Introduce attribute metadata.

CHANGELOG.rst

@@ -11,6 +11,8 @@ The third digit is only for regressions.
 Changes:
 ^^^^^^^^
 
+- Attributes now can have user-defined metadata which greatly improves ``attrs``'s extensibility.
+  `#96 <https://github.com/hynek/attrs/pull/96>`_
 - Don't overwrite ``__name__`` with ``__qualname__`` for ``attr.s(slots=True)`` classes.
   `#99 <https://github.com/hynek/attrs/issues/99>`_
 

docs/api.rst

@@ -69,7 +69,7 @@ Core
       ... class C(object):
       ...     x = attr.ib()
       >>> C.x
-      Attribute(name='x', default=NOTHING, validator=None, repr=True, cmp=True, hash=True, init=True, convert=None)
+      Attribute(name='x', default=NOTHING, validator=None, repr=True, cmp=True, hash=True, init=True, convert=None, metadata=mappingproxy({}))
 
 
 .. autofunction:: attr.make_class
@@ -125,9 +125,9 @@ Helpers
       ...     x = attr.ib()
       ...     y = attr.ib()
       >>> attr.fields(C)
-      (Attribute(name='x', default=NOTHING, validator=None, repr=True, cmp=True, hash=True, init=True, convert=None), Attribute(name='y', default=NOTHING, validator=None, repr=True, cmp=True, hash=True, init=True, convert=None))
+      (Attribute(name='x', default=NOTHING, validator=None, repr=True, cmp=True, hash=True, init=True, convert=None, metadata=mappingproxy({})), Attribute(name='y', default=NOTHING, validator=None, repr=True, cmp=True, hash=True, init=True, convert=None, metadata=mappingproxy({})))
       >>> attr.fields(C)[1]
-      Attribute(name='y', default=NOTHING, validator=None, repr=True, cmp=True, hash=True, init=True, convert=None)
+      Attribute(name='y', default=NOTHING, validator=None, repr=True, cmp=True, hash=True, init=True, convert=None, metadata=mappingproxy({}))
       >>> attr.fields(C).y is attr.fields(C)[1]
       True
 

docs/examples.rst

@@ -405,6 +405,29 @@ Converters are run *before* validators, so you can use validators to check the f
     ValueError: x must be be at least 0.
 
 
+.. _metadata:
+
+Metadata
+--------
+
+All ``attrs`` attributes may include arbitrary metadata in the form on a read-only dictionary.
+
+.. doctest::
+
+    >>> @attr.s
+    ... class C(object):
+    ...    x = attr.ib(metadata={'my_metadata': 1})
+    >>> attr.fields(C).x.metadata
+    mappingproxy({'my_metadata': 1})
+    >>> attr.fields(C).x.metadata['my_metadata']
+    1
+
+Metadata is not used by ``attrs``, and is meant to enable rich functionality in third-party libraries.
+The metadata dictionary follows the normal dictionary rules: keys need to be hashable, and both keys and values are recommended to be immutable.
+
+If you're the author of a third-party library with ``attrs`` integration, please see :ref:`Extending Metadata <extending_metadata>`.
+
+
 .. _slots:
 
 Slots
@@ -458,7 +481,7 @@ Slot classes are a little different than ordinary, dictionary-backed classes:
     ... class C(object):
     ...     x = attr.ib()
     >>> C.x
-    Attribute(name='x', default=NOTHING, validator=None, repr=True, cmp=True, hash=True, init=True, convert=None)
+    Attribute(name='x', default=NOTHING, validator=None, repr=True, cmp=True, hash=True, init=True, convert=None, metadata=mappingproxy({}))
     >>> @attr.s(slots=True)
     ... class C(object):
     ...     x = attr.ib()

docs/extending.rst

@@ -17,7 +17,7 @@ So it is fairly simple to build your own decorators on top of ``attrs``:
    ... @attr.s
    ... class C(object):
    ...     a = attr.ib()
-   (Attribute(name='a', default=NOTHING, validator=None, repr=True, cmp=True, hash=True, init=True, convert=None),)
+   (Attribute(name='a', default=NOTHING, validator=None, repr=True, cmp=True, hash=True, init=True, convert=None, metadata=mappingproxy({})),)
 
 
 .. warning::
@@ -36,3 +36,43 @@ So it is fairly simple to build your own decorators on top of ``attrs``:
          pass
 
       f = a(b(original_f))
+
+.. _extending_metadata:
+
+Metadata
+--------
+
+If you're the author of a third-party library with ``attrs`` integration, you may want to take advantage of attribute metadata.
+
+Here are some tips for effective use of metadata:
+
+- Try making your metadata keys and values immutable.
+  This keeps the entire ``Attribute`` instances immutable too.
+
+- To avoid metadata key collisions, consider exposing your metadata keys from your modules.::
+
+    from mylib import MY_METADATA_KEY
+
+    @attr.s
+    class C(object):
+      x = attr.ib(metadata={MY_METADATA_KEY: 1})
+
+  Metadata should be composable, so consider supporting this approach even if you decide implementing your metadata in one of the following ways.
+
+- Expose ``attr.ib`` wrappers for your specific metadata.
+  This is a more graceful approach if your users don't require metadata from other libraries.
+
+  .. doctest::
+
+    >>> MY_TYPE_METADATA = '__my_type_metadata'
+    >>>
+    >>> def typed(cls, default=attr.NOTHING, validator=None, repr=True, cmp=True, hash=True, init=True, convert=None, metadata={}):
+    ...     metadata = dict() if not metadata else metadata
+    ...     metadata[MY_TYPE_METADATA] = cls
+    ...     return attr.ib(default, validator, repr, cmp, hash, init, convert, metadata)
+    >>>
+    >>> @attr.s
+    ... class C(object):
+    ...     x = typed(int, default=1, init=False)
+    >>> attr.fields(C).x.metadata[MY_TYPE_METADATA]
+    <class 'int'>

src/attr/_compat.py

@@ -1,13 +1,14 @@
 from __future__ import absolute_import, division, print_function
 
 import sys
+import types
 
 
 PY2 = sys.version_info[0] == 2
 
 
 if PY2:
-    import types
+    from UserDict import IterableUserDict
 
     # We 'bundle' isclass instead of using inspect as importing inspect is
     # fairly expensive (order of 10-15 ms for a modern machine in 2016)
@@ -22,6 +23,57 @@ def iteritems(d):
 
     def iterkeys(d):
         return d.iterkeys()
+
+    # Python 2 is bereft of a read-only dict proxy, so we make one!
+    class ReadOnlyDict(IterableUserDict):
+        """
+        Best-effort read-only dict wrapper.
+        """
+
+        def __setitem__(self, key, val):
+            # We gently pretend we're a Python 3 mappingproxy.
+            raise TypeError("'mappingproxy' object does not support item "
+                            "assignment")
+
+        def update(self, _):
+            # We gently pretend we're a Python 3 mappingproxy.
+            raise AttributeError("'mappingproxy' object has no attribute "
+                                 "'update'")
+
+        def __delitem__(self, _):
+            # We gently pretend we're a Python 3 mappingproxy.
+            raise TypeError("'mappingproxy' object does not support item "
+                            "deletion")
+
+        def clear(self):
+            # We gently pretend we're a Python 3 mappingproxy.
+            raise AttributeError("'mappingproxy' object has no attribute "
+                                 "'clear'")
+
+        def pop(self, key, default=None):
+            # We gently pretend we're a Python 3 mappingproxy.
+            raise AttributeError("'mappingproxy' object has no attribute "
+                                 "'pop'")
+
+        def popitem(self):
+            # We gently pretend we're a Python 3 mappingproxy.
+            raise AttributeError("'mappingproxy' object has no attribute "
+                                 "'popitem'")
+
+        def setdefault(self, key, default=None):
+            # We gently pretend we're a Python 3 mappingproxy.
+            raise AttributeError("'mappingproxy' object has no attribute "
+                                 "'setdefault'")
+
+        def __repr__(self):
+            # Override to be identical to the Python 3 version.
+            return "mappingproxy(" + repr(self.data) + ")"
+
+    def metadata_proxy(d):
+        res = ReadOnlyDict()
+        res.data.update(d)  # We blocked update, so we have to do it like this.
+        return res
+
 else:
     def isclass(klass):
         return isinstance(klass, type)
@@ -33,3 +85,6 @@ def iteritems(d):
 
     def iterkeys(d):
         return d.keys()
+
+    def metadata_proxy(d):
+        return types.MappingProxyType(dict(d))

src/attr/_make.py

@@ -6,13 +6,14 @@
 from operator import itemgetter
 
 from . import _config
-from ._compat import iteritems, isclass, iterkeys
+from ._compat import iteritems, isclass, iterkeys, metadata_proxy
 from .exceptions import FrozenInstanceError, NotAnAttrsClassError
 
 # This is used at least twice, so cache it here.
 _obj_setattr = object.__setattr__
-_init_convert_pat = '__attr_convert_{}'
+_init_convert_pat = "__attr_convert_{}"
 _tuple_property_pat = "    {attr_name} = property(itemgetter({index}))"
+_empty_metadata_singleton = metadata_proxy({})
 
 
 class _Nothing(object):
@@ -48,7 +49,7 @@ def __hash__(self):
 
 def attr(default=NOTHING, validator=None,
          repr=True, cmp=True, hash=True, init=True,
-         convert=None):
+         convert=None, metadata={}):
     """
     Create a new attribute on a class.
 
@@ -97,6 +98,8 @@ def attr(default=NOTHING, validator=None,
         to the desired format.  It is given the passed-in value, and the
         returned value will be used as the new value of the attribute.  The
         value is converted before being passed to the validator, if any.
+    :param metadata: An arbitrary mapping, to be used by third-party
+        components.
     """
     return _CountingAttr(
         default=default,
@@ -106,6 +109,7 @@ def attr(default=NOTHING, validator=None,
         hash=hash,
         init=init,
         convert=convert,
+        metadata=metadata,
     )
 
 
@@ -132,8 +136,8 @@ class MyClassAttributes(tuple):
             ))
     else:
         attr_class_template.append("    pass")
-    globs = {'itemgetter': itemgetter}
-    eval(compile("\n".join(attr_class_template), '', 'exec'), globs)
+    globs = {"itemgetter": itemgetter}
+    eval(compile("\n".join(attr_class_template), "", "exec"), globs)
     return globs[attr_class_name]
 
 
@@ -283,7 +287,7 @@ def wrap(cls):
             for ca_name in ca_list:
                 # It might not actually be in there, e.g. if using 'these'.
                 cls_dict.pop(ca_name, None)
-            cls_dict.pop('__dict__', None)
+            cls_dict.pop("__dict__", None)
 
             qualname = getattr(cls, "__qualname__", None)
             cls = type(cls.__name__, cls.__bases__, cls_dict)
@@ -698,56 +702,65 @@ class Attribute(object):
 
     Plus *all* arguments of :func:`attr.ib`.
     """
-    __slots__ = ('name', 'default', 'validator', 'repr', 'cmp', 'hash', 'init',
-                 'convert')
-
-    _optional = {"convert": None}
+    __slots__ = ("name", "default", "validator", "repr", "cmp", "hash", "init",
+                 "convert", "metadata")
 
     def __init__(self, name, default, validator, repr, cmp, hash, init,
-                 convert=None):
+                 convert=None, metadata=None):
         # Cache this descriptor here to speed things up later.
         __bound_setattr = _obj_setattr.__get__(self, Attribute)
 
-        __bound_setattr('name', name)
-        __bound_setattr('default', default)
-        __bound_setattr('validator', validator)
-        __bound_setattr('repr', repr)
-        __bound_setattr('cmp', cmp)
-        __bound_setattr('hash', hash)
-        __bound_setattr('init', init)
-        __bound_setattr('convert', convert)
+        __bound_setattr("name", name)
+        __bound_setattr("default", default)
+        __bound_setattr("validator", validator)
+        __bound_setattr("repr", repr)
+        __bound_setattr("cmp", cmp)
+        __bound_setattr("hash", hash)
+        __bound_setattr("init", init)
+        __bound_setattr("convert", convert)
+        __bound_setattr("metadata", (metadata_proxy(metadata) if metadata
+                                     else _empty_metadata_singleton))
 
     def __setattr__(self, name, value):
         raise FrozenInstanceError()
 
     @classmethod
     def from_counting_attr(cls, name, ca):
-        return cls(name=name,
-                   **dict((k, getattr(ca, k))
-                          for k
-                          in Attribute.__slots__
-                          if k != "name"))
+        inst_dict = dict((k, getattr(ca, k))
+                         for k
+                         in Attribute.__slots__
+                         if k != "name")
+        return cls(name=name, **inst_dict)
 
     # Don't use _add_pickle since fields(Attribute) doesn't work
     def __getstate__(self):
         """
         Play nice with pickle.
         """
-        return tuple(getattr(self, name) for name in self.__slots__)
+        return tuple(getattr(self, name) if name != "metadata"
+                     else dict(self.metadata)
+                     for name in self.__slots__)
 
     def __setstate__(self, state):
         """
         Play nice with pickle.
         """
         __bound_setattr = _obj_setattr.__get__(self, Attribute)
         for name, value in zip(self.__slots__, state):
-            __bound_setattr(name, value)
+            if name != "metadata":
+                __bound_setattr(name, value)
+            else:
+                __bound_setattr(name, metadata_proxy(value) if value else
+                                _empty_metadata_singleton)
+
 
 _a = [Attribute(name=name, default=NOTHING, validator=None,
-                repr=True, cmp=True, hash=True, init=True)
+                repr=True, cmp=True, hash=(name != "metadata"), init=True)
       for name in Attribute.__slots__]
+
 Attribute = _add_hash(
-    _add_cmp(_add_repr(Attribute, attrs=_a), attrs=_a), attrs=_a
+    _add_cmp(_add_repr(Attribute, attrs=_a), attrs=_a),
+    attrs=[a for a in _a if a.hash]
 )
 
 
@@ -756,24 +769,31 @@ class _CountingAttr(object):
     Intermediate representation of attributes that uses a counter to preserve
     the order in which the attributes have been defined.
     """
-    __attrs_attrs__ = [
+    __slots__ = ("counter", "default", "repr", "cmp", "hash", "init",
+                 "metadata", "validator", "convert")
+    __attrs_attrs__ = tuple(
         Attribute(name=name, default=NOTHING, validator=None,
                   repr=True, cmp=True, hash=True, init=True)
         for name
         in ("counter", "default", "repr", "cmp", "hash", "init",)
-    ]
-    counter = 0
+    ) + (
+        Attribute(name="metadata", default=None, validator=None,
+                  repr=True, cmp=True, hash=False, init=True),
+    )
+    cls_counter = 0
 
-    def __init__(self, default, validator, repr, cmp, hash, init, convert):
-        _CountingAttr.counter += 1
-        self.counter = _CountingAttr.counter
+    def __init__(self, default, validator, repr, cmp, hash, init, convert,
+                 metadata):
+        _CountingAttr.cls_counter += 1
+        self.counter = _CountingAttr.cls_counter
         self.default = default
         self.validator = validator
         self.repr = repr
         self.cmp = cmp
         self.hash = hash
         self.init = init
         self.convert = convert
+        self.metadata = metadata
 
 
 _CountingAttr = _add_cmp(_add_repr(_CountingAttr))

tests/test_dark_magic.py

@@ -23,6 +23,7 @@ class C1Slots(object):
     x = attr.ib(validator=attr.validators.instance_of(int))
     y = attr.ib()
 
+
 foo = None