Convert class-level constants to properties for attrs compatibility

`attrs` cannot use multiple inheritance when both parents introduce their attributes (as documented). Only one side can inherit the attributes, other bases must be pure interfaces/protocols.

Reimplement the `ExprNode.type` via properties to exclude it from the sight of `attrs`.

Author: Sergey Vasilyev

data_diff/abcs/database_types.py

@@ -15,7 +15,9 @@
 
 @dataclass
 class ColType:
-    supported = True
+    @property
+    def supported(self) -> bool:
+        return True
 
 
 @dataclass
@@ -132,7 +134,9 @@ def make_value(self, value):
 
 @dataclass
 class Text(StringType):
-    supported = False
+    @property
+    def supported(self) -> bool:
+        return False
 
 
 # In majority of DBMSes, it is called JSON/JSONB. Only in Snowflake, it is OBJECT.
@@ -169,4 +173,6 @@ def __post_init__(self):
 class UnknownColType(ColType):
     text: str
 
-    supported = False
+    @property
+    def supported(self) -> bool:
+        return False

data_diff/queries/ast_classes.py

@@ -28,7 +28,9 @@ class Root:
 class ExprNode(Compilable):
     "Base class for query expression nodes"
 
-    type: Any = None
+    @property
+    def type(self) -> Optional[type]:
+        return None
 
     def _dfs_values(self):
         yield self
@@ -216,7 +218,10 @@ class Concat(ExprNode):
 class Count(ExprNode):
     expr: Expr = None
     distinct: bool = False
-    type = int
+
+    @property
+    def type(self) -> Optional[type]:
+        return int
 
 
 class LazyOps:
@@ -337,7 +342,10 @@ def then(self, then: Expr) -> CaseWhen:
 class IsDistinctFrom(ExprNode, LazyOps):
     a: Expr
     b: Expr
-    type = bool
+
+    @property
+    def type(self) -> Optional[type]:
+        return bool
 
 
 @dataclass(eq=False, order=False)
@@ -361,7 +369,9 @@ class UnaryOp(ExprNode, LazyOps):
 
 
 class BinBoolOp(BinOp):
-    type = bool
+    @property
+    def type(self) -> Optional[type]:
+        return bool
 
 
 @dataclass(eq=False, order=False)
@@ -700,7 +710,10 @@ def __getitem__(self, name):
 class In(ExprNode):
     expr: Expr
     list: Sequence[Expr]
-    type = bool
+
+    @property
+    def type(self) -> Optional[type]:
+        return bool
 
 
 @dataclass
@@ -711,7 +724,9 @@ class Cast(ExprNode):
 
 @dataclass
 class Random(ExprNode, LazyOps):
-    type = float
+    @property
+    def type(self) -> Optional[type]:
+        return float
 
 
 @dataclass
@@ -722,11 +737,16 @@ class ConstantTable(ExprNode):
 @dataclass
 class Explain(ExprNode, Root):
     select: Select
-    type = str
+
+    @property
+    def type(self) -> Optional[type]:
+        return str
 
 
 class CurrentTimestamp(ExprNode):
-    type = datetime
+    @property
+    def type(self) -> Optional[type]:
+        return datetime
 
 
 @dataclass
@@ -742,7 +762,9 @@ class TimeTravel(ITable):
 
 
 class Statement(Compilable, Root):
-    type = None
+    @property
+    def type(self) -> Optional[type]:
+        return None
 
 
 @dataclass

data_diff/queries/extras.py

@@ -1,5 +1,5 @@
 "Useful AST classes that don't quite fall within the scope of regular SQL"
-from typing import Callable, Sequence
+from typing import Callable, Optional, Sequence
 from runtype import dataclass
 
 from data_diff.abcs.database_types import ColType
@@ -11,7 +11,10 @@
 class NormalizeAsString(ExprNode):
     expr: ExprNode
     expr_type: ColType = None
-    type = str
+
+    @property
+    def type(self) -> Optional[type]:
+        return str
 
 
 @dataclass