Support loading configuration from text files

TOML is a very popular format now, and is taking hold in the Python
ecosystem via pyproject.toml (among others). This allows toml config
files via,

    app.config.from_file("config.toml", toml.loads)

it also allows for any other file format whereby there is a loader
that takes a string and returns a mapping.

src/flask/config.py

@@ -176,31 +176,48 @@ class and has ``@property`` attributes, it needs to be
             if key.isupper():
                 self[key] = getattr(obj, key)
 
-    def from_json(self, filename, silent=False):
-        """Updates the values in the config from a JSON file. This function
-        behaves as if the JSON object was a dictionary and passed to the
-        :meth:`from_mapping` function.
+    def from_file(self, filename, loads, silent=False):
+        """Update the values in the config from a file that is loaded using
+        the *loads* argument.  This method passes the loaded Mapping
+        to the :meth:`from_mapping` function.
 
         :param filename: the filename of the JSON file.  This can either be an
                          absolute filename or a filename relative to the
                          root path.
+        :param loads: a callable that loads the file text and returned a
+                      Mapping.
+        :type loads: Callable[[str], Mapping].
         :param silent: set to ``True`` if you want silent failure for missing
                        files.
 
-        .. versionadded:: 0.11
+        .. versionadded:: 1.2
         """
         filename = os.path.join(self.root_path, filename)
-
         try:
-            with open(filename) as json_file:
-                obj = json.loads(json_file.read())
+            with open(filename) as file_:
+                obj = loads(file_.read())
         except IOError as e:
             if silent and e.errno in (errno.ENOENT, errno.EISDIR):
                 return False
             e.strerror = "Unable to load configuration file (%s)" % e.strerror
             raise
         return self.from_mapping(obj)
 
+    def from_json(self, filename, silent=False):
+        """Updates the values in the config from a JSON file. This function
+        behaves as if the JSON object was a dictionary and passed to the
+        :meth:`from_mapping` function.
+
+        :param filename: the filename of the JSON file.  This can either be an
+                         absolute filename or a filename relative to the
+                         root path.
+        :param silent: set to ``True`` if you want silent failure for missing
+                       files.
+
+        .. versionadded:: 0.11
+        """
+        return self.from_file(filename, json.loads, silent=silent)
+
     def from_mapping(self, *mapping, **kwargs):
         """Updates the config like :meth:`update` ignoring items with non-upper
         keys.

tests/static/config.toml

@@ -0,0 +1,2 @@
+TEST_KEY = "foo"
+SECRET_KEY = "config"

tests/test_config.py

@@ -11,6 +11,7 @@
 from datetime import timedelta
 
 import pytest
+import toml
 
 import flask
 from flask._compat import PY2
@@ -46,6 +47,13 @@ def test_config_from_json():
     common_object_test(app)
 
 
+def test_config_from_file():
+    app = flask.Flask(__name__)
+    current_dir = os.path.dirname(os.path.abspath(__file__))
+    app.config.from_file(os.path.join(current_dir, "static", "config.toml"), toml.loads)
+    common_object_test(app)
+
+
 def test_config_from_mapping():
     app = flask.Flask(__name__)
     app.config.from_mapping({"SECRET_KEY": "config", "TEST_KEY": "foo"})

tox.ini

@@ -13,6 +13,7 @@ deps =
     greenlet
     blinker
     python-dotenv
+    toml
 
     lowest: Werkzeug==0.15
     lowest: Jinja2==2.10