diff --git a/mercurial/context.py b/mercurial/context.py
--- a/mercurial/context.py
+++ b/mercurial/context.py
@@ -7,6 +7,7 @@
 
 from __future__ import absolute_import
 
+import abc
 import errno
 import filecmp
 import os
@@ -717,7 +718,36 @@
     def matches(self, match):
         return self.walk(match)
 
-class basefilectx(object):
+class abstractfilectx(object):
+    """Defines the mininum required methods for filectxs"""
+    __metaclass__ = abc.ABCMeta
+
+    @abc.abstractmethod
+    def data(self): pass
+    @abc.abstractmethod
+    def cmp(self, other): pass
+    @abc.abstractmethod
+    def changectx(self): pass
+    @abc.abstractmethod
+    def flags(self): pass
+    @abc.abstractmethod
+    def path(self): pass
+    @abc.abstractmethod
+    def size(self): pass
+    @abc.abstractmethod
+    def isbinary(self): pass
+    @abc.abstractmethod
+    def islink(self): pass
+    @abc.abstractmethod
+    def isexec(self): pass
+    @abc.abstractmethod
+    def isabsent(self): pass
+    @abc.abstractmethod
+    def write(self, data, flags): pass
+    @abc.abstractmethod
+    def setflags(self, l, x): pass
+
+class basefilectx(abstractfilectx):
     """A filecontext object represents the common logic for its children:
     filectx: read-only access to a filerevision that is already present
              in the repo,
@@ -1302,6 +1332,12 @@
         return [filectx(self._repo, self._path, fileid=x,
                         filelog=self._filelog) for x in c]
 
+    def setflags(self, l, x):
+        raise error.ProgrammingError("filectx is immutable")
+
+    def write(self, data, flags):
+        raise error.ProgrammingError("filectx is immutable")
+
 class committablectx(basectx):
     """A committablectx object provides common functionality for a context that
     wants the ability to commit, e.g. workingctx or memctx."""
@@ -2408,6 +2444,9 @@
         """wraps repo.wwrite"""
         self._data = data
 
+    def setflags(self, l, x):
+        raise NotImplementedError
+
 class overlayfilectx(committablefilectx):
     """Like memfilectx but take an original filectx and optional parameters to
     override parts of it. This is useful when fctx.data() is expensive (i.e.
@@ -2577,7 +2616,7 @@
 
         return scmutil.status(modified, added, removed, [], [], [], [])
 
-class arbitraryfilectx(object):
+class arbitraryfilectx(abstractfilectx):
     """Allows you to use filectx-like functions on a file in an arbitrary
     location on disk, possibly not in the working directory.
     """
@@ -2597,12 +2636,36 @@
             return not filecmp.cmp(self.path(), self._repo.wjoin(fctx.path()))
         return self.data() != fctx.data()
 
+    def changectx(self):
+        raise error.ProgrammingError("an arbitraryfilectx cannot have a "
+                                     "changectx")
+
+    def isabsent(self):
+        return False
+
+    # TODO deduplicate these from ``basefilectx``
+    def isbinary(self):
+        try:
+            return util.binary(self.data())
+        except IOError:
+            return False
+    def isexec(self):
+        return 'x' in self.flags()
+    def islink(self):
+        return 'l' in self.flags()
+
+    def size(self):
+        return os.stat(self._path).st_size
+
     def path(self):
         return self._path
 
     def flags(self):
         return ''
 
+    def setflags(self, l, x):
+        raise NotImplementedError
+
     def data(self):
         return util.readfile(self._path)
 
diff --git a/mercurial/filemerge.py b/mercurial/filemerge.py
--- a/mercurial/filemerge.py
+++ b/mercurial/filemerge.py
@@ -104,6 +104,18 @@
     def isabsent(self):
         return True
 
+    def isexec(self):
+        return False
+
+    def islink(self):
+        return False
+
+    def setflags(self, l, x):
+        raise error.ProgrammingError("absentfilectx is immutable")
+
+    def write(self, data, flags):
+        raise error.ProgrammingError("absentfilectx is immutable")
+
 def _findtool(ui, tool):
     if tool in internals:
         return tool