diff --git a/packages/subiquity_client/generator/Makefile b/packages/subiquity_client/generator/Makefile
index 0704c5994..386d4f497 100644
--- a/packages/subiquity_client/generator/Makefile
+++ b/packages/subiquity_client/generator/Makefile
@@ -2,7 +2,7 @@ types_py = ../subiquity/subiquity/common/types.py
 types_dart = ../lib/src/types.dart
 
 generate:
-	python3 generator.py $(types_py) $(types_dart)
+	python3 generator.py $(types_py) --output $(types_dart)
 	dart format $(types_dart)
 
 check:
diff --git a/packages/subiquity_client/generator/generator.py b/packages/subiquity_client/generator/generator.py
index 9d725653a..cab74d9a7 100755
--- a/packages/subiquity_client/generator/generator.py
+++ b/packages/subiquity_client/generator/generator.py
@@ -1,5 +1,6 @@
 #!/usr/bin/env python3
 
+import argparse
 import ast
 import fileinput
 import os
@@ -355,17 +356,43 @@ def __str__(self):
 }}
 """
 
+class PythonInputFile(argparse.FileType):
+    def __init__(self):
+        super().__init__("r", encoding="utf-8")
+
+    def __call__(self, path: str):
+        if path != "-" and not path.endswith(".py"):
+            raise argparse.ArgumentTypeError(f"{path} does not have the .py extension")
+
+        return super().__call__(path)
+
+
+def DartOutputFile(path: str | None) -> str | None:
+    if path is None or path == "-":
+        return None
+
+    if not path.endswith(".dart"):
+        raise argparse.ArgumentTypeError(f"{path} does not have a .dart extension")
+    return path
+
 
 def main():
-    input = sys.argv[1] if len(sys.argv) > 1 else ""
-    output = sys.argv[2] if len(sys.argv) == 3 else None
+    parser = argparse.ArgumentParser()
+
+    # FileType is supposed to work with default="-" but it does not when used
+    # with positional arguments.
+    parser.add_argument("python-input-files", nargs="+",
+                        type=PythonInputFile(), metavar="input.py")
+    parser.add_argument("--output", default="-", metavar="output.dart",
+                        type=DartOutputFile)
+
+    args = vars(parser.parse_args())
 
-    if not input.endswith(".py") or (output is not None and not output.endswith(".dart")):
-        print("usage: generator <input.py> (<output.dart>)")
-        return
+    input_files = args["python-input-files"]
+    output = args["output"]
 
     generator = Generator()
-    with open(sys.argv[1], "r") as file:
+    for file in input_files:
         generator.parse(file.read())
 
     data = generator.to_dart()