Reduce dependency declaration verbosity

Use metaprogramming so that dependencies can be declared without
redundantly specifying the name of the dependency.

Fixes #38

metsrw/di.py

@@ -19,6 +19,8 @@
 See http://code.activestate.com/recipes/413268/
 """
 
+from six import with_metaclass
+
 from .plugins import premisrw
 
 
@@ -81,15 +83,14 @@ class Dependency(object):
         >>> from .di import is_class, has_methods, Dependency
         >>> class FSEntry(object):
         ...     premis_object_class = Dependency(
-        ...         'premis_object_class',
         ...         has_methods('serialize'),
         ...         has_class_methods('fromtree'),
         ...         is_class)
 
     """
 
-    def __init__(self, dependency_name, *assertions):
-        self.dependency_name = dependency_name
+    def __init__(self, *assertions):
+        self.dependency_name = None
         self.assertions = assertions
 
     def __get__(self, instance, owner):
@@ -101,6 +102,27 @@ def __get__(self, instance, owner):
         return obj
 
 
+class DependencyPossessorMeta(type):
+    """Metaclass for ``DependencyPossessor``. Classes that can have
+    dependencies---i.e., classes that have class attributes whose values are
+    instances of ``Dependency`` above---must inherit from
+    ``DependencyPossessor``. This allows us to tell the ``Dependency`` instance
+    that its ``dependency_name`` value should be the same as the name of the
+    class attribute it was assigned to in the managed class. In short, it
+    allows us to write ``premis_object_class = Dependency()`` instead of
+    ``premis_object_class = Dependency('premis_object_class')``.
+    """
+    def __init__(cls, name, bases, attr_dict):
+        super(DependencyPossessorMeta, cls).__init__(name, bases, attr_dict)
+        for key, attr in attr_dict.items():
+            if isinstance(attr, Dependency):
+                attr.dependency_name = key
+
+
+class DependencyPossessor(with_metaclass(DependencyPossessorMeta, object)):
+    pass
+
+
 # ==============================================================================
 #   Assertions for declaring dependencies using Dependency
 # ==============================================================================

metsrw/fsentry.py

@@ -8,15 +8,21 @@
 from lxml import etree
 import six
 
-from .di import is_class, has_methods, has_class_methods, Dependency
+from .di import (
+    is_class,
+    has_methods,
+    has_class_methods,
+    Dependency,
+    DependencyPossessor,
+)
 from . import exceptions
 from .metadata import MDWrap, MDRef, SubSection, AMDSec
 from . import utils
 
 LOGGER = logging.getLogger(__name__)
 
 
-class FSEntry(object):
+class FSEntry(DependencyPossessor):
     """A class representing a filesystem entry - either a file or a directory.
 
     When passed to a :class:`metsrw.mets.METSDocument` instance, the tree of
@@ -75,17 +81,14 @@ class FSEntry(object):
     # ``FSEntry`` instance must be able to call ``self.premis_object_class`` and
     # get a class with methods ``fromtree`` and ``serialize``.
     premis_object_class = Dependency(
-        'premis_object_class',
         has_methods('serialize'),
         has_class_methods('fromtree'),
         is_class)
     premis_event_class = Dependency(
-        'premis_event_class',
         has_methods('serialize'),
         has_class_methods('fromtree'),
         is_class)
     premis_agent_class = Dependency(
-        'premis_agent_class',
         has_methods('serialize'),
         has_class_methods('fromtree'),
         is_class)

tests/test_dependency_injection.py

@@ -142,7 +142,6 @@ def test_dependency_injection(self):
         with ``fromtree`` and ``serialize`` methods::
 
             >>> premis_object_class = Dependency(
-            ...     'premis_object_class',
             ...     has_methods('serialize'),
             ...     has_class_methods('fromtree'),
             ...     is_class)