Fix type narrowing for Python 3.10

pyproject.toml

@@ -1,6 +1,6 @@
 [tool.poetry]
 name = "tyro"
-version = "0.3.26"
+version = "0.3.27"
 description = "Strongly typed, zero-effort CLI interfaces"
 authors = ["brentyi <[email protected]>"]
 include = ["./tyro/**/*"]

tyro/_parsers.py

@@ -83,7 +83,7 @@ def from_callable_or_type(
             field = dataclasses.replace(
                 field,
                 # Resolve generic types.
-                typ=_resolver.narrow_type(
+                typ=_resolver.narrow_container_types(
                     _resolver.type_from_typevar_constraints(  # type: ignore
                         _resolver.apply_type_from_typevar(
                             field.typ,

tyro/_resolver.py

@@ -2,7 +2,6 @@
 import collections.abc
 import copy
 import dataclasses
-import pathlib
 import sys
 from typing import (
     Any,
@@ -118,14 +117,10 @@ def narrow_type(typ: TypeT, default_instance: Any) -> TypeT:
     """Type narrowing: if we annotate as Animal but specify a default instance of Cat, we
     should parse as Cat.
 
-    Note that Union types are intentionally excluded here."""
-
-    # Don't apply narrowing for pathlib.PosixPath, pathlib.WindowsPath, etc.
-    # This is mostly an aesthetic decision, and is needed because pathlib.Path() hacks
-    # __new__ to dynamically choose between path types.
-    if typ is pathlib.Path:
-        return typ
-
+    This should generally only be applied to fields used as nested structures, not
+    individual arguments/fields. (if a field is annotated as Union[int, str], and a
+    string default is passed in, we don't want to narrow the type to always be
+    strings!)"""
     try:
         potential_subclass = type(default_instance)
 
@@ -135,6 +130,11 @@ def narrow_type(typ: TypeT, default_instance: Any) -> TypeT:
             return typ
 
         superclass = unwrap_annotated(typ)[0]
+
+        # For Python 3.10: don't narrow union types.
+        if get_origin(superclass) is Union:
+            return typ
+
         if superclass is Any or issubclass(potential_subclass, superclass):  # type: ignore
             if get_origin(typ) is Annotated:
                 return Annotated.__class_getitem__(  # type: ignore
@@ -144,13 +144,17 @@ def narrow_type(typ: TypeT, default_instance: Any) -> TypeT:
     except TypeError:
         pass
 
+    return typ
+
+
+def narrow_container_types(typ: TypeT, default_instance: Any) -> TypeT:
+    """Type narrowing for containers. Infers types of container contents."""
     if typ is list and isinstance(default_instance, list):
         typ = List.__getitem__(Union.__getitem__(tuple(map(type, default_instance))))  # type: ignore
     elif typ is set and isinstance(default_instance, set):
         typ = Set.__getitem__(Union.__getitem__(tuple(map(type, default_instance))))  # type: ignore
     elif typ is tuple and isinstance(default_instance, tuple):
         typ = Tuple.__getitem__(tuple(map(type, default_instance)))  # type: ignore
-
     return typ