this:

In [32]: 5 - (+(-(+(-(+(2))))))
Out[32]: 3

Answer 2

This is just equal to

5 - (+(-(+(-(+2)))))

where all the + and - beyond the first are unary operators. For numbers, + returns the operand unchanged. But, its meaning can be overridden using the __pos__ special method on your own classes.

It's useless obfuscation to do this with numbers, but it may be useful (if incredibly confusing) if you are using class instances with custom __neg__ and/or __pos__ operations.

---

You can abuse this to get C-like pre-increment -- and ++ operators. Don't ever actually do this.

class IncrementableInteger(object):
    def __init__(self, val=0):
        self.val = val
    def __pos__(self):
        class PlusOne:
            def __pos__(_self):
                self.val += 1
        return PlusOne()
    def __neg__(self):
        class MinusOne:
            def __neg__(_self):
                self.val -= 1
        return MinusOne()
    def __str__(self):
        return str(self.val)
    def __repr__(self):
        return repr(self.val)

Example:

>>> IncrementableInteger(4)
4
>>> v=_
>>> ++v
>>> v
5