diff --git a/securedrop/upload-screenshots.py b/securedrop/upload-screenshots.py
index d8baafecadb..72f0b31b5ad 100755
--- a/securedrop/upload-screenshots.py
+++ b/securedrop/upload-screenshots.py
@@ -24,7 +24,7 @@
 # filename into the canonical title we give that screenshot in Weblate.
 #
 # Example conversion: "source-session_timeout.png" -> "source: session timeout"
-CANONICALIZATION_RULES = [(r"\.png$", ""), (r"-", ": "), (r"_", " ")]
+CANONICALIZATION_RULES = ((r"\.png$", ""), (r"-", ": "), (r"_", " "))
 
 # Weblate organizes internationalization work into projects and components,
 # which are part of many URLs, and need to be referenced in some API requests.
@@ -94,7 +94,7 @@ def __init__(
         component,
         files,
         request_limit,
-        canonicalization_rules=[],
+        canonicalization_rules=(),
     ):
 
         if len(token) != 40:
@@ -169,7 +169,7 @@ def _canonicalize(self, filename):
             filename = re.sub(pattern, repl, filename)
         return filename
 
-    def upload(self, existing_screenshots=[]):
+    def upload(self, existing_screenshots=None):
         """
         Uploads all files using the screenshots endpoint. Optionally, checks
         files against a list of existing screenshots and replaces them rather
@@ -179,11 +179,14 @@ def upload(self, existing_screenshots=[]):
             basename = os.path.basename(file)
             canonical_name = self._canonicalize(basename)
             existing_screenshot_url = None
-            for screenshot in existing_screenshots:
-                if screenshot["name"] == canonical_name:
-                    existing_screenshot_url = screenshot["file_url"]
-                    break
-            image = {"image": open(file, "rb")}
+
+            if existing_screenshots is not None:
+                for screenshot in existing_screenshots:
+                    if screenshot["name"] == canonical_name:
+                        existing_screenshot_url = screenshot["file_url"]
+                        break
+                image = {"image": open(file, "rb")}
+
             if existing_screenshot_url is not None:
                 print("Replacing existing screenshot {}".format(basename))
                 response = self.session.post(existing_screenshot_url, files=image)