diff --git a/merlion/models/base.py b/merlion/models/base.py
index 1cf7f3aea..e8e3cbe4b 100644
--- a/merlion/models/base.py
+++ b/merlion/models/base.py
@@ -103,7 +103,9 @@ def __reduce__(self):
     def __copy__(self):
         return self.from_dict(self.to_dict())
 
-    def __deepcopy__(self, memodict={}):
+    def __deepcopy__(self, memodict=None):
+        if memodict is None:
+            memodict = {}
         return self.__copy__()
 
     def get_unused_kwargs(self, **kwargs):
@@ -467,7 +469,9 @@ def __copy__(self):
         new_model.__setstate__(state_dict)
         return new_model
 
-    def __deepcopy__(self, memodict={}):
+    def __deepcopy__(self, memodict=None):
+        if memodict is None:
+            memodict = {}
         new_model = self.__class__(config=copy.deepcopy(self.config))
         state_dict = self.__getstate__()
         state_dict.pop("config", None)
diff --git a/merlion/models/ensemble/base.py b/merlion/models/ensemble/base.py
index cb17bed95..04df643e2 100644
--- a/merlion/models/ensemble/base.py
+++ b/merlion/models/ensemble/base.py
@@ -71,7 +71,9 @@ def __copy__(self):
         config_dict["models"] = self.models
         return self.from_dict(config_dict)
 
-    def __deepcopy__(self, memodict={}):
+    def __deepcopy__(self, memodict=None):
+        if memodict is None:
+            memodict = {}
         copied = copy.copy(self)
         copied.models = copy.deepcopy(self.models)
         return copied
diff --git a/merlion/models/ensemble/combine.py b/merlion/models/ensemble/combine.py
index 0dd4d1956..799785fdf 100644
--- a/merlion/models/ensemble/combine.py
+++ b/merlion/models/ensemble/combine.py
@@ -83,7 +83,9 @@ def from_dict(cls, state):
     def __copy__(self):
         return self.from_dict(self.to_dict())
 
-    def __deepcopy__(self, memodict={}):
+    def __deepcopy__(self, memodict=None):
+        if memodict is None:
+            memodict = {}
         return self.__copy__()
 
     @abstractmethod
diff --git a/merlion/models/layers.py b/merlion/models/layers.py
index d3f769658..f148b61e3 100644
--- a/merlion/models/layers.py
+++ b/merlion/models/layers.py
@@ -129,7 +129,9 @@ def __copy__(self):
         config_dict["model"] = self.model
         return self.from_dict(config_dict)
 
-    def __deepcopy__(self, memodict={}):
+    def __deepcopy__(self, memodict=None):
+        if memodict is None:
+            memodict = {}
         config_dict = super().to_dict(_skipped_keys={"model"})
         config_dict["model"] = copy.deepcopy(self.model)
         return self.from_dict(config_dict)
diff --git a/merlion/post_process/base.py b/merlion/post_process/base.py
index 14d21ec2d..a1dcc3762 100644
--- a/merlion/post_process/base.py
+++ b/merlion/post_process/base.py
@@ -38,7 +38,9 @@ def from_dict(cls, state_dict):
     def __copy__(self):
         return self.from_dict(self.to_dict())
 
-    def __deepcopy__(self, memodict={}):
+    def __deepcopy__(self, memodict=None):
+        if memodict is None:
+            memodict = {}
         return self.__copy__()
 
     def __repr__(self):
diff --git a/merlion/utils/conj_priors.py b/merlion/utils/conj_priors.py
index a73b8f0a6..a44ec4c3a 100644
--- a/merlion/utils/conj_priors.py
+++ b/merlion/utils/conj_priors.py
@@ -88,7 +88,9 @@ def __copy__(self):
             setattr(ret, k, copy.deepcopy(v))
         return ret
 
-    def __deepcopy__(self, memodict={}):
+    def __deepcopy__(self, memodict=None):
+        if memodict is None:
+            memodict = {}
         return self.__copy__()
 
     @staticmethod