implement 'config.accumulate()' (#994)

gallery_dl/config.py

@@ -140,6 +140,25 @@ def interpolate_common(common, paths, key, default=None, *, conf=_config):
     return default
 
 
+def accumulate(path, key, *, conf=_config):
+    """Accumulate the values of 'key' along 'path'"""
+    result = []
+    try:
+        if key in conf:
+            value = conf[key]
+            if value:
+                result.extend(value)
+        for p in path:
+            conf = conf[p]
+            if key in conf:
+                value = conf[key]
+                if value:
+                    result[:0] = value
+    except Exception:
+        pass
+    return result
+
+
 def set(path, key, value, *, conf=_config):
     """Set the value of property 'key' for this session"""
     for p in path:

test/test_config.py

@@ -96,6 +96,28 @@ def test(path, value, expected=None):
         test(("Z1", "Z2", "A1", "A2", "A3"), 999, 8)
         test((), 9)
 
+    def test_accumulate(self):
+        self.assertEqual(config.accumulate((), "l"), [])
+
+        config.set(()        , "l", [5, 6])
+        config.set(("c",)    , "l", [3, 4])
+        config.set(("c", "c"), "l", [1, 2])
+        self.assertEqual(
+            config.accumulate((), "l")        , [5, 6])
+        self.assertEqual(
+            config.accumulate(("c",), "l")    , [3, 4, 5, 6])
+        self.assertEqual(
+            config.accumulate(("c", "c"), "l"), [1, 2, 3, 4, 5, 6])
+
+        config.set(("c",), "l", None)
+        config.unset(("c", "c"), "l")
+        self.assertEqual(
+            config.accumulate((), "l")        , [5, 6])
+        self.assertEqual(
+            config.accumulate(("c",), "l")    , [5, 6])
+        self.assertEqual(
+            config.accumulate(("c", "c"), "l"), [5, 6])
+
     def test_set(self):
         config.set(()        , "c", [1, 2, 3])
         config.set(("b",)    , "c", [1, 2, 3])