Add Config.env_prefix option (#1990)

* add Config.env_prefix option

* fix variable name in docs

* simplify test case

* rollback markdown formatting

* Update docs/config.md

Co-authored-by: Marcelo Trylesinski <marcelotryle@gmail.com>

Co-authored-by: Marcelo Trylesinski <marcelotryle@gmail.com>

docs/config.md

@@ -113,6 +113,23 @@ from starlette.config import environ
 environ['TESTING'] = 'TRUE'
 ```
 
+## Reading prefixed environment variables
+
+You can namespace the environment variables by setting `env_prefix` argument.
+
+```python title="myproject/settings.py"
+import os
+from starlette.config import Config
+
+os.environ['APP_DEBUG'] = 'yes'
+os.environ['ENVIRONMENT'] = 'dev'
+
+config = Config(env_prefix='APP_')
+
+DEBUG = config('DEBUG') # lookups APP_DEBUG, returns "yes"
+ENVIRONMENT = config('ENVIRONMENT') # lookups APP_ENVIRONMENT, raises KeyError as variable is not defined
+```
+
 ## A full example
 
 Structuring large applications can be complex. You need proper separation of

starlette/config.py

@@ -54,8 +54,10 @@ def __init__(
         self,
         env_file: typing.Optional[typing.Union[str, Path]] = None,
         environ: typing.Mapping[str, str] = environ,
+        env_prefix: str = "",
     ) -> None:
         self.environ = environ
+        self.env_prefix = env_prefix
         self.file_values: typing.Dict[str, str] = {}
         if env_file is not None and os.path.isfile(env_file):
             self.file_values = self._read_file(env_file)
@@ -103,6 +105,7 @@ def get(
         cast: typing.Optional[typing.Callable] = None,
         default: typing.Any = undefined,
     ) -> typing.Any:
+        key = self.env_prefix + key
         if key in self.environ:
             value = self.environ[key]
             return self._perform_cast(key, value, cast)

tests/test_config.py

@@ -127,3 +127,13 @@ def test_environ():
     environ = Environ()
     assert list(iter(environ)) == list(iter(os.environ))
     assert len(environ) == len(os.environ)
+
+
+def test_config_with_env_prefix(tmpdir, monkeypatch):
+    config = Config(
+        environ={"APP_DEBUG": "value", "ENVIRONMENT": "dev"}, env_prefix="APP_"
+    )
+    assert config.get("DEBUG") == "value"
+
+    with pytest.raises(KeyError):
+        config.get("ENVIRONMENT")