Use a context variable to pass Schema context

CHANGELOG.rst

@@ -26,7 +26,7 @@ Other changes:
 As a consequence of this change:
   - Time with time offsets are now supported.
   - YYYY-MM-DD is now accepted as a datetime and deserialized as naive 00:00 AM.
-  - `from_iso_date`, `from_iso_time` and `from_iso_datetime` are removed from `marshmallow.utils`
+  - `from_iso_date`, `from_iso_time` and `from_iso_datetime` are removed from `marshmallow.utils`.
 
 - *Backwards-incompatible*: Custom validators must raise a `ValidationError <marshmallow.exceptions.ValidationError>` for invalid values.
   Returning `False` is no longer supported (:issue:`1775`).
@@ -56,6 +56,33 @@ As a consequence of this change:
 
 Thanks :user:`ddelange` for the PR.
 
+- *Backwards-incompatible*: Remove `Schema <marshmallow.schema.Schema>`'s ``context`` attribute. Passing a context
+  should be done using `contextvars.ContextVar` (:issue:`1826`).
+  marshmallow 4 provides an experimental `Context <marshmallow.experimental.context.Context>`
+  manager class that can be used to both set and retrieve context.
+
+.. code-block:: python
+
+    import typing
+
+    from marshmallow import Schema, fields
+    from marshmallow.experimental.context import Context
+
+
+    class UserContext(typing.TypedDict):
+        suffix: str
+
+
+    class UserSchema(Schema):
+        name_suffixed = fields.Function(
+            lambda obj: obj["name"] + Context[UserContext].get()["suffix"]
+        )
+
+
+    with Context[UserContext]({"suffix": "bar"}):
+        UserSchema().dump({"name": "foo"})
+        # {'name_suffixed': 'foobar'}
+
 Deprecations/Removals:
 
 - *Backwards-incompatible*: Remove implicit field creation, i.e. using the ``fields`` or ``additional`` class Meta options with undeclared fields (:issue:`1356`).

docs/api_reference.rst

@@ -10,6 +10,7 @@ API Reference
     marshmallow.decorators
     marshmallow.validate
     marshmallow.utils
+    marshmallow.experimental.context
     marshmallow.error_store
     marshmallow.class_registry
     marshmallow.exceptions

docs/custom_fields.rst

@@ -95,38 +95,72 @@ Both :class:`Function <marshmallow.fields.Function>` and :class:`Method <marshma
     result = schema.load({"balance": "100.00"})
     result["balance"]  # => 100.0
 
-.. _adding-context:
+.. _using_context:
 
-Adding context to `Method` and `Function` fields
-------------------------------------------------
+Using context
+-------------
 
-A :class:`Function <marshmallow.fields.Function>` or :class:`Method <marshmallow.fields.Method>` field may need information about its environment to know how to serialize a value.
+A field may need information about its environment to know how to (de)serialize a value.
 
-In these cases, you can set the ``context`` attribute (a dictionary) of a `Schema`. :class:`Function <marshmallow.fields.Function>` and :class:`Method <marshmallow.fields.Method>` fields will have access to this dictionary.
+You can use the experimental `Context <marshmallow.experimental.context.Context>` class
+to set and retrieve context.
 
-As an example, you might want your ``UserSchema`` to output whether or not a ``User`` is the author of a ``Blog`` or whether a certain word appears in a ``Blog's`` title.
+Let's say your ``UserSchema`` needs to output
+whether or not a ``User`` is the author of a ``Blog`` or
+whether a certain word appears in a ``Blog's`` title.
 
 .. code-block:: python
 
+    import typing
+    from dataclasses import dataclass
+
+    from marshmallow import Schema, fields
+    from marshmallow.experimental.context import Context
+
+
+    @dataclass
+    class User:
+        name: str
+
+
+    @dataclass
+    class Blog:
+        title: str
+        author: User
+
+
+    class ContextDict(typing.TypedDict):
+        blog: Blog
+
+
     class UserSchema(Schema):
         name = fields.String()
-        # Function fields optionally receive context argument
-        is_author = fields.Function(lambda user, context: user == context["blog"].author)
+
+        is_author = fields.Function(
+            lambda user: user == Context[ContextDict].get()["blog"].author
+        )
         likes_bikes = fields.Method("writes_about_bikes")
 
-        def writes_about_bikes(self, user):
-            return "bicycle" in self.context["blog"].title.lower()
+        def writes_about_bikes(self, user: User) -> bool:
+            return "bicycle" in Context[ContextDict].get()["blog"].title.lower()
 
+.. note::
+    You can use `Context.get <marshmallow.experimental.context.Context.get>`
+    within custom fields, pre-/post-processing methods, and validators.
+
+When (de)serializing, set the context by using `Context <marshmallow.experimental.context.Context>` as a context manager.
+
+.. code-block:: python
 
-    schema = UserSchema()
 
     user = User("Freddie Mercury", "[email protected]")
     blog = Blog("Bicycle Blog", author=user)
 
-    schema.context = {"blog": blog}
-    result = schema.dump(user)
-    result["is_author"]  # => True
-    result["likes_bikes"]  # => True
+    schema = UserSchema()
+    with Context({"blog": blog}):
+        result = schema.dump(user)
+        print(result["is_author"])  # => True
+        print(result["likes_bikes"])  # => True
 
 
 Customizing error messages

docs/extending.rst

@@ -454,19 +454,6 @@ Our application schemas can now inherit from our custom schema class.
     result = ser.dump(user)
     result  # {"user": {"name": "Keith", "email": "[email protected]"}}
 
-Using context
--------------
-
-The ``context`` attribute of a `Schema` is a general-purpose store for extra information that may be needed for (de)serialization. It may be used in both ``Schema`` and ``Field`` methods.
-
-.. code-block:: python
-
-    schema = UserSchema()
-    # Make current HTTP request available to
-    # custom fields, schema methods, schema validators, etc.
-    schema.context["request"] = request
-    schema.dump(user)
-
 Custom error messages
 ---------------------
 

docs/marshmallow.experimental.context.rst

@@ -0,0 +1,5 @@
+Context (experimental)
+======================
+
+.. automodule:: marshmallow.experimental.context
+    :members:

docs/upgrading.rst

@@ -124,6 +124,58 @@ If you want to use anonymous functions, you can use this helper function.
     class UserSchema(Schema):
         password = fields.String(validate=predicate(lambda x: x == "password"))
 
+New context API
+***************
+
+Passing context to `Schema <marshmallow.schema.Schema>` classes is no longer supported. Use `contextvars.ContextVar` for passing context to
+fields, pre-/post-processing methods, and validators instead.
+
+marshmallow 4 provides an experimental `Context <marshmallow.experimental.context.Context>`
+manager class that can be used to both set and retrieve context.
+
+.. code-block:: python
+
+    # 3.x
+    from marshmallow import Schema, fields
+
+
+    class UserSchema(Schema):
+        name_suffixed = fields.Function(
+            lambda obj, context: obj["name"] + context["suffix"]
+        )
+
+
+    user_schema = UserSchema()
+    user_schema.context = {"suffix": "bar"}
+    user_schema.dump({"name": "foo"})
+    # {'name_suffixed': 'foobar'}
+
+    # 4.x
+    import typing
+
+    from marshmallow import Schema, fields
+    from marshmallow.experimental.context import Context
+
+
+    class UserContext(typing.TypedDict):
+        suffix: str
+
+
+    UserSchemaContext = Context[UserContext]
+
+
+    class UserSchema(Schema):
+        name_suffixed = fields.Function(
+            lambda obj: obj["name"] + UserSchemaContext.get()["suffix"]
+        )
+
+
+    with UserSchemaContext({"suffix": "bar"}):
+        UserSchema().dump({"name": "foo"})
+        # {'name_suffixed': 'foobar'}
+
+See :ref:`using_context` for more information.
+
 Implicit field creation is removed
 **********************************
 
@@ -237,8 +289,8 @@ if you need to change the final output type.
 ``pass_many`` is renamed to ``pass_collection`` in decorators
 *************************************************************
 
-The ``pass_many`` argument to `pre_load <marshmallow.decorators.pre_load>`, 
-`post_load <marshmallow.decorators.post_load>`, `pre_dump <marshmallow.decorators.pre_dump>`, 
+The ``pass_many`` argument to `pre_load <marshmallow.decorators.pre_load>`,
+`post_load <marshmallow.decorators.post_load>`, `pre_dump <marshmallow.decorators.pre_dump>`,
 and `post_dump <marshmallow.decorators.post_dump>` is renamed to ``pass_collection``.
 
 The behavior is unchanged.
@@ -309,7 +361,7 @@ Upgrading to 3.13
 ``load_default`` and ``dump_default``
 +++++++++++++++++++++++++++++++++++++
 
-The ``missing`` and ``default`` parameters of fields are renamed to 
+The ``missing`` and ``default`` parameters of fields are renamed to
 ``load_default`` and ``dump_default``, respectively.
 
 .. code-block:: python
@@ -330,6 +382,7 @@ The ``missing`` and ``default`` parameters of fields are renamed to
 
 ``load_default`` and ``dump_default`` are passed to the field constructor as keyword arguments.
 
+
 Upgrading to 3.3
 ++++++++++++++++
 

docs/why.rst

@@ -55,39 +55,6 @@ In this example, a single schema produced three different outputs! The dynamic n
 .. _Django REST Framework: https://www.django-rest-framework.org/
 .. _Flask-RESTful: https://flask-restful.readthedocs.io/
 
-
-Context-aware serialization
----------------------------
-
-marshmallow schemas can modify their output based on the context in which they are used. Field objects have access to a ``context`` dictionary that can be changed at runtime.
-
-Here's a simple example that shows how a `Schema <marshmallow.Schema>` can anonymize a person's name when a boolean is set on the context.
-
-.. code-block:: python
-
-    class PersonSchema(Schema):
-        id = fields.Integer()
-        name = fields.Method("get_name")
-
-        def get_name(self, person, context):
-            if context.get("anonymize"):
-                return "<anonymized>"
-            return person.name
-
-
-    person = Person(name="Monty")
-    schema = PersonSchema()
-    schema.dump(person)  # {'id': 143, 'name': 'Monty'}
-
-    # In a different context, anonymize the name
-    schema.context["anonymize"] = True
-    schema.dump(person)  # {'id': 143, 'name': '<anonymized>'}
-
-
-.. seealso::
-
-    See the relevant section of the :ref:`usage guide <adding-context>` to learn more about context-aware serialization.
-
 Advanced schema nesting
 -----------------------
 

src/marshmallow/experimental/__init__.py

@@ -0,0 +1,5 @@
+"""Experimental features.
+
+The features in this subpackage are experimental. Breaking changes may be
+introduced in minor marshmallow versions.
+"""

src/marshmallow/experimental/context.py

@@ -0,0 +1,61 @@
+"""Helper API for setting serialization/deserialization context.
+
+Example usage:
+
+.. code-block:: python
+
+    import typing
+
+    from marshmallow import Schema, fields
+    from marshmallow.experimental.context import Context
+
+
+    class UserContext(typing.TypedDict):
+        suffix: str
+
+
+    UserSchemaContext = Context[UserContext]
+
+
+    class UserSchema(Schema):
+        name_suffixed = fields.Function(
+            lambda user: user["name"] + UserSchemaContext.get()["suffix"]
+        )
+
+
+    with UserSchemaContext({"suffix": "bar"}):
+        print(UserSchema().dump({"name": "foo"}))
+        # {'name_suffixed': 'foobar'}
+"""
+
+import contextlib
+import contextvars
+import typing
+
+_T = typing.TypeVar("_T")
+_CURRENT_CONTEXT: contextvars.ContextVar = contextvars.ContextVar("context")
+
+
+class Context(contextlib.AbstractContextManager, typing.Generic[_T]):
+    """Context manager for setting and retrieving context."""
+
+    def __init__(self, context: _T) -> None:
+        self.context = context
+        self.token: contextvars.Token | None = None
+
+    def __enter__(self) -> None:
+        self.token = _CURRENT_CONTEXT.set(self.context)
+
+    def __exit__(self, *args, **kwargs) -> None:
+        _CURRENT_CONTEXT.reset(typing.cast(contextvars.Token, self.token))
+
+    @classmethod
+    def get(cls, default=...) -> _T:
+        """Get the current context.
+
+        :param default: Default value to return if no context is set.
+            If not provided and no context is set, a :exc:`LookupError` is raised.
+        """
+        if default is not ...:
+            return _CURRENT_CONTEXT.get(default)
+        return _CURRENT_CONTEXT.get()