[Python] Add [<ClassAttributes>] to control Python class generation

[dbrattli]

Overview

Introduce a new [<ClassAttributes>] attribute that provides Python-specific control over how F# types are transpiled to Python classes. This attribute implies member attachment (similar to AttachMembers) while offering Python-specific parameters for better integration with Python frameworks like Pydantic, dataclasses, and SQLAlchemy.

Current AttachMembers Behavior (For Context)

Without AttachMembers (Default)

type User() =
    member val Name: string = "" with get, set

Generates:

class User:
    def __init__(self, __unit: None=None) -> None:
        pass

def User_get_Name(self) -> str:
    return self._Name

def User_set_Name(self, value: str) -> None:
    self._Name = value

With AttachMembers (Current)

[<AttachMembers>]
type User() =
    member val Name: string = "" with get, set

Generates:

class User:
    def __init__(self, __unit: None=None) -> None:
        self._Name: str = ""

    @property
    def Name(self) -> str:
        return self._Name

    @Name.setter
    def Name(self, value: str) -> None:
        self._Name = value

---

New ClassAttributes Attribute

Basic Syntax

// Replaces [<AttachMembers>] for Python with same behavior (properties style)
[<ClassAttributes>]
type User() = ...

// New: Generate class attributes instead of @property
[<ClassAttributes(style="attributes")>]
type User() = ...

// With additional Python-specific parameters
[<ClassAttributes(style="attributes", init=true)>]
type Config() = ...

Style: "attributes" (New Feature)

F# Input

[<ClassAttributes(style="attributes")>]
type User() =
    member val Name: string = "" with get, set
    member val Age: int = 0 with get, set
    member val Email: string option = None with get, set

Generated Output

class User:
    Name: str
    Age: int
    Email: str | None

    def __init__(self, Name: str = "", Age: int = 0, Email: str | None = None):
        self.Name = Name
        self.Age = Age
        self.Email = Email

---

Parameter Reference

Parameter	Type	Default	Description
style	string	"properties"	"properties" (current behavior) or "attributes"
init	bool	true	Generate __init__ method

Parameter Examples

Suppress init Generation

[<ClassAttributes(style="attributes", init=false)>]
type Config() =
    member val Debug: bool = false with get, set
    member val Timeout: int = 30 with get, set

class Config:
    Debug: bool = False
    Timeout: int = 30
    # No __init__ method generated

[MangelMaxime]

@dbrattli We probably need to update the documentation too.

Also, wouldn't it be better to replace the string with a DU, for example something like:

[<ClassAttributes(style=ClassAttributesStyles.Attributes, init=false)>]

this way it avoid potential typo error?

[dbrattli]

But attribute values need afaik to be basic types. Then the alternative is to use an enum: https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/language-specification/attributes#2224-attribute-parameter-types

[MangelMaxime]

I though I remembered something on that line but didn't check 🙈

However, we can use an enum to achieve something similar to DUs. This is what we do for StringEnum for example

[<RequireQualifiedAccess>]
type ClassAttributeStyle =
    | Properties = 0
    | Attributes = 1

[<AttributeUsage(AttributeTargets.Class)>]
type ClassAttributes() =
    inherit Attribute()

    new(style: ClassAttributeStyle) = ClassAttributes()

    new(style: ClassAttributeStyle, init: bool) = ClassAttributes()

[<ClassAttributes(style = ClassAttributeStyle.Properties)>]
type MyClass () =
    class end

[dbrattli]

Thanks. Will fix