diff --git a/WFacer/specie_decorators/__init__.py b/WFacer/specie_decorators/__init__.py
index f739973..46c3029 100644
--- a/WFacer/specie_decorators/__init__.py
+++ b/WFacer/specie_decorators/__init__.py
@@ -11,7 +11,7 @@
     PmgGuessChargeDecorator,
 )
 
-allowed_decorators = defaultdict(lambda: [])
+allowed_decorators = defaultdict(list)
 for subclass in get_subclasses(BaseDecorator).values():
     prop = subclass.decorated_prop_name
     if subclass.required_prop_names is not None:
diff --git a/WFacer/specie_decorators/base.py b/WFacer/specie_decorators/base.py
index b7e669a..d75c681 100644
--- a/WFacer/specie_decorators/base.py
+++ b/WFacer/specie_decorators/base.py
@@ -116,7 +116,7 @@ def group_site_by_species(entries):
             defaultdict:
                (Entry index, site index) belonging to each species.
         """
-        groups_by_species = defaultdict(lambda: [])
+        groups_by_species = defaultdict(list)
 
         # These entries should not contain Vacancy.
         for e_id, entry in enumerate(entries):
@@ -249,7 +249,7 @@ def _process(self, entries, decorate_rules):
                     species_decor.append(sp)
 
             # Preserve all information.
-            site_properties = defaultdict(lambda: [])
+            site_properties = defaultdict(list)
             for site in s_undecor:
                 for p in site.properties:
                     site_properties[p].append(site.properties[p])
diff --git a/tests/test_enumeration.py b/tests/test_enumeration.py
index 9e943b3..b9539b4 100644
--- a/tests/test_enumeration.py
+++ b/tests/test_enumeration.py
@@ -170,7 +170,7 @@ def test_enumerate_structures(single_expansion):
         for occu, mid in zip(occus, mids)
     ]
     # When keeping the ground states, each composition must have at least 1 state.
-    count_occurences = defaultdict(lambda: 0)
+    count_occurences = defaultdict(int)
     for n in ns:
         count_occurences[n] += 1