Config.from_prefixed_env: Allow to not trim prefix

Fixes #5060

CHANGES.rst

@@ -44,6 +44,8 @@ Unreleased
     to set the domain, which modern browsers interpret as an exact match rather than
     a subdomain match. Warnings about ``localhost`` and IP addresses are also removed.
     :issue:`5051`
+-   ``config.from_prefixed_env`` supports not trimming the prefix using
+    ``trim_prefix=False``. :issue:`5060`
 
 
 Version 2.2.4

src/flask/config.py

@@ -99,15 +99,21 @@ class Config(dict):
         return self.from_pyfile(rv, silent=silent)
 
     def from_prefixed_env(
-        self, prefix: str = "FLASK", *, loads: t.Callable[[str], t.Any] = json.loads
+        self,
+        prefix: str = "FLASK",
+        *,
+        loads: t.Callable[[str], t.Any] = json.loads,
+        trim_prefix: bool = True,
     ) -> bool:
-        """Load any environment variables that start with ``FLASK_``,
+        """Load any environment variables that start with ``FLASK_``.
-        dropping the prefix from the env key for the config key. Values
+        Values are passed through a loading function to attempt to
-        are passed through a loading function to attempt to convert them
+        convert them to more specific types than strings.
-        to more specific types than strings.
 
         Keys are loaded in :func:`sorted` order.
 
+        The default is to drop the prefix from the env key for the
+        config key.
+
         The default loading function attempts to parse values as any
         valid JSON type, including dicts and lists.
 
@@ -121,6 +127,11 @@ class Config(dict):
             the returned value as the config value. If any error is
             raised it is ignored and the value remains a string. The
             default is :func:`json.loads`.
+        :param trim_prefix: A flag indicating if the prefix should be
+            dropped for the config key. Defaults to ``True``
+
+        .. versionchanged:: 2.3
+            The ``trim_prefix`` parameter was added.
 
         .. versionadded:: 2.1
         """
@@ -139,8 +150,9 @@ class Config(dict):
                 # Keep the value as a string if loading failed.
                 pass
 
-            # Change to key.removeprefix(prefix) on Python >= 3.9.
+            if trim_prefix:
-            key = key[len_prefix:]
+                # Change to key.removeprefix(prefix) on Python >= 3.9.
+                key = key[len_prefix:]
 
             if "__" not in key:
                 # A non-nested key, set directly.

tests/test_config.py

@@ -108,6 +108,18 @@ def test_from_prefixed_env_nested(monkeypatch):
     assert app.config["NEW"] == {"K": "v"}
 
 
+def test_from_prefixed_env_no_trim(monkeypatch):
+    monkeypatch.setenv("FLASK_SECRET_KEY", "asdf")
+    monkeypatch.setenv("APP_DEBUG", "true")
+
+    app = flask.Flask(__name__)
+    app.config.from_prefixed_env(trim_prefix=True)
+    app.config.from_prefixed_env("APP", trim_prefix=False)
+
+    assert app.config["SECRET_KEY"] == "asdf"
+    assert app.config["APP_DEBUG"] is True
+
+
 def test_config_from_mapping():
     app = flask.Flask(__name__)
     app.config.from_mapping({"SECRET_KEY": "config", "TEST_KEY": "foo"})