[Python] Add Decorate attribute

[dbrattli]

Overview

Introduce a new [<Decorate>] attribute that adds Python decorators to generated classes, enabling integration with Python frameworks like dataclasses, attrs, functools, and any other decorator-based libraries.

Purpose

F# doesn't have a direct equivalent to Python decorators, but they're essential for integrating with the Python ecosystem. The [<Decorate>] attribute bridges this gap by allowing F# developers to specify Python decorators that should be applied to the generated class.

⚠️ Important: Python Interop Only

The [<Decorate>] attribute is purely for Python interop and does NOT affect F# compilation behavior.

What it DOES (Python interop):

• Adds decorators to the generated Python code
• Enables integration with Python libraries that require decorators
• Makes the transpiled Python work with Python ecosystem tools

What it DOESN'T do (F# side):

• Does NOT change how your F# code compiles or behaves
• Does NOT add decorator functionality to F# code
• Does NOT make Python decorator features available in F# compilation

[<Decorate("dataclasses.dataclass")>]
type User() =
    member val Name: string = "" with get, set

// This is still regular F# - no dataclass behavior here
let user = User()
user.Name <- "Alice"  // Standard F# property access

// The @dataclass behavior only exists in the generated Python

Basic Usage

Simple Decorator

F# Input

[<Decorate("dataclasses.dataclass")>]
type User() =
    member val Name: string = "" with get, set
    member val Age: int = 0 with get, set

Generated Output

from dataclasses import dataclass

@dataclass
class User:
    Name: str = ""
    Age: int = 0

Multiple Decorators

F# Input

[<Decorate("dataclasses.dataclass")>]
[<Decorate("functools.total_ordering")>]
type User() =
    member val Name: string = "" with get, set

Generated Output

from dataclasses import dataclass
from functools import total_ordering

@total_ordering
@dataclass
class User:
    Name: str = ""

---

Decorators with Parameters

Basic Parameters

F# Input

[<Decorate("functools.lru_cache", "maxsize=128")>]
type CachedClass() =
    member val Value: string = "" with get, set

Generated Output

from functools import lru_cache

@lru_cache(maxsize=128)
class CachedClass:
    Value: str = ""

Complex Parameters

F# Input

[<Decorate("attrs.define", "auto_attribs=True, slots=True")>]
type OptimizedClass() =
    member val Data: string = "" with get, set

Generated Output

import attrs

@attrs.define(auto_attribs=True, slots=True)
class OptimizedClass:
    Data: str = ""

---

Framework Integration Examples

Dataclasses

[<Decorate("dataclasses.dataclass")>]
type Point() =
    member val X: float = 0.0 with get, set
    member val Y: float = 0.0 with get, set

from dataclasses import dataclass

@dataclass
class Point:
    X: float = 0.0
    Y: float = 0.0

Attrs Library

[<Decorate("attrs.frozen")>]
type ImmutablePoint() =
    member val X: float = 0.0 with get, set
    member val Y: float = 0.0 with get, set

import attrs

@attrs.frozen
class ImmutablePoint:
    X: float = 0.0
    Y: float = 0.0

[MangelMaxime]

@dbrattli We probably need to update the documentation on Fable website too

[dbrattli]

@MangelMaxime Yes, I was wondering if it should be done before or after Fable 5 is released, but I can probably do it now and tag it with (New in Fable 5)?

[MangelMaxime]

@dbrattli IHMO we can document it now, so we don't forget about it.

If you want to you can add a "tag" with the next version in it:

[dbrattli]

Thanks, I'll add it later today