diff --git a/CHANGES.md b/CHANGES.md
index d96bd12..888f029 100644
--- a/CHANGES.md
+++ b/CHANGES.md
@@ -1,6 +1,7 @@
 ### 0.24.0 (202x-xx-xx xx:xx:00 UTC)
 
 * Change abbreviate long titles under menu tab
+* Update attr 20.2.0 (4f74fba) to 20.3.0 (f3762ba)
 * Update Requests library 2.24.0 (2f70990) to 2.25.0 (03957eb)
 * Update urllib3 1.25.11 (00f1769) to 1.26.1 (7675532)
 
diff --git a/lib/attr/__init__.py b/lib/attr/__init__.py
index 7a79e57..bf329ca 100644
--- a/lib/attr/__init__.py
+++ b/lib/attr/__init__.py
@@ -21,7 +21,7 @@ from ._make import (
 from ._version_info import VersionInfo
 
 
-__version__ = "20.2.0"
+__version__ = "20.3.0"
 __version_info__ = VersionInfo._from_version_string(__version__)
 
 __title__ = "attrs"
diff --git a/lib/attr/__init__.pyi b/lib/attr/__init__.pyi
index 0869914..442d6e7 100644
--- a/lib/attr/__init__.pyi
+++ b/lib/attr/__init__.pyi
@@ -46,6 +46,7 @@ _OnSetAttrType = Callable[[Any, Attribute[Any], Any], Any]
 _OnSetAttrArgType = Union[
     _OnSetAttrType, List[_OnSetAttrType], setters._NoOpType
 ]
+_FieldTransformer = Callable[[type, List[Attribute]], List[Attribute]]
 # FIXME: in reality, if multiple validators are passed they must be in a list
 # or tuple, but those are invariant and so would prevent subtypes of
 # _ValidatorType from working when passed in a list or tuple.
@@ -272,8 +273,10 @@ def attrs(
     eq: Optional[bool] = ...,
     order: Optional[bool] = ...,
     auto_detect: bool = ...,
+    collect_by_mro: bool = ...,
     getstate_setstate: Optional[bool] = ...,
     on_setattr: Optional[_OnSetAttrArgType] = ...,
+    field_transformer: Optional[_FieldTransformer] = ...,
 ) -> _C: ...
 @overload
 def attrs(
@@ -295,8 +298,10 @@ def attrs(
     eq: Optional[bool] = ...,
     order: Optional[bool] = ...,
     auto_detect: bool = ...,
+    collect_by_mro: bool = ...,
     getstate_setstate: Optional[bool] = ...,
     on_setattr: Optional[_OnSetAttrArgType] = ...,
+    field_transformer: Optional[_FieldTransformer] = ...,
 ) -> Callable[[_C], _C]: ...
 @overload
 def define(
@@ -319,6 +324,7 @@ def define(
     auto_detect: bool = ...,
     getstate_setstate: Optional[bool] = ...,
     on_setattr: Optional[_OnSetAttrArgType] = ...,
+    field_transformer: Optional[_FieldTransformer] = ...,
 ) -> _C: ...
 @overload
 def define(
@@ -341,6 +347,7 @@ def define(
     auto_detect: bool = ...,
     getstate_setstate: Optional[bool] = ...,
     on_setattr: Optional[_OnSetAttrArgType] = ...,
+    field_transformer: Optional[_FieldTransformer] = ...,
 ) -> Callable[[_C], _C]: ...
 
 mutable = define
@@ -381,7 +388,9 @@ def make_class(
     auto_exc: bool = ...,
     eq: Optional[bool] = ...,
     order: Optional[bool] = ...,
+    collect_by_mro: bool = ...,
     on_setattr: Optional[_OnSetAttrArgType] = ...,
+    field_transformer: Optional[_FieldTransformer] = ...,
 ) -> type: ...
 
 # _funcs --
@@ -397,6 +406,7 @@ def asdict(
     filter: Optional[_FilterType[Any]] = ...,
     dict_factory: Type[Mapping[Any, Any]] = ...,
     retain_collection_types: bool = ...,
+    value_serializer: Optional[Callable[[type, Attribute, Any], Any]] = ...,
 ) -> Dict[str, Any]: ...
 
 # TODO: add support for returning NamedTuple from the mypy plugin
diff --git a/lib/attr/_compat.py b/lib/attr/_compat.py
index bed5b13..b0ead6e 100644
--- a/lib/attr/_compat.py
+++ b/lib/attr/_compat.py
@@ -91,7 +91,7 @@ if PY2:
         res.data.update(d)  # We blocked update, so we have to do it like this.
         return res
 
-    def just_warn(*args, **kw):  # pragma: nocover
+    def just_warn(*args, **kw):  # pragma: no cover
         """
         We only warn on Python 3 because we are not aware of any concrete
         consequences of not setting the cell on Python 2.
@@ -132,7 +132,7 @@ def make_set_closure_cell():
     """
     # pypy makes this easy. (It also supports the logic below, but
     # why not do the easy/fast thing?)
-    if PYPY:  # pragma: no cover
+    if PYPY:
 
         def set_closure_cell(cell, value):
             cell.__setstate__((value,))
diff --git a/lib/attr/_funcs.py b/lib/attr/_funcs.py
index ca92f9f..e6c930c 100644
--- a/lib/attr/_funcs.py
+++ b/lib/attr/_funcs.py
@@ -13,6 +13,7 @@ def asdict(
     filter=None,
     dict_factory=dict,
     retain_collection_types=False,
+    value_serializer=None,
 ):
     """
     Return the ``attrs`` attribute values of *inst* as a dict.
@@ -32,6 +33,10 @@ def asdict(
     :param bool retain_collection_types: Do not convert to ``list`` when
         encountering an attribute whose type is ``tuple`` or ``set``.  Only
         meaningful if ``recurse`` is ``True``.
+    :param Optional[callable] value_serializer: A hook that is called for every
+        attribute or dict key/value.  It receives the current instance, field
+        and value and must return the (updated) value.  The hook is run *after*
+        the optional *filter* has been applied.
 
     :rtype: return type of *dict_factory*
 
@@ -40,6 +45,7 @@ def asdict(
 
     ..  versionadded:: 16.0.0 *dict_factory*
     ..  versionadded:: 16.1.0 *retain_collection_types*
+    ..  versionadded:: 20.3.0 *value_serializer*
     """
     attrs = fields(inst.__class__)
     rv = dict_factory()
@@ -47,17 +53,30 @@ def asdict(
         v = getattr(inst, a.name)
         if filter is not None and not filter(a, v):
             continue
+
+        if value_serializer is not None:
+            v = value_serializer(inst, a, v)
+
         if recurse is True:
             if has(v.__class__):
                 rv[a.name] = asdict(
-                    v, True, filter, dict_factory, retain_collection_types
+                    v,
+                    True,
+                    filter,
+                    dict_factory,
+                    retain_collection_types,
+                    value_serializer,
                 )
-            elif isinstance(v, (tuple, list, set)):
+            elif isinstance(v, (tuple, list, set, frozenset)):
                 cf = v.__class__ if retain_collection_types is True else list
                 rv[a.name] = cf(
                     [
                         _asdict_anything(
-                            i, filter, dict_factory, retain_collection_types
+                            i,
+                            filter,
+                            dict_factory,
+                            retain_collection_types,
+                            value_serializer,
                         )
                         for i in v
                     ]
@@ -67,10 +86,18 @@ def asdict(
                 rv[a.name] = df(
                     (
                         _asdict_anything(
-                            kk, filter, df, retain_collection_types
+                            kk,
+                            filter,
+                            df,
+                            retain_collection_types,
+                            value_serializer,
                         ),
                         _asdict_anything(
-                            vv, filter, df, retain_collection_types
+                            vv,
+                            filter,
+                            df,
+                            retain_collection_types,
+                            value_serializer,
                         ),
                     )
                     for kk, vv in iteritems(v)
@@ -82,19 +109,36 @@ def asdict(
     return rv
 
 
-def _asdict_anything(val, filter, dict_factory, retain_collection_types):
+def _asdict_anything(
+    val,
+    filter,
+    dict_factory,
+    retain_collection_types,
+    value_serializer,
+):
     """
     ``asdict`` only works on attrs instances, this works on anything.
     """
     if getattr(val.__class__, "__attrs_attrs__", None) is not None:
         # Attrs class.
-        rv = asdict(val, True, filter, dict_factory, retain_collection_types)
-    elif isinstance(val, (tuple, list, set)):
+        rv = asdict(
+            val,
+            True,
+            filter,
+            dict_factory,
+            retain_collection_types,
+            value_serializer,
+        )
+    elif isinstance(val, (tuple, list, set, frozenset)):
         cf = val.__class__ if retain_collection_types is True else list
         rv = cf(
             [
                 _asdict_anything(
-                    i, filter, dict_factory, retain_collection_types
+                    i,
+                    filter,
+                    dict_factory,
+                    retain_collection_types,
+                    value_serializer,
                 )
                 for i in val
             ]
@@ -103,13 +147,20 @@ def _asdict_anything(val, filter, dict_factory, retain_collection_types):
         df = dict_factory
         rv = df(
             (
-                _asdict_anything(kk, filter, df, retain_collection_types),
-                _asdict_anything(vv, filter, df, retain_collection_types),
+                _asdict_anything(
+                    kk, filter, df, retain_collection_types, value_serializer
+                ),
+                _asdict_anything(
+                    vv, filter, df, retain_collection_types, value_serializer
+                ),
             )
             for kk, vv in iteritems(val)
         )
     else:
         rv = val
+        if value_serializer is not None:
+            rv = value_serializer(None, None, rv)
+
     return rv
 
 
@@ -164,7 +215,7 @@ def astuple(
                         retain_collection_types=retain,
                     )
                 )
-            elif isinstance(v, (tuple, list, set)):
+            elif isinstance(v, (tuple, list, set, frozenset)):
                 cf = v.__class__ if retain is True else list
                 rv.append(
                     cf(
@@ -209,6 +260,7 @@ def astuple(
                 rv.append(v)
         else:
             rv.append(v)
+
     return rv if tuple_factory is list else tuple_factory(rv)
 
 
diff --git a/lib/attr/_make.py b/lib/attr/_make.py
index 0fbbd7c..49484f9 100644
--- a/lib/attr/_make.py
+++ b/lib/attr/_make.py
@@ -12,6 +12,7 @@ from operator import itemgetter
 from . import _config, setters
 from ._compat import (
     PY2,
+    PYPY,
     isclass,
     iteritems,
     metadata_proxy,
@@ -223,6 +224,7 @@ def attrib(
     .. deprecated:: 19.2.0 *cmp* Removal on or after 2021-06-01.
     .. versionadded:: 19.2.0 *eq* and *order*
     .. versionadded:: 20.1.0 *on_setattr*
+    .. versionchanged:: 20.3.0 *kw_only* backported to Python 2
     """
     eq, order = _determine_eq_order(cmp, eq, order, True)
 
@@ -373,7 +375,7 @@ def _collect_base_attrs(cls, taken_attr_names):
             if a.inherited or a.name in taken_attr_names:
                 continue
 
-            a = a._assoc(inherited=True)
+            a = a.evolve(inherited=True)
             base_attrs.append(a)
             base_attr_map[a.name] = base_cls
 
@@ -411,7 +413,7 @@ def _collect_base_attrs_broken(cls, taken_attr_names):
             if a.name in taken_attr_names:
                 continue
 
-            a = a._assoc(inherited=True)
+            a = a.evolve(inherited=True)
             taken_attr_names.add(a.name)
             base_attrs.append(a)
             base_attr_map[a.name] = base_cls
@@ -419,7 +421,9 @@ def _collect_base_attrs_broken(cls, taken_attr_names):
     return base_attrs, base_attr_map
 
 
-def _transform_attrs(cls, these, auto_attribs, kw_only, collect_by_mro):
+def _transform_attrs(
+    cls, these, auto_attribs, kw_only, collect_by_mro, field_transformer
+):
     """
     Transform all `_CountingAttr`s on a class into `Attribute`s.
 
@@ -451,6 +455,7 @@ def _transform_attrs(cls, these, auto_attribs, kw_only, collect_by_mro):
                 continue
             annot_names.add(attr_name)
             a = cd.get(attr_name, NOTHING)
+
             if not isinstance(a, _CountingAttr):
                 if a is NOTHING:
                     a = attrib()
@@ -498,8 +503,8 @@ def _transform_attrs(cls, these, auto_attribs, kw_only, collect_by_mro):
     AttrsClass = _make_attr_tuple_class(cls.__name__, attr_names)
 
     if kw_only:
-        own_attrs = [a._assoc(kw_only=True) for a in own_attrs]
-        base_attrs = [a._assoc(kw_only=True) for a in base_attrs]
+        own_attrs = [a.evolve(kw_only=True) for a in own_attrs]
+        base_attrs = [a.evolve(kw_only=True) for a in base_attrs]
 
     attrs = AttrsClass(base_attrs + own_attrs)
 
@@ -518,14 +523,34 @@ def _transform_attrs(cls, these, auto_attribs, kw_only, collect_by_mro):
         if had_default is False and a.default is not NOTHING:
             had_default = True
 
+    if field_transformer is not None:
+        attrs = field_transformer(cls, attrs)
     return _Attributes((attrs, base_attrs, base_attr_map))
 
 
-def _frozen_setattrs(self, name, value):
-    """
-    Attached to frozen classes as __setattr__.
-    """
-    raise FrozenInstanceError()
+if PYPY:
+
+    def _frozen_setattrs(self, name, value):
+        """
+        Attached to frozen classes as __setattr__.
+        """
+        if isinstance(self, BaseException) and name in (
+            "__cause__",
+            "__context__",
+        ):
+            BaseException.__setattr__(self, name, value)
+            return
+
+        raise FrozenInstanceError()
+
+
+else:
+
+    def _frozen_setattrs(self, name, value):
+        """
+        Attached to frozen classes as __setattr__.
+        """
+        raise FrozenInstanceError()
 
 
 def _frozen_delattrs(self, name):
@@ -574,9 +599,15 @@ class _ClassBuilder(object):
         collect_by_mro,
         on_setattr,
         has_custom_setattr,
+        field_transformer,
     ):
         attrs, base_attrs, base_map = _transform_attrs(
-            cls, these, auto_attribs, kw_only, collect_by_mro
+            cls,
+            these,
+            auto_attribs,
+            kw_only,
+            collect_by_mro,
+            field_transformer,
         )
 
         self._cls = cls
@@ -1001,6 +1032,7 @@ def attrs(
     collect_by_mro=False,
     getstate_setstate=None,
     on_setattr=None,
+    field_transformer=None,
 ):
     r"""
     A class decorator that adds `dunder
@@ -1093,12 +1125,14 @@ def attrs(
         argument name.  If a ``__attrs_post_init__`` method exists on the
         class, it will be called after the class is fully initialized.
     :param bool slots: Create a `slotted class <slotted classes>` that's more
-        memory-efficient.
+        memory-efficient. Slotted classes are generally superior to the default
+        dict classes, but have some gotchas you should know about, so we
+        encourage you to read the `glossary entry <slotted classes>`.
     :param bool frozen: Make instances immutable after initialization.  If
         someone attempts to modify a frozen instance,
         `attr.exceptions.FrozenInstanceError` is raised.
 
-        Please note:
+        .. note::
 
             1. This is achieved by installing a custom ``__setattr__`` method
                on your class, so you can't implement your own.
@@ -1184,7 +1218,7 @@ def attrs(
 
     :param on_setattr: A callable that is run whenever the user attempts to set
         an attribute (either by assignment like ``i.x = 42`` or by using
-        `setattr` like ``setattr(i, "x", 42)``). It receives the same argument
+        `setattr` like ``setattr(i, "x", 42)``). It receives the same arguments
         as validators: the instance, the attribute that is being modified, and
         the new value.
 
@@ -1194,6 +1228,11 @@ def attrs(
         If a list of callables is passed, they're automatically wrapped in an
         `attr.setters.pipe`.
 
+    :param Optional[callable] field_transformer:
+        A function that is called with the original class object and all
+        fields right before ``attrs`` finalizes the class.  You can use
+        this, e.g., to automatically add converters or validators to
+        fields based on their types.  See `transform-fields` for more details.
 
     .. versionadded:: 16.0.0 *slots*
     .. versionadded:: 16.1.0 *frozen*
@@ -1223,6 +1262,7 @@ def attrs(
     .. versionadded:: 20.1.0 *collect_by_mro*
     .. versionadded:: 20.1.0 *getstate_setstate*
     .. versionadded:: 20.1.0 *on_setattr*
+    .. versionadded:: 20.3.0 *field_transformer*
     """
     if auto_detect and PY2:
         raise PythonTooOldError(
@@ -1269,6 +1309,7 @@ def attrs(
             collect_by_mro,
             on_setattr,
             has_own_setattr,
+            field_transformer,
         )
         if _determine_whether_to_implement(
             cls, repr, auto_detect, ("__repr__",)
@@ -1903,6 +1944,63 @@ def _assign_with_converter(attr_name, value_var, has_on_setattr):
     )
 
 
+if PY2:
+
+    def _unpack_kw_only_py2(attr_name, default=None):
+        """
+        Unpack *attr_name* from _kw_only dict.
+        """
+        if default is not None:
+            arg_default = ", %s" % default
+        else:
+            arg_default = ""
+        return "%s = _kw_only.pop('%s'%s)" % (
+            attr_name,
+            attr_name,
+            arg_default,
+        )
+
+    def _unpack_kw_only_lines_py2(kw_only_args):
+        """
+        Unpack all *kw_only_args* from _kw_only dict and handle errors.
+
+        Given a list of strings "{attr_name}" and "{attr_name}={default}"
+        generates list of lines of code that pop attrs from _kw_only dict and
+        raise TypeError similar to builtin if required attr is missing or
+        extra key is passed.
+
+        >>> print("\n".join(_unpack_kw_only_lines_py2(["a", "b=42"])))
+        try:
+            a = _kw_only.pop('a')
+            b = _kw_only.pop('b', 42)
+        except KeyError as _key_error:
+            raise TypeError(
+                ...
+        if _kw_only:
+            raise TypeError(
+                ...
+        """
+        lines = ["try:"]
+        lines.extend(
+            "    " + _unpack_kw_only_py2(*arg.split("="))
+            for arg in kw_only_args
+        )
+        lines += """\
+except KeyError as _key_error:
+    raise TypeError(
+        '__init__() missing required keyword-only argument: %s' % _key_error
+    )
+if _kw_only:
+    raise TypeError(
+        '__init__() got an unexpected keyword argument %r'
+        % next(iter(_kw_only))
+    )
+""".split(
+            "\n"
+        )
+        return lines
+
+
 def _attrs_to_init_script(
     attrs,
     frozen,
@@ -2157,14 +2255,14 @@ def _attrs_to_init_script(
     args = ", ".join(args)
     if kw_only_args:
         if PY2:
-            raise PythonTooOldError(
-                "Keyword-only arguments only work on Python 3 and later."
-            )
+            lines = _unpack_kw_only_lines_py2(kw_only_args) + lines
 
-        args += "{leading_comma}*, {kw_only_args}".format(
-            leading_comma=", " if args else "",
-            kw_only_args=", ".join(kw_only_args),
-        )
+            args += "%s**_kw_only" % (", " if args else "",)  # leading comma
+        else:
+            args += "%s*, %s" % (
+                ", " if args else "",  # leading comma
+                ", ".join(kw_only_args),  # kw_only args
+            )
     return (
         """\
 def __init__(self, {args}):
@@ -2181,6 +2279,13 @@ class Attribute(object):
     """
     *Read-only* representation of an attribute.
 
+    Instances of this class are frequently used for introspection purposes
+    like:
+
+    - `fields` returns a tuple of them.
+    - Validators get them passed as the first argument.
+    - The *field transformer* hook receives a list of them.
+
     :attribute name: The name of the attribute.
     :attribute inherited: Whether or not that attribute has been inherited from
         a base class.
@@ -2303,10 +2408,17 @@ class Attribute(object):
 
         return self.eq and self.order
 
-    # Don't use attr.assoc since fields(Attribute) doesn't work
-    def _assoc(self, **changes):
+    # Don't use attr.evolve since fields(Attribute) doesn't work
+    def evolve(self, **changes):
         """
         Copy *self* and apply *changes*.
+
+        This works similarly to `attr.evolve` but that function does not work
+        with ``Attribute``.
+
+        It is mainly meant to be used for `transform-fields`.
+
+        .. versionadded:: 20.3.0
         """
         new = copy.copy(self)
 
diff --git a/lib/attr/_next_gen.py b/lib/attr/_next_gen.py
index b5ff60e..2b5565c 100644
--- a/lib/attr/_next_gen.py
+++ b/lib/attr/_next_gen.py
@@ -33,6 +33,7 @@ def define(
     auto_detect=True,
     getstate_setstate=None,
     on_setattr=None,
+    field_transformer=None,
 ):
     r"""
     The only behavioral differences are the handling of the *auto_attribs*
@@ -72,6 +73,7 @@ def define(
             collect_by_mro=True,
             getstate_setstate=getstate_setstate,
             on_setattr=on_setattr,
+            field_transformer=field_transformer,
         )
 
     def wrap(cls):