I've been playing with dictionary subtypes for custom environments, and I encountered a strange interaction between exec, dictionary subtypes, and global variables. I've attached a test program, but first I'd like to give some background.
Python uses dictionary objects as symbol tables in it's execution contexts. Since these objects used to be basic types, which were not subclassable, some of the interpreter code accessed them using low-level C functions. Then dictionary types were made subclassable, and I was pretty excited, because I wanted to use them to set up custom execution environments with extended semantics. This didn't work, because when the dictionary types were made subclassable, the interpreter code which used them as symbol tables was not changed, so subclassed dictionaries used in exec statements or eval calls bypassed their subclass functions, and jumped straight to the underlying C functions. In python 2.4, this was finally fixed, and you can now use subclassed dictionaries in exec statements and eval calls. Except that there is some niggling edge case dealing with variables which have been marked 'global'. It seems that if a compiled chunk of python contains a 'global VAR' statement, anywhere, then that VAR, and only that VAR, will bypass the subclassed functions when it is accessed. I'd like to fix this, and send it upstream, and I've worked on extensive python/C bindings before, but I've not hacked the interpreter. I took a look around, and I'm not sure where the symtable lookup is happening. However, I have some belief that someone on this list _may_ be able to point me in the right direction. #!/usr/bin/python2.4 class TypedDictionary(dict): def __setitem__(self, key, value): print '__setitem__(%s, %s)' % (key, value) if self.has_key(key): t = type(self[key]) try: value = t(value) except Exception: raise TypeError, \ "illegal assignment to '%s': %s cannot be coerced to %s" \ % (key, type(value), t) dict.__setitem__(self, key, value) test_script = """ foo = 0 bar = 'xyz' print 'foo:', repr(foo), '; bar:', repr(bar) foo = '1' bar = 42 print 'foo:', repr(foo), '; bar:', repr(bar) """ print "Without 'global foo':" exec test_script in TypedDictionary() print print "With 'global foo':" exec test_script + """ def f(): global foo pass """ in TypedDictionary() This program produces this output: Without 'global foo': __setitem__(foo, 0) __setitem__(bar, xyz) foo: 0 ; bar: 'xyz' __setitem__(foo, 1) __setitem__(bar, 42) foo: 1 ; bar: '42' With 'global foo': __setitem__(bar, xyz) foo: 0 ; bar: 'xyz' __setitem__(bar, 42) foo: '1' ; bar: '42' __setitem__(f, <function f at 0x40230454>) -- http://mail.python.org/mailman/listinfo/python-list