diff --git a/pkgs/servers/home-assistant/parse-requirements.py b/pkgs/servers/home-assistant/parse-requirements.py
index bf81ce2afbe..3efdc01c914 100755
--- a/pkgs/servers/home-assistant/parse-requirements.py
+++ b/pkgs/servers/home-assistant/parse-requirements.py
@@ -24,6 +24,7 @@ import sys
 import tarfile
 import tempfile
 from io import BytesIO
+from typing import Dict, Optional
 from urllib.request import urlopen
 
 COMPONENT_PREFIX = "homeassistant.components"
@@ -78,22 +79,23 @@ def get_reqs(components, component):
     return requirements
 
 
-# Store a JSON dump of Nixpkgs' python3Packages
-output = subprocess.check_output(
-    [
-        "nix-env",
-        "-f",
-        os.path.dirname(sys.argv[0]) + "/../../..",
-        "-qa",
-        "-A",
-        PKG_SET,
-        "--json",
-    ]
-)
-packages = json.loads(output)
+def dump_packages() -> Dict[str, Dict[str, str]]:
+    # Store a JSON dump of Nixpkgs' python3Packages
+    output = subprocess.check_output(
+        [
+            "nix-env",
+            "-f",
+            os.path.dirname(sys.argv[0]) + "/../../..",
+            "-qa",
+            "-A",
+            PKG_SET,
+            "--json",
+        ]
+    )
+    return json.loads(output)
 
 
-def name_to_attr_path(req):
+def name_to_attr_path(req: str, packages: Dict[str, Dict[str, str]]) -> Optional[str]:
     attr_paths = set()
     names = [req]
     # E.g. python-mpd2 is actually called python3.6-mpd2
@@ -124,41 +126,50 @@ def name_to_attr_path(req):
         return None
 
 
-version = get_version()
-print("Generating component-packages.nix for version {}".format(version))
-components = parse_components(version=version)
-build_inputs = {}
-for component in sorted(components.keys()):
-    attr_paths = []
-    missing_reqs = []
-    reqs = sorted(get_reqs(components, component))
-    for req in reqs:
-        # Some requirements are specified by url, e.g. https://example.org/foobar#xyz==1.0.0
-        # Therefore, if there's a "#" in the line, only take the part after it
-        req = req[req.find("#") + 1 :]
-        name = req.split("==")[0]
-        attr_path = name_to_attr_path(name)
-        if attr_path is not None:
-            # Add attribute path without "python3Packages." prefix
-            attr_paths.append(attr_path[len(PKG_SET + ".") :])
+def main() -> None:
+    packages = dump_packages()
+    version = get_version()
+    print("Generating component-packages.nix for version {}".format(version))
+    components = parse_components(version=version)
+    build_inputs = {}
+    for component in sorted(components.keys()):
+        attr_paths = []
+        missing_reqs = []
+        reqs = sorted(get_reqs(components, component))
+        for req in reqs:
+            # Some requirements are specified by url, e.g. https://example.org/foobar#xyz==1.0.0
+            # Therefore, if there's a "#" in the line, only take the part after it
+            req = req[req.find("#") + 1 :]
+            name = req.split("==")[0]
+            attr_path = name_to_attr_path(name, packages)
+            if attr_path is not None:
+                # Add attribute path without "python3Packages." prefix
+                attr_paths.append(attr_path[len(PKG_SET + ".") :])
+            else:
+                missing_reqs.append(name)
         else:
-            missing_reqs.append(name)
-    else:
-        build_inputs[component] = attr_paths
-    n_diff = len(reqs) > len(build_inputs[component])
-    if n_diff > 0:
-        print("Component {} is missing {} dependencies".format(component, n_diff))
-        print("missing requirements: {}".format(missing_reqs))
+            build_inputs[component] = (attr_paths, missing_reqs)
+        n_diff = len(reqs) > len(build_inputs[component])
+        if n_diff > 0:
+            print("Component {} is missing {} dependencies".format(component, n_diff))
+            print("missing requirements: {}".format(missing_reqs))
 
-with open(os.path.dirname(sys.argv[0]) + "/component-packages.nix", "w") as f:
-    f.write("# Generated by parse-requirements.py\n")
-    f.write("# Do not edit!\n\n")
-    f.write("{\n")
-    f.write(f'  version = "{version}";\n')
-    f.write("  components = {\n")
-    for component, attr_paths in build_inputs.items():
-        f.write('    "{component}" = ps: with ps; [ ')
-        f.write(" ".join(attr_paths))
-        f.write(" ];\n")
-    f.write("  };\n")
-    f.write("}\n")
+    with open(os.path.dirname(sys.argv[0]) + "/component-packages.nix", "w") as f:
+        f.write("# Generated by parse-requirements.py\n")
+        f.write("# Do not edit!\n\n")
+        f.write("{\n")
+        f.write(f'  version = "{version}";\n')
+        f.write("  components = {\n")
+        for component, deps in build_inputs.items():
+            available, missing = deps
+            f.write(f"    \"{component}\" = ps: with ps; [ ")
+            f.write(" ".join(available))
+            f.write("];")
+            if len(missing) > 0:
+                f.write(f" # missing inputs: {' '.join(missing)}")
+            f.write("\n")
+        f.write("  };\n")
+        f.write("}\n")
+
+if __name__ == '__main__':
+    main()