Index: tools/opt-viewer/opt-diff.py
===================================================================
--- tools/opt-viewer/opt-diff.py
+++ tools/opt-viewer/opt-diff.py
@@ -20,24 +20,17 @@
 import argparse
 from collections import defaultdict
 from multiprocessing import cpu_count, Pool
-import os, os.path
-import fnmatch
-
-def find_files(dir_or_file):
-    if os.path.isfile(dir_or_file):
-        return [dir_or_file]
-
-    all = []
-    for dir, subdirs, files in os.walk(dir_or_file):
-        for file in files:
-            if fnmatch.fnmatch(file, "*.opt.yaml"):
-                all.append( os.path.join(dir, file))
-    return all
 
 if __name__ == '__main__':
     parser = argparse.ArgumentParser(description=desc)
-    parser.add_argument('yaml_dir_or_file_1')
-    parser.add_argument('yaml_dir_or_file_2')
+    parser.add_argument(
+        'yaml_dir_or_file_1',
+        help='An optmization record file or a directory searched for optmization '
+             'record files that are used as the old version for the comparison')
+    parser.add_argument(
+        'yaml_dir_or_file_2',
+        help='An optmization record file or a directory searched for optmization '
+             'record files that are used as the new version for the comparison')
     parser.add_argument(
         '--jobs',
         '-j',
@@ -53,8 +46,8 @@
     parser.add_argument('--output', '-o', default='diff.opt.yaml')
     args = parser.parse_args()
 
-    files1 = find_files(args.yaml_dir_or_file_1)
-    files2 = find_files(args.yaml_dir_or_file_2)
+    files1 = optrecord.find_opt_files([args.yaml_dir_or_file_1])
+    files2 = optrecord.find_opt_files([args.yaml_dir_or_file_2])
 
     print_progress = not args.no_progress_indicator
     all_remarks1, _, _ = optrecord.gather_results(files1, args.jobs, print_progress)
Index: tools/opt-viewer/opt-stats.py
===================================================================
--- tools/opt-viewer/opt-stats.py
+++ tools/opt-viewer/opt-stats.py
@@ -15,7 +15,11 @@
 
 if __name__ == '__main__':
     parser = argparse.ArgumentParser(description=desc)
-    parser.add_argument('yaml_files', nargs='+')
+    parser.add_argument(
+        'yaml_dirs_or_files',
+        nargs='+',
+        help='List of optimization record files or directories searched '
+             'for optimization record files.')
     parser.add_argument(
         '--jobs',
         '-j',
@@ -31,8 +35,14 @@
     args = parser.parse_args()
 
     print_progress = not args.no_progress_indicator
+
+    files = optrecord.find_opt_files(args.yaml_dirs_or_files)
+    if not files:
+        parser.error("No *.opt.yaml files found")
+        sys.exit(1)
+
     all_remarks, file_remarks, _ = optrecord.gather_results(
-        args.yaml_files, args.jobs, print_progress)
+        files, args.jobs, print_progress)
     if print_progress:
         print('\n')
 
Index: tools/opt-viewer/opt-viewer.py
===================================================================
--- tools/opt-viewer/opt-viewer.py
+++ tools/opt-viewer/opt-viewer.py
@@ -219,7 +219,11 @@
 
 if __name__ == '__main__':
     parser = argparse.ArgumentParser(description=desc)
-    parser.add_argument('yaml_files', nargs='+')
+    parser.add_argument(
+        'yaml_dirs_or_files',
+        nargs='+',
+        help='List of optimization record files or directories searched '
+             'for optimization record files.')
     parser.add_argument(
         '--output-dir',
         '-o',
@@ -248,8 +252,14 @@
     args = parser.parse_args()
 
     print_progress = not args.no_progress_indicator
+
+    files = optrecord.find_opt_files(args.yaml_dirs_or_files)
+    if not files:
+        parser.error("No *.opt.yaml files found")
+        sys.exit(1)
+
     all_remarks, file_remarks, should_display_hotness = \
-        optrecord.gather_results(args.yaml_files, args.jobs, print_progress)
+        optrecord.gather_results(files, args.jobs, print_progress)
 
     map_remarks(all_remarks)
 
Index: tools/opt-viewer/optrecord.py
===================================================================
--- tools/opt-viewer/optrecord.py
+++ tools/opt-viewer/optrecord.py
@@ -12,8 +12,10 @@
 
 import cgi
 from collections import defaultdict
+import fnmatch
 import functools
 from multiprocessing import Lock
+import os, os.path
 import subprocess
 
 import optpmap
@@ -233,3 +235,16 @@
         all_remarks.update(all_remarks_job)
 
     return all_remarks, file_remarks, max_hotness != 0
+
+
+def find_opt_files(dirs_or_files):
+    all = []
+    for dir_or_file in dirs_or_files:
+        if os.path.isfile(dir_or_file):
+            all.append(dir_or_file)
+        else:
+            for dir, subdirs, files in os.walk(dir_or_file):
+                for file in files:
+                    if fnmatch.fnmatch(file, "*.opt.yaml"):
+                        all.append(os.path.join(dir, file))
+    return all