diff --git a/swh/indexer/metadata_dictionary/__init__.py b/swh/indexer/metadata_dictionary/__init__.py
--- a/swh/indexer/metadata_dictionary/__init__.py
+++ b/swh/indexer/metadata_dictionary/__init__.py
@@ -27,7 +27,7 @@
 @click.command()
 @click.argument("mapping_name")
 @click.argument("file_name")
-def main(mapping_name, file_name):
+def main(mapping_name: str, file_name: str):
     from pprint import pprint
 
     with open(file_name, "rb") as fd:
diff --git a/swh/indexer/metadata_dictionary/base.py b/swh/indexer/metadata_dictionary/base.py
--- a/swh/indexer/metadata_dictionary/base.py
+++ b/swh/indexer/metadata_dictionary/base.py
@@ -5,7 +5,7 @@
 
 import json
 import logging
-from typing import List
+from typing import List, Union
 
 from swh.indexer.codemeta import SCHEMA_URI, compact, merge_values
 
@@ -32,7 +32,7 @@
         raise NotImplementedError(f"{self.__class__.__name__}.name")
 
     @classmethod
-    def detect_metadata_files(cls, files):
+    def detect_metadata_files(cls, files: list):
         """
         Detects files potentially containing metadata
 
@@ -150,7 +150,7 @@
 class JsonMapping(DictMapping, SingleFileMapping):
     """Base class for all mappings that use a JSON file as input."""
 
-    def translate(self, raw_content):
+    def translate(self, raw_content) -> Union[dict, None]:
         """
         Translates content by parsing content from a bytestring containing
         json data and translating with the appropriate mapping
@@ -167,11 +167,12 @@
             raw_content = raw_content.decode()
         except UnicodeDecodeError:
             self.log.warning("Error unidecoding from %s", self.log_suffix)
-            return
+            return None
         try:
             content_dict = json.loads(raw_content)
         except json.JSONDecodeError:
             self.log.warning("Error unjsoning from %s", self.log_suffix)
-            return
+            return None
         if isinstance(content_dict, dict):
             return self._translate_dict(content_dict)
+        return None
diff --git a/swh/indexer/metadata_dictionary/cff.py b/swh/indexer/metadata_dictionary/cff.py
--- a/swh/indexer/metadata_dictionary/cff.py
+++ b/swh/indexer/metadata_dictionary/cff.py
@@ -1,3 +1,5 @@
+from typing import Dict, List, Optional, Union
+
 import yaml
 
 from swh.indexer.codemeta import CODEMETA_CONTEXT_URL, CROSSWALK_TABLE, SCHEMA_URI
@@ -18,19 +20,23 @@
     mapping = CROSSWALK_TABLE["Citation File Format Core (CFF-Core) 1.0.2"]
     string_fields = ["keywords", "license", "abstract", "version", "doi"]
 
-    def translate(self, raw_content):
-        raw_content = raw_content.decode()
-        content_dict = yaml.load(raw_content, Loader=yaml.SafeLoader)
+    def translate(self, raw_content: bytes) -> Dict[str, str]:
+        raw_content_string: str = raw_content.decode()
+        content_dict = yaml.load(raw_content_string, Loader=yaml.SafeLoader)
         metadata = self._translate_dict(content_dict)
 
         metadata["@context"] = CODEMETA_CONTEXT_URL
 
         return metadata
 
-    def normalize_authors(self, d):
+    def normalize_authors(
+        self, d: List[dict]
+    ) -> Optional[Union[List, Dict[str, list]]]:
         result = []
         for author in d:
-            author_data = {"@type": SCHEMA_URI + "Person"}
+            author_data: Dict[str, Optional[Union[str, dict]]] = {
+                "@type": SCHEMA_URI + "Person"
+            }
             if "orcid" in author:
                 author_data["@id"] = author["orcid"]
             if "affiliation" in author:
@@ -45,21 +51,21 @@
 
             result.append(author_data)
 
-        result = {"@list": result}
-        return result
+        result_final = {"@list": result}
+        return result_final
 
-    def normalize_doi(self, s):
+    def normalize_doi(self, s: str) -> Dict[str, str]:
         if isinstance(s, str):
             return {"@id": "https://doi.org/" + s}
 
-    def normalize_license(self, s):
+    def normalize_license(self, s: str) -> Dict[str, str]:
         if isinstance(s, str):
             return {"@id": "https://spdx.org/licenses/" + s}
 
-    def normalize_repository_code(self, s):
+    def normalize_repository_code(self, s: str) -> Dict[str, str]:
         if isinstance(s, str):
             return {"@id": s}
 
-    def normalize_date_released(self, s):
+    def normalize_date_released(self, s: str) -> Dict[str, str]:
         if isinstance(s, str):
             return {"@value": s, "@type": SCHEMA_URI + "Date"}