asdict(): Optionally retain collection types

CHANGELOG.rst

@@ -41,6 +41,8 @@ Changes:
 - Add ``attr.attrs`` and ``attr.attrib`` as a more consistent aliases for ``attr.s`` and ``attr.ib``.
 - Add ``frozen`` option to ``attr.s`` that will make instances best-effort immutable.
   `#60 <https://github.com/hynek/attrs/issues/60>`_
+- ``attr.asdict`` now takes ``retain_collection_types`` as an argument. If ``True``, it does not convert attributes of type ``tuple`` or ``set`` to ``list``.
+  `#69 <https://github.com/hynek/attrs/issues/69>`_
 
 
 ----

src/attr/_funcs.py

@@ -6,7 +6,8 @@
 from ._make import Attribute, NOTHING, fields
 
 
-def asdict(inst, recurse=True, filter=None, dict_factory=dict):
+def asdict(inst, recurse=True, filter=None, dict_factory=dict,
+           retain_collection_types=False):
     """
     Return the ``attrs`` attribute values of *inst* as a dict. Optionally
     recurse into other ``attrs``-decorated classes.
@@ -25,6 +26,11 @@ def asdict(inst, recurse=True, filter=None, dict_factory=dict):
         example, to produce ordered dictionaries instead of normal Python
         dictionaries, pass in ``collections.OrderedDict``.
 
+    :param bool retain_collection_types: Do not convert to ``list`` when
+        encountering an attribute which is type ``tuple`` or ``set``.
+        Only meaningful if ``recurse`` is ``True``.
+
+
     :rtype: :class:`dict`
 
     .. versionadded:: 16.0.0
@@ -41,12 +47,13 @@ def asdict(inst, recurse=True, filter=None, dict_factory=dict):
                 rv[a.name] = asdict(v, recurse=True, filter=filter,
                                     dict_factory=dict_factory)
             elif isinstance(v, (tuple, list, set)):
-                rv[a.name] = [
+                cf = v.__class__ if retain_collection_types is True else list
+                rv[a.name] = cf([
                     asdict(i, recurse=True, filter=filter,
                            dict_factory=dict_factory)
                     if has(i.__class__) else i
                     for i in v
-                ]
+                ])
             elif isinstance(v, dict):
                 df = dict_factory
                 rv[a.name] = df((

tests/test_funcs.py

@@ -105,6 +105,18 @@ def test_lists_tuples(self, container, C):
             "y": [{"x": 2, "y": 3}, {"x": 4, "y": 5}, "a"],
         } == asdict(C(1, container([C(2, 3), C(4, 5), "a"])))
 
+    @given(container=st.sampled_from(SEQUENCE_TYPES))
+    def test_lists_tuples_retain_type(self, container, C):
+        """
+        If recurse and retain_collection_types are True, also recurse
+        into lists and do not convert them into list.
+        """
+        assert {
+            "x": 1,
+            "y": container([{"x": 2, "y": 3}, {"x": 4, "y": 5}, "a"]),
+        } == asdict(C(1, container([C(2, 3), C(4, 5), "a"])),
+                    retain_collection_types=True)
+
     @given(st.sampled_from(MAPPING_TYPES))
     def test_dicts(self, C, dict_factory):
         """