[2/2] Support isinstance in conditional-if

We simply check for isinstance when visiting conditionals
and correctly push for the if/else expressions, giving them new types
only for those particular expressions. After that, the type of the
variable being modified returns to its original type.

Author: jhance

mypy/checkexpr.py

@@ -1275,8 +1275,25 @@ def check_for_comp(self, e: Union[GeneratorExpr, DictionaryComprehension]) -> No
     def visit_conditional_expr(self, e: ConditionalExpr) -> Type:
         cond_type = self.accept(e.cond)
         self.check_not_void(cond_type, e)
-        if_type = self.accept(e.if_expr)
-        else_type = self.accept(e.else_expr, context=if_type)
+
+        # Gain type information from isinstance if it is there
+        # but only for the current expression
+        variable, inst_if_type, inst_else_type, kind = mypy.checker.find_isinstance_check(
+            e.cond,
+            self.chk.type_map,
+            self.chk.typing_mode_weak())
+        if variable:
+            self.chk.binder.push_frame()
+            self.chk.binder.push(variable, inst_if_type)
+            if_type = self.accept(e.if_expr)
+            self.chk.binder.pop_frame()
+            self.chk.binder.push_frame()
+            self.chk.binder.push(variable, inst_else_type)
+            else_type = self.accept(e.else_expr, context=if_type)
+            self.chk.binder.pop_frame()
+        else:
+            if_type = self.accept(e.if_expr)
+            else_type = self.accept(e.else_expr, context=if_type)
         return join.join_types(if_type, else_type)
 
     #

mypy/test/data/check-isinstance.test

@@ -708,3 +708,16 @@ x = B() # type: A
 if isinstance(x, B) and x.flag:
     pass
 [builtins fixtures/isinstancelist.py]
+[case testIsinstanceExpression]
+class A:
+    pass
+
+class B(A):
+    flag = 1
+
+x = B() # type: A
+
+x.flag if isinstance(x, B) else 0
+0 if not isinstance(x, B) else x.flag
+0 if isinstance(x, B) else x.flag # E: "A" has no attribute "flag"
+[builtins fixtures/isinstancelist.py]