feat(stdlib): add functools module bindings (#260)

* feat(stdlib): add functools module bindings (reduce, lruCache, cache)

Add Fable.Python.Functools module binding Python's functools stdlib:
- reduce: fold-left with and without seed value (2 overloads)
- lruCache: LRU-memoised wrapper with configurable maxsize
- cache: unbounded memoised wrapper (Python 3.9+, lru_cache(maxsize=None))

Includes 6 tests covering all bindings.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* fix(stdlib): revert CHANGELOG edit and align reduce callback with Itertools

- Revert CHANGELOG.md (AGENTS.md forbids PR edits to it).
- Use System.Func<_,_,_> for reduce's binary callback to match the
  accumulate convention in Itertools and ensure Fable uncurries the
  callable before Python invokes it with two positional arguments.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

---------

Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Co-authored-by: Dag Brattli <dag@brattli.net>
Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: 4 people

src/Fable.Python.fsproj

@@ -25,6 +25,7 @@
     <Compile Include="stdlib/Os.fs" />
     <Compile Include="stdlib/Heapq.fs" />
     <Compile Include="stdlib/Itertools.fs" />
+    <Compile Include="stdlib/Functools.fs" />
     <Compile Include="stdlib/Queue.fs" />
     <Compile Include="stdlib/String.fs" />
     <Compile Include="stdlib/Sys.fs" />

src/stdlib/Functools.fs

@@ -0,0 +1,44 @@
+/// Type bindings for Python functools module: https://docs.python.org/3/library/functools.html
+module Fable.Python.Functools
+
+open Fable.Core
+
+// fsharplint:disable MemberNames
+
+[<Erase>]
+type IExports =
+    // ========================================================================
+    // Higher-order functions
+    // ========================================================================
+
+    /// Apply a function of two arguments cumulatively to the items of an iterable,
+    /// reducing it to a single value (fold-left without a seed).
+    /// See https://docs.python.org/3/library/functools.html#functools.reduce
+    abstract reduce: func: System.Func<'T, 'T, 'T> * iterable: 'T seq -> 'T
+
+    /// Apply a function of two arguments cumulatively to the items of an iterable,
+    /// starting with the initializer as the seed value (fold-left with a seed).
+    /// See https://docs.python.org/3/library/functools.html#functools.reduce
+    [<Emit("$0.reduce($1, $2, $3)")>]
+    abstract reduce: func: System.Func<'State, 'T, 'State> * iterable: 'T seq * initializer: 'State -> 'State
+
+    // ========================================================================
+    // Caching decorators
+    // ========================================================================
+
+    /// Wrap func with an LRU (least-recently-used) cache of at most maxsize entries.
+    /// Returns a memoised callable with the same signature as func.
+    /// Requires Python 3.8+.
+    /// See https://docs.python.org/3/library/functools.html#functools.lru_cache
+    [<Emit("$0.lru_cache(maxsize=int($1))($2)")>]
+    abstract lruCache: maxsize: int * func: ('T -> 'R) -> ('T -> 'R)
+
+    /// Wrap func with an unbounded cache (equivalent to lru_cache(maxsize=None)).
+    /// Requires Python 3.9+.
+    /// See https://docs.python.org/3/library/functools.html#functools.cache
+    [<Emit("$0.cache($1)")>]
+    abstract cache: func: ('T -> 'R) -> ('T -> 'R)
+
+/// Higher-order functions and operations on callable objects
+[<ImportAll("functools")>]
+let functools: IExports = nativeOnly

test/Fable.Python.Test.fsproj

@@ -18,6 +18,7 @@
     <Compile Include="TestBuiltinsAttr.fs" />
     <Compile Include="TestHeapq.fs" />
     <Compile Include="TestItertools.fs" />
+    <Compile Include="TestFunctools.fs" />
     <Compile Include="TestQueue.fs" />
     <Compile Include="TestThreading.fs" />
     <Compile Include="TestTraceback.fs" />

test/TestFunctools.fs

@@ -0,0 +1,48 @@
+module Fable.Python.Tests.Functools
+
+open Fable.Python.Testing
+open Fable.Python.Functools
+
+[<Fact>]
+let ``test reduce sum works`` () =
+    functools.reduce ((fun a b -> a + b), [ 1; 2; 3; 4; 5 ])
+    |> equal 15
+
+[<Fact>]
+let ``test reduce product works`` () =
+    functools.reduce ((fun a b -> a * b), [ 1; 2; 3; 4; 5 ])
+    |> equal 120
+
+[<Fact>]
+let ``test reduce with initializer works`` () =
+    functools.reduce ((fun acc x -> acc + x), [ 1; 2; 3 ], 10)
+    |> equal 16
+
+[<Fact>]
+let ``test reduce string fold with initializer works`` () =
+    functools.reduce ((fun acc s -> acc + s), [ "b"; "c"; "d" ], "a")
+    |> equal "abcd"
+
+[<Fact>]
+let ``test lruCache memoises results`` () =
+    let callCount = ResizeArray<int>()
+    let expensive (x: int) =
+        callCount.Add x
+        x * x
+    let cached = functools.lruCache (128, expensive)
+    cached 5 |> equal 25
+    cached 5 |> equal 25
+    cached 3 |> equal 9
+    callCount.Count |> equal 2
+
+[<Fact>]
+let ``test cache memoises results`` () =
+    let callCount = ResizeArray<int>()
+    let expensive (x: int) =
+        callCount.Add x
+        x * 2
+    let cached = functools.cache expensive
+    cached 7 |> equal 14
+    cached 7 |> equal 14
+    cached 4 |> equal 8
+    callCount.Count |> equal 2