python - And-ing , OR-ing for Sets in python

Question

So I have

a=set(["a"])
b=set(["b"])

c=a&b would return a empty set, c=set([]) But when i type c = a and b, why does it give me a set(["b"])

What is the "and" and "or" doing when used in two sets?

Answer 1

A few things.

1. I'd recommend using set literal syntax if you're on Python 2.7 or above. You can just do a = {"a"}.
2. The keywords and and or are logical operators. They're used for Python's conditional logic. The | and & operators are bitwise; they are overloaded by the set() class. What you should be doing is a & b. You can read more here: http://docs.python.org/2/library/sets.html#set-objects

If you try a = 5 and 6 you will see the result is 6. That is how boolean operators work in many programming languages, including Python. It's essentially saying "check if the value on the left evaluates to True, if so, return the value on the right."

Answer 2

The answers here regarding or and and are a little wanting here. It's true that they're boolean operators and have nothing to do with sets, though. They're short-circuit boolean operators and are fairly commonly used as a short cut for assigning values with fallback values.

Here's how it works...

If you have a statement saying a or b and you had to evaluate if that whole statement was true, you'd first start by figuring out if a was true. If you found a was true, then you don't need to bother evaluating b because true or'ed with anything is always true. On the other hand, if a is false, you have to evaluate b to determine if the whole statement is false or not. Also, whatever b is will be the result of the whole a or b statement.

So, x = a or b will first test if a is true and if it is the whole right side of the statement will become a. Likewise, if a is false, the whole right side becomes b because b will determine if a or b is true since we know a is false.

and is also a short circuit operator. With a and b, if a is false we can stop evaluating and say the whole statement is false and ignore b. If a is true, the whole statement relies on the boolean value of b.

So, x = a and b will first test if a is true. If a is true, then the result of a and b can be simplified to b and then the statement becomes x = b. If a is false, then we can stop at a and just replace a and b with a making it x = a.

I don't think I've ever seen the use of and in this fashion, but it does work.

Answer 3

a and b is returning b. This is boolean logic, not a set intersection. In Python, the following are "falsy" objects: None, False, 0, "", [], tuple(), set([]), etc. Since a and b are both "truthy" objects, b is returned from a and b.

http://docs.python.org/2/tutorial/datastructures.html#more-on-conditions

One use of this is to initialize a variable like this:

def f(x):
    x = x or MyObject()

Answer 4

c=a&b returns the intersection of the two sets, which in this case is nothing. c=a and b uses the standard Python logical operators. So when you say c=a and b you are basically saying c=(a is True) and (b is True). Naively, since both a and b in this case are not empty and evaluate to True, you would expect c to be True. However, when you use the logical operators to set a value, Python returns not just True but the value of the last True object, which in this case is b.