Add Config field for dumping Flask configuration values (#281)

* Add Config field for dumping Flask configuration values

Migrate the Config field from APIFlask.

* Test error case; update changelog

---------

Co-authored-by: Steven Loria <[email protected]>

CHANGELOG.rst

@@ -4,6 +4,11 @@ Changelog
 0.16.0 (unreleased)
 *******************
 
+Features:
+
+* Add field ``Config`` for serializing Flask configuration values (:issue:`280`, :pr:`281`).
+  Thanks :user:`greyli` for the PR.
+
 Support:
 
 * Support marshmallow-sqlalchemy>=0.29.0.

src/flask_marshmallow/fields.py

@@ -10,11 +10,19 @@
 import re
 
 from flask import url_for
+from flask import current_app
 from marshmallow import fields
 from marshmallow import missing
 
 
-__all__ = ["URLFor", "UrlFor", "AbsoluteURLFor", "AbsoluteUrlFor", "Hyperlinks"]
+__all__ = [
+    "URLFor",
+    "UrlFor",
+    "AbsoluteURLFor",
+    "AbsoluteUrlFor",
+    "Hyperlinks",
+    "Config",
+]
 
 
 _tpl_pattern = re.compile(r"\s*<\s*(\S*)\s*>\s*")
@@ -178,3 +186,35 @@ def __init__(self, schema, **kwargs):
 
     def _serialize(self, value, attr, obj):
         return _rapply(self.schema, _url_val, key=attr, obj=obj)
+
+
+class Config(fields.Field):
+    """A field for Flask configuration values.
+
+    Examples: ::
+
+        from flask import Flask
+
+        app = Flask(__name__)
+        app.config['API_TITLE'] = 'Pet API'
+
+        class FooSchema(Schema):
+            user = String()
+            title = Config('API_TITLE')
+
+    This field should only be used in an output schema. A ``ValueError`` will
+    be raised if the config key is not found in the app config.
+
+    :param str key: The key of the configuration value.
+    """
+
+    _CHECK_ATTRIBUTE = False
+
+    def __init__(self, key, **kwargs):
+        fields.Field.__init__(self, **kwargs)
+        self.key = key
+
+    def _serialize(self, value, attr, obj, **kwargs):
+        if self.key not in current_app.config:
+            raise ValueError(f"The key {self.key!r} is not found in the app config.")
+        return current_app.config[self.key]

tests/test_fields.py

@@ -137,3 +137,15 @@ def test_aliases(ma):
 
     assert UrlFor is URLFor
     assert AbsoluteUrlFor is AbsoluteURLFor
+
+
+def test_config_field(ma, app, mockauthor):
+    app.config["NAME"] = "test"
+    field = ma.Config(key="NAME")
+
+    result = field.serialize("config_value", mockauthor)
+    assert result == "test"
+
+    field = ma.Config(key="DOES_NOT_EXIST")
+    with pytest.raises(ValueError, match="not found in the app config"):
+        field.serialize("config_value", mockauthor)