[Python] Add Pythonic import path syntax for relative imports (#4292)

* [Python] Add Pythonic import path syntax for relative imports

* Update changelogs

Author: dbrattli

src/Fable.Cli/CHANGELOG.md

@@ -18,6 +18,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 * [Python] Fix regression `[<Erase>]` on class types not preventing them from being emitted to Python (by @dbrattli)
 * [Python] Fix regression `%A` format specifier to output booleans as lowercase `true`/`false` (by @dbrattli)
 
+### Added
+
+* [Python] Add Pythonic import path syntax for relative imports (`.module`, `..parent`, `...grandparent`) (by @dbrattli)
+
 ### Changed
 
 * [Python] `[<Py.Decorate>]` now emits decorator strings verbatim and adds `importFrom` parameter for explicit import control (by @dbrattli)

src/Fable.Cli/Pipeline.fs

@@ -276,6 +276,28 @@ module Python =
                     )
                     |> String.concat "."
 
+                // Convert Python-style relative module paths to file paths for resolution.
+                // Paths starting with dots are treated as relative imports (Pythonic convention):
+                // e.g., ".native_code" -> "./native_code.py", ".py.native_code" -> "./py/native_code.py"
+                //       "..native_code" -> "../native_code.py", "...module" -> "../../module.py"
+                // Paths without "." prefix are treated as absolute/package imports and pass through as-is.
+                let path =
+                    if path.Contains('/') || path.EndsWith(".py") then
+                        path
+                    elif path.StartsWith(".") then
+                        let trimmed = path.TrimStart('.')
+                        let dotCount = path.Length - trimmed.Length
+                        // Build the relative path prefix: "." -> "./", ".." -> "../", "..." -> "../../", etc.
+                        let prefix =
+                            if dotCount = 1 then
+                                "./"
+                            else
+                                String.replicate (dotCount - 1) "../"
+
+                        prefix + trimmed.Replace(".", "/") + ".py"
+                    else
+                        path
+
                 if path.Contains('/') then
                     // If inside fable-library then use relative path
                     if isFableLibrary then

src/Fable.Compiler/CHANGELOG.md

@@ -19,6 +19,10 @@ 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)
 
+### Added
+
+* [Python] Add Pythonic import path syntax for relative imports (`.module`, `..parent`, `...grandparent`) (by @dbrattli)
+
 ### Changed
 
 * [Python] `[<Py.Decorate>]` now emits decorator strings verbatim and adds `importFrom` parameter for explicit import control (by @dbrattli)

src/Fable.Core/Fable.Core.PyInterop.fs

@@ -57,15 +57,27 @@ let pyInstanceof (x: obj) (cons: obj) : bool = nativeOnly
 
 /// Works like `ImportAttribute` (same semantics as ES6 imports).
 /// You can use "*" or "default" selectors.
+/// Path can be:
+/// - Relative (Pythonic): ".my_module", "..parent", "...grandparent" (resolved relative to current file)
+/// - Absolute/Package: "pydantic" or "collections.abc" (used as-is)
+/// - File path (legacy): "./my_module.py" (still supported for backwards compatibility)
 let import<'T> (selector: string) (path: string) : 'T = nativeOnly
 
-/// F#: let myMember = importMember<string> "myModule"
+/// F#: let myMember = importMember<string> ".my_module"
 /// Py: from my_module import my_member
+/// Path can be:
+/// - Relative (Pythonic): ".my_module", "..parent", "...grandparent" (resolved relative to current file)
+/// - Absolute/Package: "pydantic" or "collections.abc" (used as-is)
+/// - File path (legacy): "./my_module.py" (still supported for backwards compatibility)
 /// Note the import must be immediately assigned to a value in a let binding
 let importMember<'T> (path: string) : 'T = nativeOnly
 
-/// F#: let myLib = importAll<obj> "myLib"
-/// Py: from my_lib import *
+/// F#: let myLib = importAll<obj> ".my_lib"
+/// Py: import my_lib as myLib
+/// Path can be:
+/// - Relative (Pythonic): ".my_module", "..parent", "...grandparent" (resolved relative to current file)
+/// - Absolute/Package: "pydantic" or "collections.abc" (used as-is)
+/// - File path (legacy): "./my_module.py" (still supported for backwards compatibility)
 let importAll<'T> (path: string) : 'T = nativeOnly
 
 /// Imports a file only for its side effects

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

@@ -3848,7 +3848,20 @@ let transformImports (_com: IPythonCompiler) (imports: Import list) : Statement
 let getIdentForImport (ctx: Context) (moduleName: string) (name: string option) =
     // printfn "getIdentForImport: %A" (moduleName, name)
     match name with
-    | None -> Path.GetFileNameWithoutExtension(moduleName)
+    | None ->
+        // Handle Python-style relative imports like ".native_code", "..native_code", "...module" etc.
+        // Path.GetFileNameWithoutExtension(".native_code") returns "" which is wrong
+        let moduleName = moduleName.TrimStart('.')
+
+        let lastPart =
+            match moduleName.LastIndexOf('.') with
+            | -1 -> moduleName
+            | idx -> moduleName.Substring(idx + 1)
+
+        if lastPart.Length > 0 then
+            lastPart
+        else
+            Path.GetFileNameWithoutExtension(moduleName)
     | Some name -> name |> Naming.toPythonNaming
     |> getUniqueNameInRootScope ctx
     |> Identifier

tests/Python/TestPyInterop.fs

@@ -283,26 +283,27 @@ type NativeCode =
 
 [<Fact>]
 let ``test importAll`` () =
-    let nativeCode: NativeCode = importAll "./py/native_code.py"
+    let nativeCode: NativeCode = importAll ".py.native_code"
     3 |> nativeCode.add5 |> equal 8
 
+// Deliberately using legacy relative path syntax to ensure it still works
 let add5 (x: int): int = importMember "./py/native_code.py"
 
 [<Fact>]
 let ``test importMember`` () =
     add5 -1 |> equal 4
 
     // Cannot use the same name as Fable will mangle the identifier
-    let add7: int -> int = importMember "./py/native_code.py"
+    let add7: int -> int = importMember ".py.native_code"
     add7 12 |> equal 19
 
-    let add5': int -> int = import "add5" "./py/native_code.py"
+    let add5': int -> int = import "add5" ".py.native_code"
     add5' 12 |> equal 17
 
-    let multiply3 (x: int): int = importMember "./py/more_native_code.py"
+    let multiply3 (x: int): int = importMember ".py.more_native_code"
     multiply3 9 |> equal 27
 
-[<ImportAll("./py/native_code.py")>]
+[<ImportAll(".py.native_code")>]
 let nativeCode: NativeCode = nativeOnly
 
 [<Fact>]