don't uvloop.install on import (#2303)

CHANGES.md

@@ -6,6 +6,7 @@
 
 - Correct max string length calculation when there are string operators (#2292)
 - Fixed option usage when using the `--code` flag (#2259)
+- Do not call `uvloop.install()` when _Black_ is used as a library (#2303)
 
 ## 21.5b2
 

src/black/__init__.py

@@ -38,7 +38,7 @@
 from black.mode import Mode, TargetVersion
 from black.mode import Feature, supports_feature, VERSION_TO_FEATURES
 from black.cache import read_cache, write_cache, get_cache_info, filter_cached, Cache
-from black.concurrency import cancel, shutdown
+from black.concurrency import cancel, shutdown, maybe_install_uvloop
 from black.output import dump_to_file, diff, color_diff, out, err
 from black.report import Report, Changed
 from black.files import find_project_root, find_pyproject_toml, parse_pyproject_toml
@@ -54,14 +54,6 @@
 
 from _black_version import version as __version__
 
-# If our environment has uvloop installed lets use it
-try:
-    import uvloop
-
-    uvloop.install()
-except ImportError:
-    pass
-
 # types
 FileContent = str
 Encoding = str
@@ -1112,6 +1104,7 @@ def patch_click() -> None:
 
 
 def patched_main() -> None:
+    maybe_install_uvloop()
     freeze_support()
     patch_click()
     main()

src/black/concurrency.py

@@ -6,6 +6,21 @@
 from black.output import err
 
 
+def maybe_install_uvloop() -> None:
+    """If our environment has uvloop installed we use it.
+
+    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
+
+        uvloop.install()
+    except ImportError:
+        pass
+
+
 def cancel(tasks: Iterable["asyncio.Task[Any]"]) -> None:
     """asyncio signal handler that cancels all `tasks` and reports to stderr."""
     err("Aborted!")

src/blackd/__init__.py

@@ -20,16 +20,9 @@
     sys.exit(-1)
 
 import black
+from black.concurrency import maybe_install_uvloop
 import click
 
-# If our environment has uvloop installed lets use it
-try:
-    import uvloop
-
-    uvloop.install()
-except ImportError:
-    pass
-
 from _black_version import version as __version__
 
 # This is used internally by tests to shut down the server prematurely
@@ -210,6 +203,7 @@ def parse_python_variant_header(value: str) -> Tuple[bool, Set[black.TargetVersi
 
 
 def patched_main() -> None:
+    maybe_install_uvloop()
     freeze_support()
     black.patch_click()
     main()