add winloop support and remove deprecated functionality from uvloop (psf#4996)

Vizonex · cobaltt7 · pre-commit-ci[bot] · web-flow · commit 8b9d3e36e8b9 · 2026-03-03T09:28:27.000-06:00
* add winloop support and remove deprecated functionality from uvloop * summarize changes made in pull request psf#4996 * close after running with web.run_app * [pre-commit.ci] auto fixes from pre-commit.com hooks for more information, see https://pre-commit.ci * reformat * add windows-latest and windows-11-arm to uvloop test --------- Co-authored-by: cobalt <61329810+cobaltt7@users.noreply.github.com> Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
diff --git a/.github/workflows/test.yml b/.github/workflows/test.yml
@@ -105,7 +105,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        os: [ubuntu-latest, macOS-latest]
+        os: [ubuntu-latest, macOS-latest, windows-latest, windows-11-arm]
 
     steps:
       - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
diff --git a/.pre-commit-config.yaml b/.pre-commit-config.yaml
@@ -52,6 +52,8 @@ repos:
           - platformdirs>=2
           - pytokens~=0.4.0
           - tomli>=1.1.0
+          - uvloop>=0.15.2; sys_platform != 'win32'
+          - winloop>=0.5.0; sys_platform == 'win32'
 
           # blackd
           - aiohttp>=3.10
diff --git a/CHANGES.md b/CHANGES.md
@@ -43,7 +43,12 @@
 
 ### Performance
 
-<!-- Changes that improve Black's performance. -->
+- Introduce winloop for windows as an alternative to uvloop (#4996)
+- Remove deprecated function `uvloop.install()` in favor of `uvloop.new_event_loop()`
+  (#4996)
+- Rename `maybe_install_uvloop` function to `maybe_use_uvloop` to simplify loop
+  installation and creation of either a uvloop/winloop evenloop or default eventloop
+  (#4996)
 
 ### Output
 
@@ -56,7 +61,8 @@
 
 ### _Blackd_
 
-<!-- Changes to blackd -->
+- Introduce winloop to be used when windows in use which enables blackd to run faster on
+  windows when winloop is installed. (#4996)
 
 ### Integrations
 
diff --git a/pyproject.toml b/pyproject.toml
@@ -66,7 +66,10 @@ dynamic = ["readme", "version"]
 
 [project.optional-dependencies]
 colorama = ["colorama>=0.4.3"]
-uvloop = ["uvloop>=0.15.2"]
+uvloop = [
+    "uvloop>=0.15.2; sys_platform != 'win32'",
+    "winloop>=0.5.0; sys_platform == 'win32'"
+]
 d = ["aiohttp>=3.10"]
 jupyter = ["ipython>=7.8.0", "tokenize-rt>=3.2.0"]
 
diff --git a/src/black/concurrency.py b/src/black/concurrency.py
@@ -27,18 +27,24 @@
 from black.report import Changed, Report
 
 
-def maybe_install_uvloop() -> None:
-    """If our environment has uvloop installed we use it.
+def maybe_use_uvloop() -> asyncio.AbstractEventLoop:
+    """If our environment has uvloop or winloop installed we use it otherwise
+    a normal asyncio eventloop is called as fallback.
 
     This is called only from command-line entry points to avoid
     interfering with the parent process if Black is used as a library.
     """
     try:
-        import uvloop
+        if sys.platform != "win32":
+            import uvloop
 
-        uvloop.install()
+            return uvloop.new_event_loop()
+        else:
+            import winloop
+
+            return winloop.new_event_loop()
     except ImportError:
-        pass
+        return asyncio.new_event_loop()
 
 
 def cancel(tasks: Iterable[asyncio.Future[Any]]) -> None:
@@ -81,7 +87,6 @@ def reformat_many(
     no_cache: bool = False,
 ) -> None:
     """Reformat multiple files using a ProcessPoolExecutor."""
-    maybe_install_uvloop()
 
     if workers is None:
         workers = int(os.environ.get("BLACK_NUM_WORKERS", 0))
@@ -110,7 +115,7 @@ def reformat_many(
     if executor is None:
         executor = ThreadPoolExecutor(max_workers=1)
 
-    loop = asyncio.new_event_loop()
+    loop = maybe_use_uvloop()
     asyncio.set_event_loop(loop)
     try:
         loop.run_until_complete(
diff --git a/src/blackd/__init__.py b/src/blackd/__init__.py
@@ -21,7 +21,7 @@
 
 import black
 from _black_version import version as __version__
-from black.concurrency import maybe_install_uvloop
+from black.concurrency import maybe_use_uvloop
 
 # This is used internally by tests to shut down the server prematurely
 _stop_signal = asyncio.Event()
@@ -82,7 +82,19 @@ def main(bind_host: str, bind_port: int) -> None:
     app = make_app()
     ver = black.__version__
     black.out(f"blackd version {ver} listening on {bind_host} port {bind_port}")
-    web.run_app(app, host=bind_host, port=bind_port, handle_signals=True, print=None)
+    loop = maybe_use_uvloop()
+    try:
+        web.run_app(
+            app,
+            host=bind_host,
+            port=bind_port,
+            handle_signals=True,
+            print=None,
+            loop=loop,
+        )
+    finally:
+        if not loop.is_closed():
+            loop.close()
 
 
 @cache
@@ -245,7 +257,6 @@ def parse_python_variant_header(value: str) -> tuple[bool, set[black.TargetVersi
 
 
 def patched_main() -> None:
-    maybe_install_uvloop()
     freeze_support()
     main()
 
