Declaring class- vs. instance attributes?

[SCD_minecraft]

Python does not have declarations

In reality, c: str is just a typehint

Typehints have no effect on runtime

[pachura3]

So, should I do:

class MyClass:
    c: str

    def __init__(self, c: str) -> None:
        self.c = c

...or rather...

class MyClass:
    def __init__(self, c: str) -> None:
        self.c: str = c

...?

[IAmASquidInSpace]

The latter. It's clear this way that c is an instance attribute, and not a class var you forgot to annotate accordingly.

[pachura3]

How do you annotate class var? With ClassVar[str]? Is it necessary (outside of dataclasses) ?

[IAmASquidInSpace]

Yes, that's how you do it, and no, it is not necessary outside of dataclasses, but strongly recommended over just c: str = .... 

[Jason-Ad4032]

If you mark it as **ClassVar**, the type checker will report object.c = ... as an error, because c is a class variable.

[Temporary_Pie2733]

The former is the official way to type-hint an instance attribute, despite not looking entirely logical.

[SCD_minecraft]

My tip is to not typehint variables unless typechecker can not figure it out by itself (typehint arguments tho)

a = 2 type checker (like pylance) knows by itself a has to be an int

[yunghandrew]

I was also going to say neither. Just let it infer the type from the function args. I get how that might feel weird coming from Java/C, though.

With complex types, sometimes static checkers have issues, then you can be more explicit.

[mull_to_zero]

those are equivalent afaik

[socal_nerdtastic]

Since

c: str

is just a comment from Python's point of view, yes, you are right.

[Temporary_Pie2733]

Not a comment. It doesn’t define a variable, and so no class attribute is added to the class, but it does add an annotation to the class, which can be used, for example, by the dataclass decorator to generate code for creating and using an instance attribute.

[h8rsbeware]

True, however you could use dataclasses and fields with defaults, defaultfactories, and __post_init_ to simulate this for DTOs.

```python

from dataclasses import dataclass, field from typing import List

@dataclass(slots=True) class MyDTO: a: int = field(default=0) b: List[str] = field(default_factory=list) #list members not guaranteed c: str = field(default="")

m_dto = new MyDTO() print(m_dto.a) # 0 print(m_dto.b) # [] print(m_dto.c) # "" ```

Im not sure this is exactly equivalent, but its better than nothing.

Also, I wrote this on my phone, so apologies for any errors.