improve 'parse_query()' and add tests

- another irrelevant micro-optimization !
- use urllib.parse.parse_qsl directly instead of parse_qs, which
  just packs the results of parse_qsl in a different data structure
- reduced memory requirements since no additional dict and lists are
  created

gallery_dl/text.py

@@ -130,7 +130,11 @@ def extract_iter(txt, begin, end, pos=0):
 
 def parse_query(qs):
     """Parse a query string into key-value pairs"""
-    return {key: vlist[0] for key, vlist in urllib.parse.parse_qs(qs).items()}
+    result = {}
+    for key, value in urllib.parse.parse_qsl(qs):
+        if key not in result:
+            result[key] = value
+    return result
 
 
 if os.name == "nt":

test/test_text.py

@@ -123,6 +123,37 @@ class TestText(unittest.TestCase):
         result = ["c", "b", "a", "d"]
         self.assertEqual(list(text.extract_iter(txt, "[", "]")), result)
 
+    def test_parse_query(self):
+        # standard stuff
+        self.assertEqual(
+            text.parse_query(""), {})
+        self.assertEqual(
+            text.parse_query("foo=1"), {"foo": "1"})
+        self.assertEqual(
+            text.parse_query("foo=1&bar=2"), {"foo": "1", "bar": "2"})
+
+        # missing value
+        self.assertEqual(
+            text.parse_query("bar"), {})
+        self.assertEqual(
+            text.parse_query("foo=1&bar"), {"foo": "1"})
+        self.assertEqual(
+            text.parse_query("foo=1&bar&baz=3"), {"foo": "1", "baz": "3"})
+
+        # keys with identical names
+        self.assertEqual(
+            text.parse_query("foo=1&foo=2"), {"foo": "1"})
+        self.assertEqual(
+            text.parse_query("foo=1&bar=2&foo=3&bar=4"),
+            {"foo": "1", "bar": "2"},
+        )
+
+        # non-string arguments
+        self.assertEqual(text.parse_query(()), {})
+        self.assertEqual(text.parse_query([]), {})
+        self.assertEqual(text.parse_query({}), {})
+        self.assertEqual(text.parse_query(None), {})
+
 
 if __name__ == '__main__':
     unittest.main()