diff --git a/t5x/trainer.py b/t5x/trainer.py
index 2fa7cd45f..752beb379 100644
--- a/t5x/trainer.py
+++ b/t5x/trainer.py
@@ -293,11 +293,7 @@ def __init__(
     """
     self._name = name
     if jax.process_index() == 0:
-      self._writer = metric_writers.create_default_writer(
-          summary_dir,
-          collection=name,
-          asynchronous=True,
-      )
+      self._writer = self._create_writer(name, summary_dir)
     else:
       self._writer = metric_writers.MultiWriter([])
     self.summary_dir = os.path.join(summary_dir, name) if summary_dir else None
@@ -305,10 +301,21 @@ def __init__(
     # We use a thread pool with a single worker to ensure that calls to the
     # function are run in order (but in a background thread).
     self._summary_pool = asynclib.Pool(
-        thread_name_prefix="MetricsManager", max_workers=1)
+        thread_name_prefix="MetricsManager", max_workers=1
+    )
     # Times the duration between steps.
     self._duration_timer = _AsyncTimer()
 
+  def _create_writer(
+      self, name: str, summary_dir: Optional[str] = None
+  ) -> metric_writers.MetricWriter:
+    """Creates the writer for host 0."""
+    return metric_writers.create_default_writer(
+        summary_dir,
+        collection=name,
+        asynchronous=True,
+    )
+
   def __del__(self):
     self.close()
 
@@ -327,6 +334,7 @@ def close(self):
   def summary_writer(self) -> metric_writers.MetricWriter:
     """Returns the MetricWriter used by this class."""
     # TODO(adarob): Make returned writer threadsafe.
+    assert self._writer is not None
     return self._writer
 
   def write_scalar(self, key: str, val: metric_writers.interface.Scalar,
@@ -674,7 +682,9 @@ def accumulate_grads_microbatched(
   # Note: Default t5x models don't support flax_mutables. One needs to subclass
   # them and return flax_mutables from `get_initial_variables` and `loss_fn`.
 
-  initial_flax_mutables = train_state.flax_mutables if train_state.flax_mutables else None
+  initial_flax_mutables = (
+      train_state.flax_mutables if train_state.flax_mutables else None
+  )
 
   if num_microbatches is None or num_microbatches <= 1: