Question

I’m learning basic Boolean logic with Python. I understand the equality operators but get confused when ands and ors get thrown in. Take a look at the following code for example:

people = 20
cats = 30
dogs = 15

if people < cats:
   print "too many cats! the world is doomed!"

if people == cats or dogs == cats:
   print "this is too hard"

I understand the first if statement and why it prints. i don’t know how the second if statement is evaluated. How could I change it to get that line to print?

Thanks for your help!

Answer 1

and requires that all of its parts in the expression evaluate to True for the whole expression to be True.

or is much less picky, as soon as any part of the expression evaluates to True the whole expression is True.

You may come across the term short-circuiting in this context. It simply means that we stop evaluating expressions as soon as we can determine its outcome.

So for an and we can stop looking at the rest of the expression as soon as we find a part that evaluates to False because at that point the expression can never evaluate to True.

Likewise, with or, as soon as we find a part that’s True, we know the whole expression will be True and there won’t be a reason to look further.

For the code example you posted:

if people == cats or dogs == cats:
    print "this is too hard"

if either one of these, or both of people == cats, dogs == cats evaluated to True, the whole expression in the if statement would become True. Since neither of them are True, the expression in the if statement fails, i.e., is False, and the print statement is not executed.

Finally, if you wanted to print statement to execute, you’d have to ensure that the Boolean expression evaluated to True. One way would be to change to value of
dogs to 30 because then dogs == cats would be True and that would be
sufficient to make the whole expression True and get the print to execute.