Reuse first-step read and add diagnostic warnings for missing timestep dirs

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: sbryngelson

toolchain/mfc/viz/reader.py

@@ -241,6 +241,12 @@ def discover_timesteps(case_dir: str, fmt: str) -> List[int]:
                         pass
             return sorted(steps)
 
+        import warnings  # pylint: disable=import-outside-toplevel
+        warnings.warn(
+            f"No timestep directories found: checked '{root_dir}' and '{p0_dir}'",
+            stacklevel=2,
+        )
+
     elif fmt == 'silo':
         p0_dir = os.path.join(case_dir, 'silo_hdf5', 'p0')
         if os.path.isdir(p0_dir):
@@ -253,7 +259,13 @@ def discover_timesteps(case_dir: str, fmt: str) -> List[int]:
                         pass
             return sorted(steps)
 
-    return []  # no timestep files found in expected directories
+        import warnings  # pylint: disable=import-outside-toplevel
+        warnings.warn(
+            f"No timestep directory found: checked '{p0_dir}'",
+            stacklevel=2,
+        )
+
+    return []
 
 
 def _discover_processors(case_dir: str, fmt: str) -> List[int]:

toolchain/mfc/viz/viz.py

@@ -186,16 +186,17 @@ def read_step(step):
             return assemble_silo(case_dir, step, var=varname)
         return assemble(case_dir, step, fmt, var=varname)
 
-    # Validate variable name by reading the first timestep (without var filter)
-    def read_step_all_vars(step):
+    # Validate variable name by reading the first timestep (without var filter),
+    # then reuse it for the first rendered frame to avoid double I/O.
+    first_assembled = read_step(requested_steps[0])
+    if varname not in first_assembled.variables:
+        # Re-read without filter to show all available variable names
         if fmt == 'silo':
             from .silo_reader import assemble_silo  # pylint: disable=import-outside-toplevel
-            return assemble_silo(case_dir, step)
-        return assemble(case_dir, step, fmt)
-
-    test_assembled = read_step_all_vars(requested_steps[0])
-    if varname not in test_assembled.variables:
-        avail = sorted(test_assembled.variables.keys())
+            all_vars_assembled = assemble_silo(case_dir, requested_steps[0])
+        else:
+            all_vars_assembled = assemble(case_dir, requested_steps[0], fmt)
+        avail = sorted(all_vars_assembled.variables.keys())
         cons.print(f"[bold red]Error:[/bold red] Variable '{varname}' not found.")
         cons.print(f"[bold]Available variables:[/bold] {', '.join(avail)}")
         sys.exit(1)
@@ -231,7 +232,7 @@ def read_step_all_vars(step):
     failures = []
     for step in step_iter:
         try:
-            assembled = read_step(step)
+            assembled = first_assembled if step == requested_steps[0] else read_step(step)
         except (FileNotFoundError, EOFError, ValueError) as exc:
             cons.print(f"[yellow]Warning:[/yellow] Skipping step {step}: {exc}")
             failures.append(step)