From 782256837879ece72a89c9343813b5b78898e099 Mon Sep 17 00:00:00 2001
From: Alex Oleshkevich <alex.oleshkevich@gmail.com>
Date: Mon, 23 Jan 2023 00:24:35 +0300
Subject: [PATCH] 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       | 17 +++++++++++++++++
 starlette/config.py  |  3 +++
 tests/test_config.py | 10 ++++++++++
 3 files changed, 30 insertions(+)

diff --git a/docs/config.md b/docs/config.md
index a879374dc..853dff3d4 100644
--- a/docs/config.md
+++ b/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
diff --git a/starlette/config.py b/starlette/config.py
index 8c58b2738..795232cf6 100644
--- a/starlette/config.py
+++ b/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)
diff --git a/tests/test_config.py b/tests/test_config.py
index d33000389..70b0a4812 100644
--- a/tests/test_config.py
+++ b/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")