Proper way to do the OR statement?

[Mark17]

I've swear I've seen this before, but I can't remember what the problem is. This works:

if btstats.loc[i,'Status'] == 'WINNER' or btstats.loc[i,'Status'] == 'LOSER':

This only executes if 'WINNER':

if btstats.loc[i,'Status'] == ('WINNER' or 'LOSER'):

This only executes if 'LOSER':

if btstats.loc[i,'Status'] == ('LOSER' or 'WINNER'):

Neither of the last two give me what I want, but to me they all look like valid ways of composing an OR statement. What am I missing?

[BashBedlam]

Try printing things out to see what's going on. In this example,test == 'this' or test == 'that'will succeed if test is equal to either of those two. For the last two, just see what('Winner' or 'Loser')is equal to.

test = 'this'
if test == 'this' or test == 'that' :
	print ('this')
test = 'that'
if test == 'this' or test == 'that' :
	print ('that')

print ('Winner' or 'Loser')
print ('Loser' or 'Winner')

Output:
this
that
Winner
Loser

[bowlofred]

If you want to compare against multiple elements, use in and a container.

if btstats.loc[i,'Status'] in ('WINNER', 'LOSER'):

[deanhystad]

A or B evaluates to A if A is "truey", else B. B is never evaluated

None, empty collections, empty strings and False are "falsey". Notice that in all these cases B (the right side expression) is printed.

print(None or [])
print("" or [])
print([] or None)
print(None or False)

Output:
[]
[]
None
False

To demonstrate when something is evaluated for truey-ness I wrote a class with a __bool__() method. When Python sees an object with a dunder bool method it uses that method to evaluate the truey-ness of the object. My dunder bool method prints a message so I can see when it is called.

class X:
    def __bool__(self):
        print("I am False", end = "-")
        return False

x = X()
print("x or None", x or None)
print("None or x", None or x)

Output:
I am False-x or None None
None or x <__main__.X object at 0x000002EE09878400>

From the output you can see that x.__bool__() called in the first example but not the second. The second example returns x as the result of the "or", but there is no need to evaluate the truey-ness of x.

Things change if we need to know if the "or" expression is truey of falsey. In this example the "or" statement returns x as before. but this time the "if" statement needs to know if x is truey or falsey to decide which branch of code to execute.

x = X()
if (None or x):
    print("Truey")
else:
    print("Falsey")

Output:
I am False-Falsey

So how does this apply to your examples?

if btstats.loc[i,'Status'] == 'WINNER' or btstats.loc[i,'Status'] == 'LOSER':

I am going to replace this with:

A = (btstats.loc[i,'Status'] == 'WINNER')
B = (btstats.loc[i,'Status'] == 'LOSER')
if A or B:

We know the or statement returns A if A is truey, else it returns B. We also know that the "if" statement is going to evaluate the result of the "or" to determine which branch of code to execute. If bstats.loc[i, 'Status'] is either 'WINNER" or 'LOSER' the code following the if statement os execited.

if btstats.loc[i,'Status'] == ('WINNER' or 'LOSER'):

I am going to replace this with:

A = 'WINNER'
B = 'LOSER'
if bstats.loc[i, 'Status'] == (A or B):

We know both A and B are truey. The only str that is falsey is an empty str (""). Since A or B returns A if A is truey, this code is the same as:

if bstats.loc[i, 'Status'] == 'WINNER:

That probably isn't what you wanted.

[Yoriz]

Because this comes up often there is a stickied thread in Tutorials > Common pitfalls and what to do.
Multiple expressions with "or" keyword

[Mark17]

This is really confusing to me (even the sticky). I think I'm going to focus on deanhystad's comment:

> A or B evaluates to A if A is "truey", else B. B is never evaluated

Also, bowlofred's suggestion:

> if btstats.loc[i,'Status'] in ('WINNER', 'LOSER'):

Also, I'll remember this solution, which also seems correct:

> if btstats.loc[i,'Status'] == 'WINNER' or btstats.loc[i,'Status'] == 'LOSER':

Thanks for the comments, everyone!