[1/2] Support 'not isinstance'

If we have an expression of the form "not instance"
then we can recurse and just switch the else and if
expressions. We might as well take this opportunity.

Author: jhance

mypy/checker.py

@@ -2097,10 +2097,23 @@ def find_isinstance_check(node: Node,
             if kind == ISINSTANCE_ALWAYS_TRUE:
                 kind = ISINSTANCE_OVERLAPPING
             return (var, type, AnyType(), kind)
+    elif isinstance(node, UnaryExpr) and node.op == 'not':
+        (var, type, elsetype, kind) = find_isinstance_check(node.expr, type_map, weak)
+        return (var, elsetype, type, invert_isinstance_kind(kind))
+
     # Not a supported isinstance check
     return None, AnyType(), AnyType(), -1
 
 
+def invert_isinstance_kind(kind: int) -> int:
+    if kind == ISINSTANCE_ALWAYS_TRUE:
+        return ISINSTANCE_ALWAYS_FALSE
+    elif kind == ISINSTANCE_ALWAYS_FALSE:
+        return ISINSTANCE_ALWAYS_TRUE
+    else:
+        return kind
+
+
 def get_isinstance_type(node: Node, type_map: Dict[Node, Type]) -> Type:
     type = type_map[node]
     if isinstance(type, FunctionLike):