Enable read of uvicorn settings from environment variables (#1279)

docs/settings.md

@@ -7,6 +7,13 @@ equivalent keyword arguments, eg. `uvicorn.run("example:app", port=5000, reload=
 Please note that in this case, if you use `reload=True` or `workers=NUM`,
 you should put `uvicorn.run` into `if __name__ == '__main__'` clause in the main module.
 
+You can also configure Uvicorn using environment variables with the prefix `UVICORN_`.
+For example, in case you want to run the app on port `5000`, just set the environment variable `UVICORN_PORT` to `5000`.
+
+!!! note
+    CLI options and the arguments for `uvicorn.run()` take precedence over environment variables.
+
+
 ## Application
 
 * `APP` - The ASGI application to run, in the format `"<module>:<attribute>"`.

mkdocs.yml

@@ -29,6 +29,7 @@ nav:
   - Contributing: "contributing.md"
 
 markdown_extensions:
+  - admonition
   - codehilite:
       css_class: highlight
   - toc:

tests/test_cli.py

@@ -1,4 +1,5 @@
 import importlib
+import os
 import platform
 import sys
 from pathlib import Path
@@ -17,6 +18,10 @@
 main = importlib.import_module("uvicorn.main")
 
 
+class App:
+    pass
+
+
 def test_cli_print_version() -> None:
     runner = CliRunner()
 
@@ -131,5 +136,26 @@ def test_cli_reloader_incomplete_app_parameter(
     ) in captured.err
 
 
-class App:
-    pass
+@pytest.fixture()
+def load_env_h11_protocol():
+    old_environ = dict(os.environ)
+    os.environ["UVICORN_HTTP"] = "h11"
+    yield
+    os.environ.clear()
+    os.environ.update(old_environ)
+
+
+def test_env_variables(load_env_h11_protocol: None):
+    runner = CliRunner(env=os.environ)
+    with mock.patch.object(main, "run") as mock_run:
+        runner.invoke(cli, ["tests.test_cli:App"])
+        _, kwargs = mock_run.call_args
+        assert kwargs["http"] == "h11"
+
+
+def test_mistmatch_env_variables(load_env_h11_protocol: None):
+    runner = CliRunner(env=os.environ)
+    with mock.patch.object(main, "run") as mock_run:
+        runner.invoke(cli, ["tests.test_cli:App", "--http=httptools"])
+        _, kwargs = mock_run.call_args
+        assert kwargs["http"] == "httptools"

uvicorn/main.py

@@ -50,7 +50,7 @@ def print_version(ctx: click.Context, param: click.Parameter, value: bool) -> No
     ctx.exit()
 
 
-@click.command()
+@click.command(context_settings={"auto_envvar_prefix": "UVICORN"})
 @click.argument("app")
 @click.option(
     "--host",