4 files changed, 35 insertions, 9 deletions
diff --git a/vk/tracking/db/backend/csv.py b/vk/tracking/db/backend/csv.py
index 5b2cc7d..6176018 100644
--- a/vk/tracking/db/backend/csv.py
+++ b/vk/tracking/db/backend/csv.py
@@ -3,13 +3,12 @@
 # For details, see https://github.com/egor-tensin/vk-scripts.
 # Distributed under the MIT License.
 
-from collections.abc import Iterable
-
+from .. import meta
 from ..io import InputReaderCSV, OutputWriterCSV
 from ..record import Record
 from ..timestamp import Timestamp
 
-class Writer:
+class Writer(meta.Writer):
     def __init__(self, fd):
         self._writer = OutputWriterCSV(fd)
 
@@ -31,7 +30,7 @@ class Writer:
     def _record_to_row(record):
         return [str(record.get_timestamp())] + [str(record[field]) for field in record]
 
-class Reader(Iterable):
+class Reader(meta.Reader):
     def __init__(self, fd):
         self._reader = InputReaderCSV(fd)
 
diff --git a/vk/tracking/db/backend/log.py b/vk/tracking/db/backend/log.py
index f248d65..4ee5ff2 100644
--- a/vk/tracking/db/backend/log.py
+++ b/vk/tracking/db/backend/log.py
@@ -5,7 +5,9 @@
 
 import logging
 
-class Writer:
+from .. import meta
+
+class Writer(meta.Writer):
     def __init__(self, fd):
         self._logger = logging.getLogger(__file__)
         self._logger.setLevel(logging.INFO)
diff --git a/vk/tracking/db/backend/null.py b/vk/tracking/db/backend/null.py
index 3cdb41e..663af10 100644
--- a/vk/tracking/db/backend/null.py
+++ b/vk/tracking/db/backend/null.py
@@ -3,9 +3,9 @@
 # For details, see https://github.com/egor-tensin/vk-scripts.
 # Distributed under the MIT License.
 
-from collections.abc import Iterable
+from .. import meta
 
-class Writer:
+class Writer(meta.Writer):
     def __init__(self):
         pass
 
@@ -18,9 +18,9 @@ class Writer:
     def on_connection_error(self, e):
         pass
 
-class Reader(Iterable):
+class Reader(meta.Reader):
     def __init__(self):
         pass
 
     def __iter__(self):
-        pass
+        return iter(())
diff --git a/vk/tracking/db/meta.py b/vk/tracking/db/meta.py
new file mode 100644
index 0000000..024d9d8
--- /dev/null
+++ b/vk/tracking/db/meta.py
@@ -0,0 +1,25 @@
+# Copyright (c) 2017 Egor Tensin <Egor.Tensin@gmail.com>
+# This file is part of the "VK scripts" project.
+# For details, see https://github.com/egor-tensin/vk-scripts.
+# Distributed under the MIT License.
+
+import abc
+from collections.abc import Iterable
+
+class Writer(metaclass=abc.ABCMeta):
+    @abc.abstractmethod
+    def on_initial_status(self, user):
+        pass
+
+    @abc.abstractmethod
+    def on_status_update(self, user):
+        pass
+
+    @abc.abstractmethod
+    def on_connection_error(self, e):
+        pass
+
+class Reader(Iterable, metaclass=abc.ABCMeta):
+    @abc.abstractmethod
+    def __iter__(self):
+        pass