jax.debug.callback now requires a Callable[..., None]

This makes the "return value is ignored" behavior explicit in the type.

PiperOrigin-RevId: 626430448

jax/_src/debugging.py

@@ -73,7 +73,8 @@ map, unsafe_map = util.safe_map, map
 def debug_callback_impl(*args, callback: Callable[..., Any],
                         effect: DebugEffect):
   del effect
-  return callback(*args)
+  callback(*args)
+  return ()
 
 @debug_callback_p.def_effectful_abstract_eval
 def debug_callback_abstract_eval(*flat_avals, callback: Callable[..., Any],
@@ -136,13 +137,13 @@ def debug_callback_lowering(ctx, *args, effect, callback, **params):
     sharding = None
 
   def _callback(*flat_args):
-    return tuple(
     debug_callback_p.impl(
-            *flat_args, effect=effect, callback=callback, **params))
+        *flat_args, effect=effect, callback=callback, **params)
+    return ()
   if effects.ordered_effects.contains(effect):
     token = ctx.tokens_in.get(effect)[0]
     result, token, _ = mlir.emit_python_callback(
-        ctx, _callback, token, list(args), ctx.avals_in, ctx.avals_out, True)
+        ctx, _callback, token, list(args), ctx.avals_in, ctx.avals_out, has_side_effect=True)
     ctx.set_tokens_out(mlir.TokenSet({effect: (token,)}))
   else:
     result, token, _ = mlir.emit_python_callback(
@@ -187,7 +188,7 @@ def _debug_callback_partial_eval_custom(saveable, unks_in, inst_in, eqn):
 pe.partial_eval_jaxpr_custom_rules[debug_callback_p] = (
     _debug_callback_partial_eval_custom)
 
-def debug_callback(callback: Callable[..., Any], *args: Any,
+def debug_callback(callback: Callable[..., None], *args: Any,
                    ordered: bool = False, **kwargs: Any) -> None:
   """Calls a stageable Python callback.
 
@@ -206,7 +207,7 @@ def debug_callback(callback: Callable[..., Any], *args: Any,
   of the computation are duplicated or dropped.
 
   Args:
-    callback: A Python callable. Its return value will be ignored.
+    callback: A Python callable returning None.
     *args: The positional arguments to the callback.
     ordered: A keyword only argument used to indicate whether or not the
       staged out computation will enforce ordering of this callback w.r.t.
@@ -231,7 +232,7 @@ def debug_callback(callback: Callable[..., Any], *args: Any,
   def _flat_callback(*flat_args):
     args, kwargs = tree_util.tree_unflatten(in_tree, flat_args)
     callback(*args, **kwargs)
-    return []
+    return ()
   debug_callback_p.bind(*flat_args, callback=_flat_callback, effect=effect)
 
 class _DebugPrintFormatChecker(string.Formatter):