Set operator precedence

Question

In python, I am unable to understand the operator precedence.

a = set([1, 2, 3])

a|set([4])-set([2])

The above expression returns {1,2,3,4}. However, I thought the operator | shall be executed before - but this doesn't seem like happening.

When I apply parenthesis, it returns me the desired output, i.e. {1,3,4}

  (a|set([4]))-set([2])

So, my question is why is this happening and what is the operator (for set operators like -, |, &, ^, etc) precedence when applying set operations.

Answer 1

python operator precedence rules give priority to - operator and then to bitwise | operator:

Now we have a set with union, overloaded with |, and difference, oveloaded with -:

a = set([1, 2, 3])

a|set([4])-set([2])

The question now became: why do the same rules of precedence apply?

This is because python evaluates operator expressions applying the same rules precedence for all classes that overload the standard operators:

class Fagiolo:

    def __init__(self, val):
        self.val = val

    def __or__(self, other):
        return Fagiolo("({}+{})".format(self.val, other.val))

    def __sub__(self, other):
        return Fagiolo("({}-{})".format(self.val, other.val))

    def __str__(self):
        return self.val

red = Fagiolo("red")
white = Fagiolo("white")

new_born = red | white - Fagiolo("blue")

print(new_born)

gives:

(red+(white-blue))