grc: fixup: dynamically update namespaces from blocks (gnuradio#3317)

Author: skoslowski

grc/core/blocks/block.py

@@ -76,6 +76,7 @@ def __init__(self, parent):
         self.active_sinks = []  # on rewrite
 
         self.states = {'state': True, 'bus_source': False, 'bus_sink': False, 'bus_structure': None}
+        self.block_namespace = {}
 
         if Flags.HAS_CPP in self.flags and self.enabled and not (self.is_virtual_source() or self.is_virtual_sink()):
             # This is a workaround to allow embedded python blocks/modules to load as there is 
@@ -131,17 +132,18 @@ def rekey(ports):
         self.active_sinks = [p for p in self.sinks if not p.hidden]
 
         # namespaces may have changed, update them
-        self.block_namespace = {}
+        self.block_namespace.clear()
+        imports = ""
         try:
-            exec(self.templates.render('imports'), self.block_namespace)
+            imports = self.templates.render('imports')
+            exec(imports, self.block_namespace)
         except ImportError:
             # We do not have a good way right now to determine if an import is for a
             # hier block, these imports will fail as they are not in the search path
             # this is ok behavior, unfortunately we could be hiding other import bugs
             pass
         except Exception:
-            log.exception('Failed to evaluate import expression "{0}"'.format(expr), exc_info=True)
-            pass
+            self.add_error_message(f'Failed to evaluate import expression {imports!r}')
 
     def update_bus_logic(self):
         ###############################