Make dataclass inheritance with defaults work with py<310

I've made TreeUndoEntry a subclass of UndoEntry. But UndoEntry sets a
default value for the `created_at` attribute, which means if any of the
extra attributes defined on the child class don't define a default an
error is thrown complaining about non-keyword arguments after a keyword
only one. Python 3.10 adds a kw_only argument you can pass to the
dataclass decorator but it looks like we currently support python 3.8+.
See https://www.trueblade.com/blogs/news/python-3-10-new-dataclass-features

So a more compatible way to do it is to set `init=False` on the
attribute with a default, if you don't need to pass it in the
constructor. Which we don't because it's only set by the caller in
tests. Hence changing the tests to set it a different way. A bit uglier
but should be fine.

Also fix my "retains_sort_order" test from #5964 that wasn't actually
being run...

3 files changed, 23 insertions, 17 deletions

diff --git a/qutebrowser/mainwindow/tabbedbrowser.py b/qutebrowser/mainwindow/tabbedbrowser.py
index 469c70c7a..b006ef892 100644
--- a/qutebrowser/mainwindow/tabbedbrowser.py
+++ b/qutebrowser/mainwindow/tabbedbrowser.py
@@ -24,7 +24,7 @@ from qutebrowser.utils import (log, usertypes, utils, qtutils,
 from qutebrowser.misc import quitter, objects
 
 
-@dataclasses.dataclass(kw_only=True)
+@dataclasses.dataclass
 class _UndoEntry:
 
     """Information needed for :undo."""
@@ -34,7 +34,9 @@ class _UndoEntry:
     index: int
     pinned: bool
     created_at: datetime.datetime = dataclasses.field(
-        default_factory=datetime.datetime.now)
+        default_factory=datetime.datetime.now,
+        init=False,  # WORKAROUND until py3.10 with kw_only: https://www.trueblade.com/blogs/news/python-3-10-new-dataclass-features
+    )
 
     def restore_into_tab(self, tab: browsertab.AbstractTab):
         """Set the url, history and state of `tab` from this undo entry."""
diff --git a/qutebrowser/mainwindow/treetabbedbrowser.py b/qutebrowser/mainwindow/treetabbedbrowser.py
index d078ff782..49d88d7a5 100644
--- a/qutebrowser/mainwindow/treetabbedbrowser.py
+++ b/qutebrowser/mainwindow/treetabbedbrowser.py
@@ -16,7 +16,7 @@ from qutebrowser.browser import browsertab
 from qutebrowser.misc import notree
 
 
-@dataclasses.dataclass(kw_only=True)
+@dataclasses.dataclass
 class _TreeUndoEntry(_UndoEntry):
     """Information needed for :undo."""
 
diff --git a/tests/unit/completion/test_models.py b/tests/unit/completion/test_models.py
index e0bce8f04..7c51a6a62 100644
--- a/tests/unit/completion/test_models.py
+++ b/tests/unit/completion/test_models.py
@@ -1410,14 +1410,15 @@ def test_forward_completion(tab_with_history, info):
 def test_undo_completion(tabbed_browser_stubs, info):
     """Test :undo completion."""
     entry1 = tabbedbrowser._UndoEntry(url=QUrl('https://example.org/'),
-                                      history=None, index=None, pinned=None,
-                                      created_at=datetime(2020, 1, 1))
+                                      history=None, index=None, pinned=None)
     entry2 = tabbedbrowser._UndoEntry(url=QUrl('https://example.com/'),
-                                      history=None, index=None, pinned=None,
-                                      created_at=datetime(2020, 1, 2))
+                                      history=None, index=None, pinned=None)
     entry3 = tabbedbrowser._UndoEntry(url=QUrl('https://example.net/'),
-                                      history=None, index=None, pinned=None,
-                                      created_at=datetime(2020, 1, 2))
+                                      history=None, index=None, pinned=None)
+
+    entry1.created_at = datetime(2020, 1, 1)
+    for entry in [entry2, entry3]:
+        entry.created_at = datetime(2020, 1, 2)
 
     # Most recently closed is at the end
     tabbed_browser_stubs[0].undo_stack = [
@@ -1442,19 +1443,22 @@ def test_undo_completion(tabbed_browser_stubs, info):
     })
 
 
-def undo_completion_retains_sort_order(tabbed_browser_stubs, info):
+def test_undo_completion_retains_sort_order(tabbed_browser_stubs, info):
     """Test :undo completion sort order with > 10 entries."""
     created_dt = datetime(2020, 1, 1)
-    created_str = "2020-01-02 00:00"
+    created_str = "2020-01-01 00:00"
 
     tabbed_browser_stubs[0].undo_stack = [
-        tabbedbrowser._UndoEntry(
-            url=QUrl(f'https://example.org/{idx}'),
-            history=None, index=None, pinned=None,
-            created_at=created_dt,
-        )
-        for idx in range(1, 11)
+        [
+            tabbedbrowser._UndoEntry(
+                url=QUrl(f'https://example.org/{idx}'),
+                history=None, index=None, pinned=None,
+            )
+        ]
+        for idx in reversed(range(1, 11))
     ]
+    for entries in tabbed_browser_stubs[0].undo_stack:
+        entries[0].created_at = created_dt
 
     model = miscmodels.undo(info=info)
     model.set_pattern('')