support dataclasses with slots (#136)

Author: samuelcolvin

devtools/prettier.py

@@ -254,7 +254,12 @@ def _format_ast_expression(self, value: ast.AST, _: str, indent_current: int, in
             self._stream.write(indent_current * self._c + line)
 
     def _format_dataclass(self, value: 'Any', _: str, indent_current: int, indent_new: int) -> None:
-        self._format_fields(value, value.__dict__.items(), indent_current, indent_new)
+        try:
+            field_items = value.__dict__.items()
+        except AttributeError:
+            # slots
+            field_items = ((f, getattr(value, f)) for f in value.__slots__)
+        self._format_fields(value, field_items, indent_current, indent_new)
 
     def _format_sqlalchemy_class(self, value: 'Any', _: str, indent_current: int, indent_new: int) -> None:
         if sa_inspect is not None:

tests/test_prettier.py

@@ -255,6 +255,30 @@ class BarDataclass:
     )
 
 
+def test_dataclass_slots():
+    try:
+        dec = dataclass(slots=True)
+    except TypeError:
+        pytest.skip('dataclasses.slots not available')
+
+    @dec
+    class FooDataclass:
+        x: int
+        y: str
+
+    f = FooDataclass(123, 'bar')
+    v = pformat(f)
+    print(v)
+    assert (
+        v
+        == """\
+FooDataclass(
+    x=123,
+    y='bar',
+)"""
+    )
+
+
 @pytest.mark.skipif(numpy is None, reason='numpy not installed')
 def test_indent_numpy():
     v = pformat({'numpy test': numpy.array(range(20))})