[Python] Changed Decorate attribute to take importFrom parameter instead of auto-importing (#4290)

* [Python] Improve Decorate attribute

* Update changelogs

* Add decorator

* Simplify code

* Removed attribute on wrong element

Author: dbrattli

src/Fable.Cli/CHANGELOG.md

@@ -11,6 +11,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 * [Python] Fix regression `%A` format specifier to output booleans as lowercase `true`/`false` (by @dbrattli)
 
+### Changed
+
+* [Python] `[<Py.Decorate>]` now emits decorator strings verbatim and adds `importFrom` parameter for explicit import control (by @dbrattli)
+
 ## 5.0.0-alpha.19 - 2025-12-04
 
 ### Fixed

src/Fable.Compiler/CHANGELOG.md

@@ -11,6 +11,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 * [Python] Fix regression `%A` format specifier to output booleans as lowercase `true`/`false` (by @dbrattli)
 
+### Changed
+
+* [Python] `[<Py.Decorate>]` now emits decorator strings verbatim and adds `importFrom` parameter for explicit import control (by @dbrattli)
+
 ## 5.0.0-alpha.18 - 2025-12-04
 
 ### Fixed

src/Fable.Core/CHANGELOG.md

@@ -7,6 +7,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## Unreleased
 
+### Changed
+
+* [Python] `[<Py.Decorate>]` now emits decorator strings verbatim and adds `importFrom` parameter for explicit import control (by @dbrattli)
+
 ## 5.0.0-beta.3 - 2025-12-04
 
 ### Added

src/Fable.Core/Fable.Core.Py.fs

@@ -33,22 +33,23 @@ module Py =
     /// libraries.
     /// </summary>
     /// <remarks>
-    /// <para>The [&lt;Decorate&gt;] attribute is purely for Python interop and does NOT
-    /// affect F# compilation behavior.</para>
+    /// <para>The decorator is emitted verbatim. Use importFrom to specify the module to import from.</para>
     /// <para>Multiple [&lt;Decorate&gt;] attributes are applied in reverse order
     /// (bottom to top), following Python's standard decorator stacking behavior.</para>
     /// <para>Examples:</para>
-    /// <para>[&lt;Decorate("dataclasses.dataclass")&gt;] - Simple class decorator</para>
-    /// <para>[&lt;Decorate("functools.lru_cache", "maxsize=128")&gt;] - Decorator with
-    /// parameters</para>
-    /// <para>[&lt;Decorate("pydantic.field_validator", "'Name'")&gt;] - Method decorator for
-    /// Pydantic validators</para>
+    /// <para>[&lt;Decorate("dataclass", "dataclasses")&gt;] - imports dataclass from dataclasses</para>
+    /// <para>[&lt;Decorate("lru_cache", importFrom="functools", parameters="maxsize=128")&gt;] - with import and parameters</para>
+    /// <para>[&lt;Decorate("app.get(\"/\")")&gt;] - Local variable decorator with parameters (no import needed)</para>
     /// </remarks>
     [<AttributeUsage(AttributeTargets.Class ||| AttributeTargets.Method, AllowMultiple = true)>]
-    type DecorateAttribute(decorator: string) =
+    type DecorateAttribute(decorator: string, importFrom: string, parameters: string) =
         inherit Attribute()
-        new(decorator: string, parameters: string) = DecorateAttribute(decorator)
+        /// Decorator without import or parameters
+        new(decorator: string) = DecorateAttribute(decorator, "", "")
+        /// Decorator with import but no parameters
+        new(decorator: string, importFrom: string) = DecorateAttribute(decorator, importFrom, "")
 
+    /// Decorator with all parameters
     /// <summary>
     /// Marks a static member to be emitted as a Python @classmethod instead of @staticmethod.
     /// </summary>
@@ -65,7 +66,6 @@ module Py =
     type ClassMethodAttribute() =
         inherit Attribute()
 
-    [<RequireQualifiedAccess>]
     type ClassAttributeStyle =
         // Translates to properties with instance attributes backing
         | Properties = 0

src/Fable.Transforms/Python/Fable2Python.Types.fs

@@ -42,6 +42,7 @@ type DecoratorInfo =
     {
         Decorator: string
         Parameters: string
+        ImportFrom: string
     }
 
 

src/Fable.Transforms/Python/Fable2Python.Util.fs

@@ -70,17 +70,8 @@ module Util =
         )
         |> Option.defaultValue defaultParams
 
-    /// Parses a decorator string to extract module and function/class name
-    let parseDecorator (decorator: string) =
-        match decorator.Split('.') with
-        | [| functionName |] -> None, functionName // No module, just function name
-        | parts when parts.Length >= 2 ->
-            let moduleName = parts.[0 .. (parts.Length - 2)] |> String.concat "."
-            let functionName = parts.[parts.Length - 1]
-            Some moduleName, functionName
-        | _ -> None, decorator // Fallback
-
     /// Extracts decorator information from entity attributes
+    /// Constructor args order: (decorator, importFrom, parameters)
     let getDecoratorInfo (atts: Fable.Attribute seq) =
         atts
         |> Seq.choose (fun att ->
@@ -91,12 +82,21 @@ module Util =
                         {
                             Decorator = decorator
                             Parameters = ""
+                            ImportFrom = ""
                         }
-                | [ :? string as decorator; :? string as parameters ] ->
+                | [ :? string as decorator; :? string as importFrom ] ->
+                    Some
+                        {
+                            Decorator = decorator
+                            Parameters = ""
+                            ImportFrom = importFrom
+                        }
+                | [ :? string as decorator; :? string as importFrom; :? string as parameters ] ->
                     Some
                         {
                             Decorator = decorator
                             Parameters = parameters
+                            ImportFrom = importFrom
                         }
                 | _ -> None // Invalid decorator
             else
@@ -105,24 +105,22 @@ module Util =
         |> Seq.toList
 
     /// Generates Python decorator expressions from DecoratorInfo
+    /// The decorator string is emitted verbatim. If ImportFrom is specified,
+    /// an import statement is generated automatically.
     let generateDecorators (com: IPythonCompiler) (ctx: Context) (decoratorInfos: DecoratorInfo list) =
         decoratorInfos
         |> List.map (fun info ->
-            let moduleName, functionName = parseDecorator info.Decorator
-
-            let decoratorExpr =
-                match moduleName with
-                | Some module_ -> com.GetImportExpr(ctx, module_, functionName)
-                | None -> Expression.name functionName
+            // Handle import if ImportFrom is specified
+            if not (String.IsNullOrEmpty info.ImportFrom) then
+                // This triggers the import: from {importFrom} import {decorator}
+                com.GetImportExpr(ctx, info.ImportFrom, info.Decorator) |> ignore
 
             if String.IsNullOrEmpty info.Parameters then
                 // Simple decorator without parameters: @decorator
-                decoratorExpr
+                Expression.emit (info.Decorator, [])
             else
                 // Decorator with parameters: @decorator(param1=value1, param2=value2)
-                // For parameters, we emit the full decorator call as raw Python code
-                // This preserves exact parameter syntax for maximum flexibility
-                Expression.emit ($"%s{functionName}(%s{info.Parameters})", [])
+                Expression.emit ($"%s{info.Decorator}(%s{info.Parameters})", [])
         )
 
     let getIdentifier (_com: IPythonCompiler) (_ctx: Context) (name: string) =

tests/Python/TestPyInterop.fs

@@ -501,7 +501,7 @@ type PydanticUserWithValidator(Name: string) =
     inherit BaseModel()
     member val Name: string = Name with get, set
 
-    [<Py.Decorate("pydantic.field_validator", "'Name'")>]
+    [<Py.Decorate("field_validator", importFrom="pydantic", parameters="'Name'")>]
     [<Py.ClassMethod>]
     static member validate_name(cls: obj, v: string) : string =
         v.ToUpper()
@@ -513,7 +513,7 @@ let ``test Pydantic field_validator with classmethod`` () =
     let user = emitPyExpr<PydanticUserWithValidator> [] "PydanticUserWithValidator(Name='john')"
     user.Name |> equal "JOHN"
 
-[<Py.Decorate("dataclasses.dataclass")>]
+[<Py.Decorate("dataclass", importFrom="dataclasses")>]
 [<Py.ClassAttributes(style=Py.ClassAttributeStyle.Attributes, init=false)>]
 type DecoratedUser() =
     member val Name: string = "" with get, set
@@ -529,19 +529,20 @@ let ``test simple decorator without parameters`` () =
     user.Name |> equal "Test User"
     user.Age |> equal 25
 
-[<Py.Decorate("functools.lru_cache", "maxsize=128")>]
-[<Py.ClassAttributes(style=Py.ClassAttributeStyle.Attributes, init=false)>]
-type DecoratedCache() =
-    member val Value: string = "cached" with get, set
+[<AttachMembers>]
+type DecoratedCacheClass() =
+    [<Py.Decorate("lru_cache", importFrom="functools", parameters="maxsize=128")>]
+    static member expensive_computation(x: int) : int =
+        x * x  // Simulated expensive computation
 
 [<Fact>]
 let ``test decorator with parameters`` () =
-    // Test that decorator with parameters is applied correctly
-    let cache = DecoratedCache()
-    cache.Value |> equal "cached"
+    // Test that decorator with parameters is applied correctly to a method
+    let result = DecoratedCacheClass.expensive_computation(5)
+    result |> equal 25
 
-[<Py.Decorate("dataclasses.dataclass")>]
-[<Py.Decorate("functools.total_ordering")>]
+[<Py.Decorate("dataclass", importFrom="dataclasses")>]
+[<Py.Decorate("total_ordering", importFrom="functools")>]
 [<Py.ClassAttributes(style=Py.ClassAttributeStyle.Attributes, init=false)>]
 type MultiDecoratedClass() =
     member val Priority: int = 0 with get, set
@@ -560,7 +561,7 @@ let ``test multiple decorators applied in correct order`` () =
     obj.Priority |> equal 1
     obj.Name |> equal "test"
 
-[<Py.Decorate("attrs.define", "auto_attribs=True, slots=True")>]
+[<Py.Decorate("define", importFrom="attrs", parameters="auto_attribs=True, slots=True")>]
 [<Py.ClassAttributes(style=Py.ClassAttributeStyle.Attributes, init=false)>]
 type AttrsDecoratedClass() =
     member val Data: string = "attrs_data" with get, set
@@ -575,7 +576,7 @@ let ``test complex decorator parameters`` () =
 
 // Test combining Decorate with existing F# features
 
-[<Py.Decorate("dataclasses.dataclass")>]
+[<Py.Decorate("dataclass", "dataclasses")>]
 [<Py.ClassAttributes(style=Py.ClassAttributeStyle.Attributes, init=false)>]
 type InheritedDecoratedClass() =
     inherit DecoratedUser()
@@ -608,13 +609,32 @@ let ``test PropertiesUserWithInit`` () =
     user.Email |> equal (Some "[email protected]")
     user.Enabled |> equal true
 
+// Test Py.Decorate without import (local variable decorator like FastAPI app.get)
+
+// Simulate a decorator factory (like FastAPI's app instance)
+let my_decorator (path: string) : (obj -> obj) = import "my_decorator" "./native_code.py"
+
+[<AttachMembers>]
+type ClassWithLocalDecorator() =
+    // Use full decorator expression since my_decorator is a local variable (no import)
+    [<Py.Decorate("my_decorator(\"/test\")")>]
+    static member decorated_method() : string =
+        "result"
+
+[<Fact>]
+let ``test Py.Decorate without import (local variable)`` () =
+    // Test that decorator is emitted verbatim without auto-import
+    // This pattern is used for FastAPI: @app.get("/")
+    let result = ClassWithLocalDecorator.decorated_method()
+    result |> equal "decorated: result"
+
 // Test Py.Decorate on static methods
 
 [<AttachMembers>]
 type ClassWithDecoratedStaticMethod() =
     member val Value: int = 0 with get, set
 
-    [<Py.Decorate("functools.lru_cache")>]
+    [<Py.Decorate("lru_cache", importFrom="functools")>]
     static member cached_function(x: int) : int =
         x * 2
 
@@ -628,7 +648,7 @@ let ``test Py.Decorate on static method`` () =
 
 [<AttachMembers>]
 type ClassWithDecoratedStaticMethodWithParams() =
-    [<Py.Decorate("functools.lru_cache", "maxsize=32")>]
+    [<Py.Decorate("lru_cache", importFrom="functools", parameters="maxsize=32")>]
     static member cached_with_params(x: int) : int =
         x * 3
 
@@ -661,10 +681,10 @@ let ``test Py.ClassMethod attribute`` () =
 
 [<AttachMembers>]
 type ClassWithMultipleDecoratedMethods() =
-    [<Py.Decorate("functools.lru_cache", "maxsize=16")>]
+    [<Py.Decorate("lru_cache", importFrom="functools", parameters="maxsize=16")>]
     static member method_a(x: int) : int = x * 2
 
-    [<Py.Decorate("functools.lru_cache", "maxsize=32")>]
+    [<Py.Decorate("lru_cache", importFrom="functools", parameters="maxsize=32")>]
     static member method_b(x: int) : int = x * 3
 
 [<Fact>]
@@ -679,7 +699,7 @@ let ``test multiple static methods with decorators`` () =
 type ClassWithDecoratedInstanceMethod() =
     member val CallCount: int = 0 with get, set
 
-    [<Py.Decorate("functools.lru_cache", "maxsize=16")>]
+    [<Py.Decorate("lru_cache", importFrom="functools", parameters="maxsize=16")>]
     member this.cached_instance_method(x: int) : int =
         x * 4
 

tests/Python/native_code.py

@@ -1,6 +1,25 @@
+from typing import Callable, ParamSpec, TypeVar
+
+P = ParamSpec("P")
+R = TypeVar("R")
+
+
 def add5(x: int) -> int:
     return x + 5
 
 
 def add7(x: int) -> int:
     return x + 7
+
+
+def my_decorator(path: str) -> Callable[[Callable[P, R]], Callable[P, str]]:
+    """A decorator factory that simulates FastAPI's app.get/post pattern."""
+
+    def decorator(func: Callable[P, R]) -> Callable[P, str]:
+        def wrapper(*args: P.args, **kwargs: P.kwargs) -> str:
+            result = func(*args, **kwargs)
+            return f"decorated: {result}"
+
+        return wrapper
+
+    return decorator

tests/Python/py/nullness.py

@@ -14,5 +14,5 @@ def maybe_null(value: str) -> str | None:
     """
     if value == "ok":
         return value
-    else:
-        return None
+
+    return None