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.

docs/config.rst

@@ -393,8 +393,8 @@ The following configuration values are used internally by Flask:
     Added :data:`MAX_COOKIE_SIZE` to control a warning from Werkzeug.
 
 
-Configuring from Files
-----------------------
+Configuring from Python Files
+-----------------------------
 
 Configuration becomes more useful if you can store it in a separate file,
 ideally located outside the actual application package. This makes
@@ -440,6 +440,20 @@ methods on the config object as well to load from individual files.  For a
 complete reference, read the :class:`~flask.Config` object's
 documentation.
 
+Configuring from files
+----------------------
+
+It is also possible to load configure from a flat file in a format of
+your choice, for example to load from a TOML (or JSON) formatted
+file::
+
+   import json
+   import toml
+
+   app.config.from_file("config.toml", load=toml.load)
+   # Alternatively, if you prefer JSON
+   app.config.from_file("config.json", load=json.load)
+
 
 Configuring from Environment Variables
 --------------------------------------

src/flask/config.py

@@ -11,6 +11,7 @@
 import errno
 import os
 import types
+import warnings
 
 from werkzeug.utils import import_string
 
@@ -176,31 +177,55 @@ 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, load, silent=False):
+        """Update the values in the config from a file that is loaded using
+        the *load* 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 load: a callable that takes a file handle and returns a mapping
+                     from the file.
+        :type load: Callable[[Reader], Mapping]. Where Reader is a Protocol
+                    that implements a read method.
         :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 = load(file_)
         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
+        """
+        warnings.warn(
+            DeprecationWarning(
+                '"from_json" is deprecated and will be removed in 2.0. Use'
+                ' "from_file(filename, load=json.load)" instead.'
+            )
+        )
+        return self.from_file(filename, json.load, silent=silent)
+
     def from_mapping(self, *mapping, **kwargs):
         """Updates the config like :meth:`update` ignoring items with non-upper
         keys.

tests/test_config.py

@@ -6,6 +6,7 @@
     :copyright: 2010 Pallets
     :license: BSD-3-Clause
 """
+import json
 import os
 import textwrap
 from datetime import timedelta
@@ -27,7 +28,7 @@ def common_object_test(app):
     assert "TestConfig" not in app.config
 
 
-def test_config_from_file():
+def test_config_from_pyfile():
     app = flask.Flask(__name__)
     app.config.from_pyfile(__file__.rsplit(".", 1)[0] + ".py")
     common_object_test(app)
@@ -39,10 +40,10 @@ def test_config_from_object():
     common_object_test(app)
 
 
-def test_config_from_json():
+def test_config_from_file():
     app = flask.Flask(__name__)
     current_dir = os.path.dirname(os.path.abspath(__file__))
-    app.config.from_json(os.path.join(current_dir, "static", "config.json"))
+    app.config.from_file(os.path.join(current_dir, "static", "config.json"), json.load)
     common_object_test(app)
 
 
@@ -116,16 +117,16 @@ def test_config_missing():
     assert not app.config.from_pyfile("missing.cfg", silent=True)
 
 
-def test_config_missing_json():
+def test_config_missing_file():
     app = flask.Flask(__name__)
     with pytest.raises(IOError) as e:
-        app.config.from_json("missing.json")
+        app.config.from_file("missing.json", load=json.load)
     msg = str(e.value)
     assert msg.startswith(
         "[Errno 2] Unable to load configuration file (No such file or directory):"
     )
     assert msg.endswith("missing.json'")
-    assert not app.config.from_json("missing.json", silent=True)
+    assert not app.config.from_file("missing.json", load=json.load, silent=True)
 
 
 def test_custom_config_class():