diff --git a/src/stpipe/step.py b/src/stpipe/step.py
index 1b725e63..11589387 100644
--- a/src/stpipe/step.py
+++ b/src/stpipe/step.py
@@ -1191,13 +1191,7 @@ def make_input_path(self, file_path):
             File path using ``input_dir`` if the input
             had no directory path.
         """
-        full_path = file_path
-        if isinstance(file_path, str):
-            original_path, file_name = split(file_path)
-            if not len(original_path):
-                full_path = join(self.input_dir, file_name)
-
-        return full_path
+        return _make_input_path(file_path, self.input_dir)
 
     def _set_input_dir(self, input_, exclusive=True):
         """Set the input directory
@@ -1466,3 +1460,12 @@ def preserve_step_pars(step):
         yield saved_pars
     finally:
         step.update_pars(saved_pars)
+
+
+def _make_input_path(file_path, input_dir):
+    full_path = file_path
+    if isinstance(file_path, str):
+        original_path, file_name = split(file_path)
+        if not len(original_path):
+            full_path = join(input_dir, file_name)
+    return full_path