Set correct type when indexing into Var[str] (#2469)

* Index into strings

* Write tests

reflex/vars.py

@@ -572,11 +572,10 @@ class Var:
                 )
 
             # Get the type of the indexed var.
-            type_ = (
-                types.get_args(self._var_type)[0]
-                if types.is_generic_alias(self._var_type)
-                else Any
-            )
+            if types.is_generic_alias(self._var_type):
+                type_ = types.get_args(self._var_type)[0]
+            elif types._issubclass(self._var_type, str):
+                type_ = str
 
             # Use `at` to support negative indices.
             return self._replace(

tests/test_var.py

@@ -406,6 +406,22 @@ def test_var_indexing_lists(var):
     assert str(var[-1]) == f"{{{var._var_name}.at(-1)}}"
 
 
+def test_var_indexing_str():
+    """Test that we can index into str vars."""
+    str_var = BaseVar(_var_name="str", _var_type=str)
+
+    # Test that indexing gives a type of Var[str].
+    assert isinstance(str_var[0], Var)
+    assert str_var[0]._var_type == str
+
+    # Test basic indexing.
+    assert str(str_var[0]) == "{str.at(0)}"
+    assert str(str_var[1]) == "{str.at(1)}"
+
+    # Test negative indexing.
+    assert str(str_var[-1]) == "{str.at(-1)}"
+
+
 @pytest.mark.parametrize(
     "var, index",
     [