hooks: Filter entries by setting entry_data['_hook_cancel'] to True.

The current method only works for plain dicts and other simple mappings.
Mapping that may still contain items after .clear(), like ChainMap, can't
rely on the old method.

CODE.rst

@@ -51,7 +51,7 @@ Hooks make arbitrary transformations to entry data dicts.  Every entry data dict
   Initializes the hook with the user's configuration.
 
 ``run(entry_data)``
-  This method makes the hook's transformations to the entry data dict, if any.  If this method clears the entry data dict, that entry will not be output.
+  This method makes the hook's transformations to the entry data dict, if any.  If this method sets ``entry_data['_hook_cancel']`` to a truthy value, that entry will not be output.
 
 Templates
 ~~~~~~~~~

import2ledger/__main__.py

@@ -41,11 +41,13 @@ class FileImporter:
                 default_date = self.config.get_default_date()
                 in_file.seek(0)
                 for entry_data in importer(in_file):
+                    entry_data['_hook_cancel'] = False
                     for hook in self.hooks:
                         hook.run(entry_data)
-                        if not entry_data:
+                        if entry_data['_hook_cancel']:
                             break
                     else:
+                        del entry_data['_hook_cancel']
                         print(template.render(**entry_data), file=out_file, end='')
 
     def import_path(self, in_path):

import2ledger/hooks/filter_by_date.py

@@ -9,4 +9,4 @@ class FilterByDateHook:
             pass
         else:
             if not self.config.date_in_want_range(date):
-                entry_data.clear()
+                entry_data['_hook_cancel'] = True

tests/test_hooks.py

@@ -58,7 +58,7 @@ def test_filter_by_date(entry_date, start_date, end_date, allowed):
     entry_data = {'date': entry_date}
     hook = filter_by_date.FilterByDateHook(DateRangeConfig(start_date, end_date))
     hook.run(entry_data)
-    assert bool(entry_data) == allowed
+    assert entry_data.get('_hook_cancel', False) == (not allowed)
 
 class DefaultDateConfig:
     ONE_DAY = datetime.timedelta(days=1)