python oop class attribute returns None when set

Question

I'm trying to learn object orientated programming and when setting an attribute why does it return None?

Any tips on writing better code also appreciated... This is my class:

BEANS = ["black", "pinto"]
RICES = ["brown", "white"]
MEATS = ["chicken", "pork", "steak", "tofu"]


class Burrito:

    def __init__(self,
                meat,
                to_go,
                rice,
                beans,
                extra_meat=False,
                guacamole=False,
                cheese=False,
                pico=False,
                corn=False
                ):

        self.meat = self.setter("meat",meat)
        self.rice = self.setter("rice",rice)
        self.beans = self.setter("beans",beans)
        self.to_go = False
        self.extra_meat = False
        self.guacamole = False
        self.cheese = False
        self.pico = False
        self.corn = False

    def setter(self,category,attribute):

        print("category: ",category)
        print("attribute: ",attribute)

        if category == "meat":
            
            if attribute in MEATS:
                self.meat = attribute
            else:
                self.meat = False

        if category == "rice":
            
            if attribute in RICES:
                self.rice = attribute
            else:
                self.rice = False

        if category == "beans":
            
            if attribute in BEANS:
                self.beans = attribute

            else:
                self.beans = False
                
        else:
            self.category = "Error"

When I run this I expected the print to be False but returning None

noodle_burrito = Burrito("spagetti", True, True, False)
print("noodle_burrito.meat: ",noodle_burrito.meat)

And when I run this I expected the print to be tofu but returning None

vegg_burrito = Burrito("tofu", True, True, False)
print("vegg_burrito.meat: ",vegg_burrito.meat)

Thanks for any learning tips! Trying to reference this other SO post but I have never used super before.

score 0 · Answer 1 · answered Jul 23 '22 at 14:41

Your setter sets the attribute, but then returns None, which you assign to the attribute again, overwriting whatever the setter itself assigned. Just drop the assignment in __init__.

def __init__(self,
            meat,
            to_go,
            rice,
            beans,
            extra_meat=False,
            guacamole=False,
            cheese=False,
            pico=False,
            corn=False
            ):

    self.setter("meat",meat)
    self.setter("rice",rice)
    self.setter("beans",beans)
    self.to_go = False
    self.extra_meat = False
    self.guacamole = False
    self.cheese = False
    self.pico = False
    self.corn = False

However, there's no reason for the setter method at all. Just assign values directly to the attributes.

class Burrito:

    def __init__(self,
                meat,
                to_go,
                rice,
                beans,
                extra_meat=False,
                guacamole=False,
                cheese=False,
                pico=False,
                corn=False
                ):

        self.meat = meat
        self.rice = rice
        self.beans = beans
        self.to_go = to_go
        self.extra_meat = extra_meat
        self.guacamole = guacamole
        self.cheese = cheese
        self.pico = pico
        self.corn = corn

How would I filter for bad attributes, what I was trying to define in `MEATS`, `RICES` ,`BEANS`? — bbartling, Jul 23 '22 at 14:45
You don't. `__init__`'s job is to simply do what the caller asks; it's their job to pass the correct arguments, and on them if they fail to do so. (But see [AKX's answer](https://stackoverflow.com/a/73091635/1126841) for using a *different* function that *only* validates the arguments; I won't repeat it here.) — chepner, Jul 23 '22 at 14:47

score 0 · Accepted Answer · answered Jul 23 '22 at 14:42

0

Any function that doesn't explicitly return anything implicitly returns None.

Your setter function internally sets things, but has no other return statement, so

self.meat = self.setter("meat",meat)

ends up setting self.meat to None right after self.setter has already set it to something.

You're also not doing anything with more than half of the parameters your constructor accepts. I would write your class as

BEANS = {"black", "pinto"}
RICES = {"brown", "white"}
MEATS = {"chicken", "pork", "steak", "tofu"}


def validate_attr(value, options):
    if value in options:
        return value
    return False


class Burrito:
    def __init__(
        self,
        *,
        meat,
        rice,
        beans,
        to_go,
        extra_meat=False,
        guacamole=False,
        cheese=False,
        pico=False,
        corn=False,
    ):
        self.meat = validate_attr(meat, MEATS)
        self.rice = validate_attr(rice, RICES)
        self.beans = validate_attr(beans, BEANS)
        self.to_go = bool(to_go)
        self.extra_meat = bool(extra_meat)
        self.guacamole = bool(guacamole)
        self.cheese = bool(cheese)
        self.pico = bool(pico)
        self.corn = bool(corn)

answered Jul 23 '22 at 14:42

AKX

152,115
15
115
172

thank you for this... Still learning here : ) – bbartling Jul 23 '22 at 14:47
Ill hit the green check when I can, what is `*,`? Never seen that before. Wildcard? – bbartling Jul 23 '22 at 14:48
[In a function signature, it means the remaining arguments must be passed kwarg-only](https://peps.python.org/pep-3102/#specification), i.e. you need to do `Burrito(meat=..., rice=...)`, etc; for a function that has many arguments, it makes it harder to accidentally pass a rice for a meat, or cheese for guacamole (which would be a no-no for a lactose-intolerant person!) – AKX Jul 23 '22 at 14:49
In your answer, `RICES`,`MEATS`,`BEANS` I notice you use dictionary instead of list. Is that a better choice than a list? – bbartling Jul 23 '22 at 15:38
It's not a dictionary, it's a set. Sets are better when you don't care about the order of the items within, and want faster inclusion lookups. – AKX Jul 23 '22 at 15:52
Ah cool and with sets the items or keys all need to be unique right? That makes sense for this use case. – bbartling Jul 24 '22 at 12:04
Yep - sets are unique-itemed and unordered; like dicts without values. – AKX Jul 24 '22 at 13:10

python oop class attribute returns None when set

2 Answers2