# HG changeset patch
# User Paul Boddie <paul@boddie.org.uk>
# Date 1328551606 -3600
# Node ID c331d79421d33118ab68cfe21af684375f2a7e73
# Parent  06082c8d4e22017f81695843e72fc9a271cdc428
Added getattr notes.

diff -r 06082c8d4e22 -r c331d79421d3 docs/getattr.txt
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/docs/getattr.txt	Mon Feb 06 19:06:46 2012 +0100
@@ -0,0 +1,46 @@
+Dynamic Attribute Access
+========================
+
+Access to attributes using specific names can either be analysed at
+compile-time or supported at run-time using the object table. However, access
+to arbitrary attributes which are not explicitly identified in the program as
+attribute access operations requires additional support. Thus, functions such
+as getattr require a means of converting string values to attribute
+references.
+
+Although a dictionary could be used to support dynamic attribute accesses, an
+alternative approach involves identifying constant strings which might refer
+to active attributes. If getattr is used in a program, such string instances
+are extended with an extra attribute containing the object table index which
+would yield an attribute in an object table lookup. For example:
+
+  class C:
+      x = 123
+
+  attrname = "x" # generates constant with attribute giving index of "x"
+                 # belonging to a special _accessor class
+
+The _accessor class is a subclass of str:
+
+  class _accessor(str):
+      pass
+
+In pseudocode, the constant would be initialised as follows:
+
+  "x" = _accessor(<data for "x">, <index of "x">)
+
+When used with getattr, an attempt is made to access the special attribute on
+the given attribute name string. Such instances are able to provide an index
+and this is used in the object table lookup:
+
+  # in the native getattr(obj, attrname) function...
+  index = attrname._index # this yields the index offset directly
+
+Where getattr is not in use, no string constants are _accessor instances.
+
+A significant limitation of this approach is that strings cannot be combined
+or modified to create strings which are then used with getattr, nor can
+strings be introduced at run-time to access attributes dynamically. However,
+since such string manipulation is potentially a source of errors and resists
+analysis of program code, a compromise offering basic support for dynamic
+access and program inspection is arguably a suitable alternative.