Add a EasyMixin and UninheritedDefaults

easypy/meta.py

@@ -1,6 +1,7 @@
-from abc import ABCMeta
+from abc import ABCMeta, abstractmethod
 from functools import wraps
 from collections import OrderedDict
+from enum import Enum
 
 from .decorations import kwargs_resilient
 
@@ -33,6 +34,11 @@ def __new__(mcs, name, bases, dct, **kwargs):
             if isinstance(base, EasyMeta):
                 hooks.extend(base._em_hooks)
 
+        if hooks.hooks['before_subclass_init']:
+            bases = list(bases)
+            hooks.before_subclass_init(name, bases, dct)
+            bases = tuple(bases)
+
         new_type = super().__new__(mcs, name, bases, dct)
 
         new_type._em_hooks = hooks
@@ -75,6 +81,30 @@ def extend(self, other):
         for k, v in other.hooks.items():
             self.hooks[k].extend(v)
 
+    @hook
+    def before_subclass_init(self, name, bases, dct):
+        """
+        Invoked before a subclass is being initialized
+
+        :param name: The name of the class. Immutable.
+        :param list bases: The bases of the class. A list, so it can be changed.
+        :param dct: The body of the class.
+
+        >>> class NoB(metaclass=EasyMeta):
+        >>>     @EasyMeta.Hook
+        >>>     def before_subclass_init(name, bases, dct):
+        >>>         dct.pop('b', None)
+        >>>
+        >>> class Foo(NoB):
+        >>>     a = 1
+        >>>     b = 2
+        >>>
+        >>> Foo.a
+        1
+        >>> Foo.b
+        AttributeError: type object 'Foo' has no attribute 'b'
+        """
+
     @hook
     def after_subclass_init(self, cls):
         """
@@ -102,3 +132,100 @@ def __setitem__(self, name, value):
             self.hooks.add(value.dlg)
         else:
             return super().__setitem__(name, value)
+
+
+class EasyMixinStage(Enum):
+    BASE_MIXIN_CLASS = 1
+    MIXIN_GENERATOR = 2
+    ACTUAL_MIXIN_SPECS = 3
+
+
+class EasyMixinMeta(ABCMeta):
+    def __new__(mcs, name, bases, dct, **kwargs):
+        try:
+            stage = dct['_easy_mixin_stage_']
+        except KeyError:
+            stage = min(b._easy_mixin_stage_ for b in bases if hasattr(b, '_easy_mixin_stage_'))
+            stage = EasyMixinStage(stage.value + 1)
+        if stage == EasyMixinStage.ACTUAL_MIXIN_SPECS:
+            base, = bases
+            return base(name, bases, dct)._generate_class()
+        else:
+            dct['_easy_mixin_stage_'] = stage
+            return super().__new__(mcs, name, bases, dct)
+
+
+class EasyMixin(metaclass=EasyMixinMeta):
+    """
+    Create mixins creators.
+
+    Direct subclasses (hereinafter "mixin creators") of this class will be
+    created normally, but subclasses of these subclasses (hereinafter "mixins")
+    will be new classes that are not subclasses of neither the mixin creator nor
+    ``EasyMixin`` nor their other base classes, and will not necessarily contain
+    the content of their bodies.  Instead, the mixin class' body and its other
+    base classes will be passed to methods of the mixin creator, which will be
+    able to affect the resulting mixin class.
+
+    >>> class Foo(EasyMixin):  # the mixin creator
+    >>>     def prepare(self):
+    >>>         # `orig_dct` contains the original body - to affect the new one
+    >>>         # we use `dct`
+    >>>         self.dct['value_of_' + self.name] = self.orig_dct['value_of_name']
+    >>>
+    >>> class Bar(Foo):  # the mixin
+    >>>     value_of_name = 'Baz'
+    >>>
+    >>> Bar.value_of_Bar
+    'Baz'
+    """
+
+    _easy_mixin_stage_ = EasyMixinStage.BASE_MIXIN_CLASS
+    metaclass = EasyMeta
+    """The metaclass for the mixin"""
+
+    def __init__(self, name, bases, dct):
+        self.name = name
+        """The name of the to-be-created mixin. Can be changed."""
+        self.bases = ()
+        """The bases of the to-be-created mixin. Can be changed."""
+        self.orig_bases = bases
+        """The bases of the mixin's body. Not carried to the created mixin."""
+        self.orig_dct = dct
+        """The the mixin's body. Not carried to the created mixin."""
+
+    @abstractmethod
+    def prepare(self):
+        """
+        Override this to control the mixin creation.
+
+        * Alter ``self.name``, ``self.bases`` and ``self.dct``.
+        * Use ``self.add_hook`` to add easymeta hooks.
+        * Access the declaration of the mixin with ``self.orig_bases`` and self.orig_dct``.
+        """
+
+    def _generate_class(self):
+        self.dct = EasyMetaDslDict()
+        self.dct.update(__module__=self.orig_dct['__module__'], __qualname__=self.orig_dct['__qualname__'])
+        self.prepare()
+        return self.metaclass(self.name, self.bases, self.dct)
+
+    def add_hook(self, fn):
+        """
+        Add EasyMeta hooks to the created mixins. Use inside ``prepare``.
+
+        >>> class Foo(EasyMixin):
+        >>>     def prepare(self):
+        >>>         @self.add_hook
+        >>>         def after_subclass_init(cls):
+        >>>             # Note that the hook will not run on Bar - only on Baz
+        >>>             print(self.orig_dct['template'].format(cls))
+        >>>
+        >>> class Bar(Foo):
+        >>>     template = 'Creating subclass {0.__name__}'
+        >>>
+        >>> class Baz(Bar):
+        >>>     pass
+        Creating subclass Baz
+        """
+        self.dct[fn.__name__] = EasyMeta.Hook(fn)

easypy/meta_mixins.py

@@ -0,0 +1,34 @@
+from .meta import EasyMixin
+
+
+class UninheritedDefaults(EasyMixin):
+    """
+    Fields declared in this class will be default in subclasses even if overwritten.
+
+
+    >>> class Foo(UninheritedDefaults):
+    >>>     a = 1
+    >>> Foo.a
+    1
+    >>>
+    >>> class Bar(Foo):
+    >>>     a = 2
+    >>> Bar.a
+    2
+    >>>
+    >>> class Baz(Bar):
+    >>>     pass
+    >>> Baz.a  # gets the value from Foo, not from Bar
+    1
+    """
+
+    def prepare(self):
+        defaults = {k: v for k, v in self.orig_dct.items() if not k.startswith('__')}
+        self.dct.update(defaults)
+
+        @self.add_hook
+        def before_subclass_init(name, bases, dct):
+            for k, v in defaults.items():
+                dct.setdefault(k, v)
+
+

tests/test_meta.py

@@ -1,4 +1,6 @@
-from easypy.meta import EasyMeta
+import pytest
+
+from easypy.meta import EasyMeta, EasyMixin
 
 
 def test_easy_meta_after_cls_init():
@@ -23,3 +25,86 @@ def after_subclass_init(cls):
     assert Baz.foo_init == 'Baz'
     assert Baz.bar_init == 'Baz'
     assert not hasattr(Baz, 'baz_init'), 'after_subclass_init declared in Baz invoked on Baz'
+
+
+def test_easy_meta_before_cls_init():
+    class Foo(metaclass=EasyMeta):
+        @EasyMeta.Hook
+        def before_subclass_init(name, bases, dct):
+            try:
+                base = dct.pop('CLASS')
+            except KeyError:
+                pass
+            else:
+                bases.insert(0, base)
+
+    class Bar:
+        pass
+
+    class Baz(Foo):
+        CLASS = Bar
+
+        a = 1
+        b = 2
+
+
+    assert not hasattr(Baz, 'CLASS')
+    assert issubclass(Baz, Bar)
+
+
+def test_easy_mixin():
+    class MyMixinCreator(EasyMixin):
+        def prepare(self):
+            verify = {k: v for k, v in self.orig_dct.items() if not k.startswith('__')}
+
+            @self.add_hook
+            def before_subclass_init(name, bases, dct):
+                for k, v in verify.items():
+                    assert v == dct[k], '%s is %s - needs to be %s' % (k, dct[k], v)
+
+    class MyMixin(MyMixinCreator):
+        a = 1
+        b = 2
+
+    class Foo(MyMixin):
+        a = 1
+        b = 2
+
+    with pytest.raises(AssertionError) as exc:
+        class Bar(MyMixin):
+            a = 2
+            b = 2
+
+    assert 'a is 2 - needs to be 1' in (str(exc.value))
+
+
+def test_uninherited_defaults():
+    from easypy.meta_mixins import UninheritedDefaults
+
+    class Defaults(UninheritedDefaults):
+        a = 1
+        b = 2
+
+    class Foo(Defaults):
+        a = 3
+        c = 4
+
+    class Bar(Foo):
+        b = 5
+        c = 6
+
+    class Baz(Bar):
+        pass
+
+
+    assert Foo.a == 3
+    assert Foo.b == 2
+    assert Foo.c == 4
+
+    assert Bar.a == 1, 'Bar.a should come from Defaults, not Foo'
+    assert Bar.b == 5
+    assert Bar.c == 6
+
+    assert Baz.a == 1, 'Baz.a should come from Defaults, not Bar'
+    assert Baz.b == 2, 'Baz.b should come from Defaults, not Bar'
+    assert Baz.c == 6, 'Baz.c is not in Defaults so it should come from Bar'