Union repr uses [] not ()

Author: ecprice

mypy/messages.py

@@ -197,7 +197,7 @@ def format_simple(self, typ: Type) -> str:
             items = []
             for t in (cast(UnionType, typ)).items:
                 items.append(strip_quotes(self.format(t)))
-            s = '"Union({})"'.format(', '.join(items))
+            s = '"Union[{}]"'.format(', '.join(items))
             if len(s) < 40:
                 return s
             else:

mypy/test/data/check-unions.test

@@ -73,7 +73,7 @@ i = Undefined # type: int
 x.foo()
 y.foo()
 i = x.foo()
-i = y.foo()   # E: Incompatible types in assignment (expression has type "Union(int, str)", variable has type "int") 
+i = y.foo()   # E: Incompatible types in assignment (expression has type "Union[int, str]", variable has type "int") 
 
 [builtins fixtures/isinstance.py]
 

mypy/types.py

@@ -388,7 +388,7 @@ def accept(self, visitor: 'TypeVisitor[T]') -> T:
 
 
 class UnionType(Type):
-    """The union type Union(T1, ..., Tn) (at least one type argument)."""
+    """The union type Union[T1, ..., Tn] (at least one type argument)."""
 
     items = Undefined(List[Type])
 
@@ -676,7 +676,7 @@ def visit_tuple_type(self, t):
 
     def visit_union_type(self, t):
         s = self.list_str(t.items)
-        return 'Union({})'.format(s)
+        return 'Union[{}]'.format(s)
 
     def visit_runtime_type_var(self, t):
         return '<RuntimeTypeVar>'