diff --git a/cij/data/__init__.py b/cij/data/__init__.py
index 4c5f9fa..8aa2b5f 100644
--- a/cij/data/__init__.py
+++ b/cij/data/__init__.py
@@ -3,6 +3,9 @@
 
 __path__ = Path(__file__).absolute().parent
 
-def get_data_fname(fname) -> str:
+def get_data_fname(fname: str) -> str:
+    '''
+    Get the full path of the data file from its relative path.
+    '''
     return pkg_resources.resource_filename(__name__, fname)