Merge lp:~salgado/lazr.delegates/extend-passthrough into lp:lazr.delegates

[Guilherme Salgado]

Extend Passthrough so that it takes an extra argument to which the context is adapted before getting/setting the attribute

This is going to be used in https://code.edge.launchpad.net/~salgado/lazr.restful/extension-interfaces

[lp:~salgado/lazr.delegates/extend-passthrough]

6. By Guilherme Salgado on 2010-07-16

A few tweaks suggested by Gary

[Gary Poster (gary)]

merge-conditional

Looks great. A few remaining trivial cleanups.

orig_interface -> adaptation in "Passthrough's third argument (orig_interface) is optional..." (src/lazr/delegates/README.txt)

"...(although in practice anything that provides a __call__() method will do)..." -> "...(although in practice any callable will do)..." (src/lazr/delegates/README.txt)

"If adaptation is not None, the context is adapted into it before getting/setting the attribute." -> "If the ``adaptation`` argument is not None, it should be a callable. It will be called with the context, and should return an object that will have the delegated attribute. The ``adaptation`` argument is expected to be used with an interface, to adapt the context." (src/lazr/delegates/_delegates.py)

Thank you!

Gary

review: Approve

[lp:~salgado/lazr.delegates/extend-passthrough]

7. By Guilherme Salgado on 2010-07-16

A couple other changes suggested by Gary

=== modified file 'src/lazr/delegates/README.txt'
--- src/lazr/delegates/README.txt	2009-04-14 16:07:52 +0000
+++ src/lazr/delegates/README.txt	2010-07-16 13:16:40 +0000
@@ -193,9 +193,9 @@
 
 The Passthrough class is the implementation machinery of ``delegates()``. It
 uses the descriptor protocol to implement the delegation behaviour provided by
-``delegates()``. It takes two arguments: the name of the attribute that is
-delegated, and the name of the attribute containing the object to which to
-delegate.
+``delegates()``. It takes at least two arguments: the name of the attribute
+that is delegated, and the name of the attribute containing the object to
+which to delegate.
 
 To illustrate, p and p2 are two Passthrough instances that use the
 instance assigned to 'mycontext' to call the 'foo' attribute and
@@ -247,6 +247,40 @@
     ...
     NotImplementedError
 
+Passthrough's third argument (adaptation) is optional and, when provided,
+should be a zope.interface.Interface subclass (although in practice any
+callable will do) to which the instance is adapted before getting/setting the
+delegated attribute.
+
+    # HasNoFoo does not have a .foo attribute...
+    >>> class HasNoFoo(object):
+    ...     _foo = 1
+    >>> no_foo = HasNoFoo()
+
+    # ... but IHasFooAdapter uses HasNoFoo._foo to provide its own .foo, so it
+    # works like an adapter for HasNoFoo into some interface that provides
+    # a 'foo' attribute.
+    >>> class IHasFooAdapter(object):
+    ...     def __init__(self, inst):
+    ...         self.inst = inst
+    ...     def _get_foo(self):
+    ...         return self.inst._foo
+    ...     def _set_foo(self, value):
+    ...         self.inst._foo = value
+    ...     foo = property(_get_foo, _set_foo)
+
+    >>> class Example(object):
+    ...     context = no_foo
+
+    >>> p = Passthrough('foo', 'context', adaptation=IHasFooAdapter)
+    >>> e = Example()
+    >>> p.__get__(e)
+    1
+    >>> p.__set__(e, 2)
+    >>> p.__get__(e)
+    2
+
+
 ===============
 Other Documents
 ===============
 
=== modified file 'src/lazr/delegates/_delegates.py'
--- src/lazr/delegates/_delegates.py	2009-04-14 16:07:52 +0000
+++ src/lazr/delegates/_delegates.py	2010-07-16 13:16:40 +0000
@@ -108,19 +108,32 @@
 
 
 class Passthrough:
-    """Call the delegated class for the decorator class."""
-    def __init__(self, name, contextvar):
+    """Call the delegated class for the decorator class.
+
+    If the ``adaptation`` argument is not None, it should be a callable. It
+    will be called with the context, and should return an object that will
+    have the delegated attribute. The ``adaptation`` argument is expected to
+    be used with an interface, to adapt the context.
+    """
+    def __init__(self, name, contextvar, adaptation=None):
         self.name = name
         self.contextvar = contextvar
+        self.adaptation = adaptation
 
     def __get__(self, inst, cls=None):
         if inst is None:
             return self
         else:
-            return getattr(getattr(inst, self.contextvar), self.name)
+            context = getattr(inst, self.contextvar)
+            if self.adaptation is not None:
+                context = self.adaptation(context)
+            return getattr(context, self.name)
 
     def __set__(self, inst, value):
-        setattr(getattr(inst, self.contextvar), self.name, value)
+        context = getattr(inst, self.contextvar)
+        if self.adaptation is not None:
+            context = self.adaptation(context)
+        setattr(context, self.name, value)
 
     def __delete__(self, inst):
         raise NotImplementedError