1 Dynamic Attribute Access
2 ========================
3
4 Access to attributes using specific names can either be analysed at
5 compile-time or supported at run-time using the object table. However, access
6 to arbitrary attributes which are not explicitly identified in the program as
7 attribute access operations requires additional support. Thus, functions such
8 as getattr require a means of converting string values to attribute
9 references.
10
11 Although a dictionary could be used to support dynamic attribute accesses, an
12 alternative approach involves identifying constant strings which might refer
13 to active attributes. If getattr is used in a program, such string instances
14 are extended with an extra attribute containing the object table index which
15 would yield an attribute in an object table lookup. For example:
16
17 class C:
18 x = 123
19
20 attrname = "x" # generates constant with attribute giving index of "x"
21 # belonging to a special _accessor class
22
23 The _accessor class is a subclass of str:
24
25 class _accessor(str):
26 pass
27
28 In pseudocode, the constant would be initialised as follows:
29
30 "x" = _accessor(<data for "x">, <index of "x">)
31
32 When used with getattr, an attempt is made to access the special attribute on
33 the given attribute name string. Such instances are able to provide an index
34 and this is used in the object table lookup:
35
36 # in the native getattr(obj, attrname) function...
37 index = attrname._index # this yields the index offset directly
38
39 Where getattr is not in use, no string constants are _accessor instances.
40
41 A significant limitation of this approach is that strings cannot be combined
42 or modified to create strings which are then used with getattr, nor can
43 strings be introduced at run-time to access attributes dynamically. However,
44 since such string manipulation is potentially a source of errors and resists
45 analysis of program code, a compromise offering basic support for dynamic
46 access and program inspection is arguably a suitable alternative.
