When dealing with more than one language, we look for similar approaches across languages. In Rust, there’s a keyword called enum to define and use enumerations. It’s easy to use, let’s look at this example from the official document:
enum IpAddrKind {
V4,
V6,
}
struct IpAddr {
kind: IpAddrKind,
address: String,
}
let home = IpAddr {
kind: IpAddrKind::V4,
address: String::from("127.0.0.1"),
};
let loopback = IpAddr {
kind: IpAddrKind::V6,
address: String::from("::1"),
};If we want to write this code in Python, we can define two classes and two variables simply. Let’s start with a simple alternative first:
class IpAddrKind:
V4 = 1
V6 = 2
class IpAddr:
def __init__(self, kind: IpAddrKind, address: str) -> None:
self.kind = kind
self.address = address
home = IpAddr(kind=IpAddrKind.V4, address="127.0.0.1")
loopback = IpAddr(kind=IpAddrKind.V6, address="::1")It’s absolutely not the same thing with the Rust one but it can help you to meet your expectations about keeping the parameter consistencies between the IP address objects and you can group or filter the objects when you need it. I could also use dataclass for IpAddr, it came with the version 3.7:
import dataclasses
@dataclasses.dataclass(frozen=True)
class IpAddrKind:
V4 = 1
V6 = 2
@dataclasses.dataclass
class IpAddr:
kind: IpAddrKind
address: strBut as you know, they’re not enumerated indeed. Let’s discuss why we need a specific keyword or a language feature for enumerations:
- The enumeration class should only do one thing. Currently it’s just a class, there’s nothing to explain that it’s an enumerator. Even if we set the values manually, we don’t know which are enumerated types. There’s no relationship between the members of the class.
- When I read the code, I should quickly understand that it’s an
enumwith unique constants, so I can use these constants to define types of some objects and compare them. In other words, if I have an enum type for colors and I want check the type of a color, it should be aColorfor example. Not integer, not string, I expect a custom enum type. - All enumerations have common basic functionalities. Members should be immutable, iterable, accessible by value and by name, and it should also be possible to get members as a list. I should not have to create a base class to add them to.
Actually there’s an Enum type since version 3.4. But the feeling of using enum in Python is like using an external package like dateutil. You’re importing Enum class, and inheriting it on your enum type. Let me show:
from enum import Enum
import dataclasses
class IpAddrKind(Enum):
V4 = 1
V6 = 2
@dataclasses.dataclass
class IpAddr:
kind: IpAddrKind
address: strI think it’s still better than using dataclass, because it gives some expectations of what we want, there’s a specific enum type, there are value and name attributes for each member, the values of members are immutable, if you try to redefine it, it will give an AttributeError, etc. All is fine.
>>> home = IpAddr(kind=IpAddrKind.V4, address="127.0.0.1")
# type type is enum.
>>> home.kind
<IpAddrKind.V4: 1>
>>> type(home.kind)
<enum 'IpAddrKind'>
# there's a value and the type of value is integer.
# the best practice is not using the value directly, because 1 is meaningless.
>>> home.kind.value
1
>>> type(home.kind.value)
<class 'int'>
# it's also possible to access to name of member.
# sometimes we need it if we're getting a data from an external service and want to remap the values, it depends.
>>> home.kind.name
'V4'Technically, we have achieved what we want, but let’s think about what we can do about usability. Do I have to assign a numerical value to the members? Partially yes, it’s not possible to define just member names without values, as in Rust. But there are some options:
from enum import auto, Enum
# option 1
class IpAddrKind(Enum):
V4, V6 = range(1, 3)
# option 2: functional way
IpAddrKind = Enum("IpAddrKind", "V4 V6")
# option 3: just assign the first value
class IpAddrKind(Enum):
V4 = 1
V6 = auto() # or you can leave it as `None`I’m not sure which option is better, I guess the first one doesn’t improve readability, option 2 is better than 1. Also I would prefer to write the values manually instead of auto() in option 3. But yeah, it depends again. It’s also possible to set the value of a member by its name. I haven’t written all the features of the enum module in this article, but it’s also possible to support bitwise operators using Flag instead of Enum, to determine the member values str instead of int using StrEnum, or to force the value types to be numbers using IntEnum, etc. See the official documentation for more information.
Before I conclude my article, I would like to add one last thing about enum use in Rust. I remember that it’s possible to use algebraic data types in Rust, but you probably won’t need it in Python. Because Rust is not a dynamically typed language and it forces you to work with types every time. Actually there’s a better example in Kobzol’s blog (see the section Algebraic data types) but let’s keep in the same sample here:
enum IpAddr {
V4(String),
V6(String),
}
let home = IpAddr::V4(String::from("127.0.0.1"));
let loopback = IpAddr::V6(String::from("::1"));How could we write it in Python? Should we use Enum or dataclass, or just a class?
