BATCHCODE WORKS:

a = {}
caller.msg(str(a))
exec("a = 'Test'")
caller.msg(str(a))

RESULT:
@batchcode test
Running Batch-code processor - Automatic mode for test ...
01/01: a = {}\nca[...]
{}
Test
End of batch file.
  Batchfile 'test' applied.

CLASS HOOK DOESN'T:

class CmdTalk(COMMAND_DEFAULT_CLASS):
    key = "talk"
    locks = "cmd:all()"

    def func(self):
        target = caller.search(self.args)
        target.at_talk(caller)

class NPC(Character):

    def at_talk(self, caller):
        a = {}
        caller.msg(str(a))
        exec("a = 'Test'")
        caller.msg(str(a))

RESULT:
*NPC Created*
talk npc
{}
{}