diff --git a/aliBuild b/aliBuild
index 0994d6b6..030581d4 100755
--- a/aliBuild
+++ b/aliBuild
@@ -28,7 +28,7 @@ def doMain(args, parser):
   # do not get fooled when parsing localized messages.
   # We set ALIBUILD_ARCHITECTURE so that it's picked up by the external
   # command which does the reporting.
-  if not "architecture" in args:
+  if "architecture" not in args:
     args.architecture = detectArch()
   ENVIRONMENT_OVERRIDES = {
     "LANG": "C",
@@ -57,7 +57,7 @@ def doMain(args, parser):
       error(e.message)
       exit(1)
 
-  if args.action == "version" or args.action == None:
+  if args.action == "version" or args.action is None:
     print("aliBuild version: {version} ({arch})".format(
       version=__version__ or "unknown", arch=args.architecture or "unknown"))
     sys.exit(0)
@@ -111,13 +111,14 @@ if __name__ == "__main__":
   else:
     os.environ["ALIBUILD_ANALYTICS_USER_UUID"] = open(expanduser("~/.config/alibuild/analytics-uuid")).read().strip()
   try:
-    profiler = "--profile" in sys.argv
-    if profiler:
+    useProfiler = "--profile" in sys.argv
+    if useProfiler:
       print("profiler started")
       import cProfile, pstats
       from io import StringIO
       pr = cProfile.Profile()
       pr.enable()
+
       def profiler():
         pr.disable()
         print("profiler stopped")
diff --git a/alibuild_helpers/__init__.py b/alibuild_helpers/__init__.py
index 0692cfb5..150d430e 100644
--- a/alibuild_helpers/__init__.py
+++ b/alibuild_helpers/__init__.py
@@ -1,6 +1,9 @@
 # This file is needed to package build_template.sh.
 
 # Single-source a PEP440-compliant version using setuptools_scm.
+
+from typing import Optional
+__version__: Optional[str]
 try:
     # This is an sdist or wheel, and it's properly installed.
     from alibuild_helpers._version import version as __version__
diff --git a/alibuild_helpers/analytics.py b/alibuild_helpers/analytics.py
index 2ecd83e3..cf0656cf 100644
--- a/alibuild_helpers/analytics.py
+++ b/alibuild_helpers/analytics.py
@@ -104,7 +104,7 @@ def report_exception(e):
     exd = e.__class__.__name__,
     exf = "1")
 
-def enable_analytics():
+def enable_analytics() -> None:
   if exists(expanduser("~/.config/alibuild/disable-analytics")):
     unlink(expanduser("~/.config/alibuild/disable-analytics"))
   if not exists(expanduser("~/.config/alibuild/analytics-uuid")):
diff --git a/alibuild_helpers/build.py b/alibuild_helpers/build.py
index 08d4f547..72ec011c 100644
--- a/alibuild_helpers/build.py
+++ b/alibuild_helpers/build.py
@@ -35,7 +35,7 @@
 import time
 
 
-def writeAll(fn, txt):
+def writeAll(fn: str, txt: str) -> None:
   f = open(fn, "w")
   f.write(txt)
   f.close()
@@ -983,7 +983,7 @@ def doBuild(args, parser):
       fp.close()
     except:
       from pkg_resources import resource_string
-      cmd_raw = resource_string("alibuild_helpers", 'build_template.sh')
+      cmd_raw = resource_string("alibuild_helpers", 'build_template.sh').decode()
 
     if args.docker:
       cachedTarball = re.sub("^" + workDir, "/sw", spec["cachedTarball"])
@@ -1077,7 +1077,7 @@ def doBuild(args, parser):
        args.develPrefix if "develPrefix" in args and spec["is_devel_pkg"] else spec["version"])
     )
     err = execute(build_command, printer=progress)
-    progress.end("failed" if err else "done", err)
+    progress.end("failed" if err else "done", bool(err))
     report_event("BuildError" if err else "BuildSuccess", spec["package"], " ".join((
       args.architecture,
       spec["version"],
diff --git a/alibuild_helpers/cmd.py b/alibuild_helpers/cmd.py
index a0ea2db5..c0c8d8b7 100644
--- a/alibuild_helpers/cmd.py
+++ b/alibuild_helpers/cmd.py
@@ -5,10 +5,11 @@
 from textwrap import dedent
 from subprocess import TimeoutExpired
 from shlex import quote
+from typing import Union, Tuple, List
 
 from alibuild_helpers.log import debug, warning, dieOnError
 
-def decode_with_fallback(data):
+def decode_with_fallback(data : Union[bytes, str]) -> str:
   """Try to decode DATA as utf-8; if that doesn't work, fall back to latin-1.
 
   This combination should cover every possible byte string, as latin-1 covers
@@ -23,7 +24,7 @@ def decode_with_fallback(data):
     return str(data)
 
 
-def getoutput(command, timeout=None):
+def getoutput(command:str, timeout: Union[int, None] = None) -> str:
   """Run command, check it succeeded, and return its stdout as a string."""
   proc = Popen(command, shell=isinstance(command, str), stdout=PIPE, stderr=PIPE)
   try:
@@ -37,16 +38,16 @@ def getoutput(command, timeout=None):
   return decode_with_fallback(stdout)
 
 
-def getstatusoutput(command, timeout=None):
+def getstatusoutput(command:str, timeout: Union[int, None] = None) -> Tuple[int, str]:
   """Run command and return its return code and output (stdout and stderr)."""
   proc = Popen(command, shell=isinstance(command, str), stdout=PIPE, stderr=STDOUT)
   try:
-    merged_output, _ = proc.communicate(timeout=timeout)
+    merged_output_bytes, _ = proc.communicate(timeout=timeout)
   except TimeoutExpired:
     warning("Process %r timed out; terminated", command)
     proc.terminate()
-    merged_output, _ = proc.communicate()
-  merged_output = decode_with_fallback(merged_output)
+    merged_output_bytes, _ = proc.communicate()
+  merged_output = decode_with_fallback(merged_output_bytes)
   # Strip a single trailing newline, if one exists, to match the behaviour of
   # subprocess.getstatusoutput.
   if merged_output.endswith("\n"):
@@ -54,9 +55,10 @@ def getstatusoutput(command, timeout=None):
   return proc.returncode, merged_output
 
 
-def execute(command, printer=debug, timeout=None):
+def execute(command: Union[str, List[str]], printer=debug, timeout:Union[int, None] =None) -> int:
   popen = Popen(command, shell=isinstance(command, str), stdout=PIPE, stderr=STDOUT)
   start_time = time.time()
+  assert popen.stdout is not None, "Could not open stdout for command"
   for line in iter(popen.stdout.readline, b""):
     printer("%s", decode_with_fallback(line).strip("\n"))
     if timeout is not None and time.time() > start_time + timeout:
diff --git a/alibuild_helpers/doctor.py b/alibuild_helpers/doctor.py
index 8c0fa42c..8fa29030 100644
--- a/alibuild_helpers/doctor.py
+++ b/alibuild_helpers/doctor.py
@@ -7,7 +7,7 @@
 from alibuild_helpers.utilities import getPackageList, parseDefaults, readDefaults, validateDefaults
 from alibuild_helpers.cmd import getstatusoutput, DockerRunner
 
-def prunePaths(workDir):
+def prunePaths(workDir: str) -> None:
   for x in ["PATH", "LD_LIBRARY_PATH", "DYLD_LIBRARY_PATH"]:
     if not x in os.environ:
       continue
@@ -52,7 +52,7 @@ def checkRequirements(spec, cmd, homebrew_replacement, getstatusoutput_docker):
           spec.get("system_requirement_missing"))
     return (err, "")
 
-def systemInfo():
+def systemInfo() -> None:
   _,out = getstatusoutput("env")
   debug("Environment:\n%s", out)
   _,out = getstatusoutput("uname -a")
diff --git a/alibuild_helpers/init.py b/alibuild_helpers/init.py
index d9d3b04e..5016baf4 100644
--- a/alibuild_helpers/init.py
+++ b/alibuild_helpers/init.py
@@ -2,13 +2,13 @@
 from alibuild_helpers.utilities import getPackageList, parseDefaults, readDefaults, validateDefaults
 from alibuild_helpers.log import debug, error, warning, banner, info
 from alibuild_helpers.log import dieOnError
-from alibuild_helpers.workarea import cleanup_git_log, updateReferenceRepoSpec
+from alibuild_helpers.workarea import updateReferenceRepoSpec
 
 from os.path import join
 import os.path as path
 import os, sys
 
-def parsePackagesDefinition(pkgname):
+def parsePackagesDefinition(pkgname: str):
   return [ dict(zip(["name","ver"], y.split("@")[0:2]))
            for y in [ x+"@" for x in list(filter(lambda y: y, pkgname.split(","))) ] ]
 
@@ -23,8 +23,10 @@ def doInit(args):
          "--dry-run / -n specified. Doing nothing.", ",".join(x["name"] for x in pkgs))
     sys.exit(0)
   try:
-    path.exists(args.develPrefix) or os.mkdir(args.develPrefix)
-    path.exists(args.referenceSources) or os.makedirs(args.referenceSources)
+    if not path.exists(args.develPrefix):
+      os.mkdir(args.develPrefix)
+    if not path.exists(args.referenceSources):
+      os.makedirs(args.referenceSources)
   except OSError as e:
     error("%s", e)
     sys.exit(1)
@@ -66,9 +68,10 @@ def doInit(args):
 
   for p in pkgs:
     spec = specs.get(p["name"])
+    dieOnError(spec is None, "cannot find recipe for package %s" % p["name"])
+    assert spec
     spec["is_devel_pkg"] = False
     spec["scm"] = Git()
-    dieOnError(spec is None, "cannot find recipe for package %s" % p["name"])
     dest = join(args.develPrefix, spec["package"])
     writeRepo = spec.get("write_repo", spec.get("source"))
     dieOnError(not writeRepo, "package %s has no source field and cannot be developed" % spec["package"])
diff --git a/alibuild_helpers/log.py b/alibuild_helpers/log.py
index 690d2b64..9b8824c9 100644
--- a/alibuild_helpers/log.py
+++ b/alibuild_helpers/log.py
@@ -3,10 +3,9 @@
 import re
 import time
 import datetime
+from typing import Any
 
-debug, error, warning, info, success = (None, None, None, None, None)
-
-def dieOnError(err, msg):
+def dieOnError(err: Any, msg: str) -> None:
   if err:
     error("%s", msg)
     sys.exit(1)
@@ -60,7 +59,7 @@ def __init__(self, begin_msg=""):
     self.lasttime = 0
     self.STAGES = ".", "..", "...", "....", ".....", "....", "...", ".."
     self.begin_msg = begin_msg
-    self.percent = -1
+    self.percent: float = -1
 
   def __call__(self, txt, *args):
     if logger.level <= logging.DEBUG or not sys.stdout.isatty():
@@ -88,7 +87,7 @@ def __call__(self, txt, *args):
     self.lasttime = time.time()
     sys.stderr.flush()
 
-  def erase(self):
+  def erase(self) -> None:
     nerase = len(self.STAGES[self.count]) if self.count > -1 else 0
     if self.percent > -1:
       nerase = nerase + 7
diff --git a/alibuild_helpers/sync.py b/alibuild_helpers/sync.py
index ae9dafd5..8f10a9e7 100644
--- a/alibuild_helpers/sync.py
+++ b/alibuild_helpers/sync.py
@@ -12,30 +12,15 @@
 from alibuild_helpers.cmd import execute
 from alibuild_helpers.log import debug, info, error, dieOnError, ProgressPrint
 from alibuild_helpers.utilities import resolve_store_path, resolve_links_path, symlink
-
-
-def remote_from_url(read_url, write_url, architecture, work_dir, insecure=False):
-  """Parse remote store URLs and return the correct RemoteSync instance for them."""
-  if read_url.startswith("http"):
-    return HttpRemoteSync(read_url, architecture, work_dir, insecure)
-  if read_url.startswith("s3://"):
-    return S3RemoteSync(read_url, write_url, architecture, work_dir)
-  if read_url.startswith("b3://"):
-    return Boto3RemoteSync(read_url, write_url, architecture, work_dir)
-  if read_url.startswith("cvmfs://"):
-    return CVMFSRemoteSync(read_url, None, architecture, work_dir)
-  if read_url:
-    return RsyncRemoteSync(read_url, write_url, architecture, work_dir)
-  return NoRemoteSync()
-
+from typing import Union
 
 class NoRemoteSync:
   """Helper class which does not do anything to sync"""
-  def fetch_symlinks(self, spec):
+  def fetch_symlinks(self, spec) -> None:
     pass
-  def fetch_tarball(self, spec):
+  def fetch_tarball(self, spec) -> None:
     pass
-  def upload_symlinks_and_tarball(self, spec):
+  def upload_symlinks_and_tarball(self, spec) -> None:
     pass
 
 class PartialDownloadError(Exception):
@@ -47,7 +32,7 @@ def __str__(self):
 
 
 class HttpRemoteSync:
-  def __init__(self, remoteStore, architecture, workdir, insecure):
+  def __init__(self, remoteStore: str, architecture: str, workdir: str, insecure: bool):
     self.remoteStore = remoteStore
     self.writeStore = ""
     self.architecture = architecture
@@ -486,14 +471,14 @@ class Boto3RemoteSync:
   time.
   """
 
-  def __init__(self, remoteStore, writeStore, architecture, workdir):
+  def __init__(self, remoteStore, writeStore, architecture, workdir) -> None:
     self.remoteStore = re.sub("^b3://", "", remoteStore)
     self.writeStore = re.sub("^b3://", "", writeStore)
     self.architecture = architecture
     self.workdir = workdir
     self._s3_init()
 
-  def _s3_init(self):
+  def _s3_init(self) -> None:
     # This is a separate method so that we can patch it out for unit tests.
     # Import boto3 here, so that if we don't use this remote store, we don't
     # have to install it in the first place.
@@ -530,7 +515,7 @@ def _s3_key_exists(self, key):
       raise
     return True
 
-  def fetch_tarball(self, spec):
+  def fetch_tarball(self, spec) -> None:
     debug("Updating remote store for package %s with hashes %s", spec["package"],
           ", ".join(spec["remote_hashes"]))
 
@@ -568,7 +553,7 @@ def fetch_tarball(self, spec):
     debug("Remote has no tarballs for %s with hashes %s", spec["package"],
           ", ".join(spec["remote_hashes"]))
 
-  def fetch_symlinks(self, spec):
+  def fetch_symlinks(self, spec) -> None:
     from botocore.exceptions import ClientError
     links_path = resolve_links_path(self.architecture, spec["package"])
     os.makedirs(os.path.join(self.workdir, links_path), exist_ok=True)
@@ -691,3 +676,21 @@ def upload_symlinks_and_tarball(self, spec):
 
     self.s3.upload_file(Bucket=self.writeStore, Key=tar_path,
                         Filename=os.path.join(self.workdir, tar_path))
+
+RemoteSync = Union[HttpRemoteSync, S3RemoteSync, Boto3RemoteSync, CVMFSRemoteSync, RsyncRemoteSync, NoRemoteSync]
+
+def remote_from_url(read_url: str, write_url: str, architecture: str, work_dir: str, insecure=False) -> RemoteSync:
+  """Parse remote store URLs and return the correct RemoteSync instance for them."""
+  if read_url.startswith("http"):
+    return HttpRemoteSync(read_url, architecture, work_dir, insecure)
+  if read_url.startswith("s3://"):
+    return S3RemoteSync(read_url, write_url, architecture, work_dir)
+  if read_url.startswith("b3://"):
+    return Boto3RemoteSync(read_url, write_url, architecture, work_dir)
+  if read_url.startswith("cvmfs://"):
+    return CVMFSRemoteSync(read_url, None, architecture, work_dir)
+  if read_url:
+    return RsyncRemoteSync(read_url, write_url, architecture, work_dir)
+  return NoRemoteSync()
+
+
diff --git a/alibuild_helpers/templating_plugin.py b/alibuild_helpers/templating_plugin.py
index 7c1c8847..94e6767d 100644
--- a/alibuild_helpers/templating_plugin.py
+++ b/alibuild_helpers/templating_plugin.py
@@ -11,7 +11,7 @@
 from jinja2.sandbox import SandboxedEnvironment
 
 
-def build_plugin(specs, args, build_order):
+def build_plugin(specs, args, build_order) -> None:
     """Read a user-provided template from stdin and render it."""
     print(SandboxedEnvironment(autoescape=False)
           .from_string(sys.stdin.read())
diff --git a/alibuild_helpers/utilities.py b/alibuild_helpers/utilities.py
index a78c175e..88ca6077 100644
--- a/alibuild_helpers/utilities.py
+++ b/alibuild_helpers/utilities.py
@@ -567,7 +567,7 @@ def getPackageList(packages, specs, configDir, preferSystem, noSystem,
 
 
 class Hasher:
-  def __init__(self):
+  def __init__(self) -> None:
     self.h = hashlib.sha1()
   def __call__(self, txt):
     if not type(txt) == bytes:
diff --git a/mypy.ini b/mypy.ini
new file mode 100644
index 00000000..3fa28b7e
--- /dev/null
+++ b/mypy.ini
@@ -0,0 +1,30 @@
+[mypy]
+python_version = 3.12
+platform = linux
+plugins = pydantic.mypy
+show_error_codes = true
+follow_imports = normal
+local_partial_types = true
+strict_equality = true
+no_implicit_optional = true
+warn_incomplete_stub = true
+warn_redundant_casts = true
+warn_unused_configs = true
+warn_unused_ignores = true
+enable_error_code = ignore-without-code, redundant-self, truthy-iterable
+disable_error_code = annotation-unchecked, import-not-found, import-untyped
+extra_checks = false
+check_untyped_defs = true
+disallow_incomplete_defs = true
+disallow_subclassing_any = true
+disallow_untyped_calls = true
+disallow_untyped_decorators = true
+disallow_untyped_defs = true
+warn_return_any = true
+warn_unreachable = true
+
+[pydantic-mypy]
+init_forbid_extra = true
+init_typed = true
+warn_required_dynamic_aliases = true
+warn_untyped_fields = true
diff --git a/tests/test_analytics.py b/tests/test_analytics.py
index 2e5d9f3b..b6f11543 100644
--- a/tests/test_analytics.py
+++ b/tests/test_analytics.py
@@ -7,11 +7,11 @@ def noAnalytics():
 def yesAnalytics():
     return True
 
-def notInvoked():
+def notInvoked() -> None:
     assert(False)
 
 class TestAnalytics(unittest.TestCase):
-    def test_analytics(self):
+    def test_analytics(self) -> None:
         self.assertEqual(False,  decideAnalytics(hasDisableFile=False,
                                                  hasUuid=False,
                                                  isTty=False,
diff --git a/tests/test_args.py b/tests/test_args.py
index c70dd762..7791dd19 100644
--- a/tests/test_args.py
+++ b/tests/test_args.py
@@ -21,7 +21,7 @@
   "build --force-unknown-architecture": [call(BUILD_MISSING_PKG_ERROR)],
   "build --force-unknown-architecture zlib --foo": [call('unrecognized arguments: --foo')],
   "init --docker-image": [call('unrecognized arguments: --docker-image')],
-  "builda --force-unknown-architecture zlib" : [call("argument action: invalid choice: 'builda' (choose from 'analytics', 'architecture', 'build', 'clean', 'deps', 'doctor', 'init', 'version')")],
+  "builda --force-unknown-architecture zlib" : [call("argument action: invalid choice: 'builda' (choose from analytics, architecture, build, clean, deps, doctor, init, version)")],
   "build --force-unknown-architecture zlib --no-system --always-prefer-system" : [call('argument --always-prefer-system: not allowed with argument --no-system')],
   "build zlib --architecture foo": ARCHITECTURE_ERROR,
   "build --force-unknown-architecture zlib --remote-store rsync://test1.local/::rw --write-store rsync://test2.local/::rw ": [call('cannot specify ::rw and --write-store at the same time')],
@@ -95,15 +95,20 @@ def test_actionParsing(self, mock_commands):
 
   @mock.patch("alibuild_helpers.utilities.getoutput", new=lambda cmd: "x86_64")   # for uname -m
   @mock.patch('alibuild_helpers.args.argparse.ArgumentParser.error')
-  def test_failingParsing(self, mock_print):
+  def test_failingParsing(self, mock_print) -> None:
     mock_print.side_effect = FakeExit("raised")
     for (cmd, calls) in PARSER_ERRORS.items():
       mock_print.mock_calls = []
       with patch.object(sys, "argv", ["alibuild"] + shlex.split(cmd)):
         self.assertRaises(FakeExit, doParseArgs)
+        if mock_print.mock_calls != calls:
+          import json
+          print('Failed test:', cmd)
+          print("Expected calls: ", json.dumps(calls, indent=2, default=str))
+          print("Actual calls: ", json.dumps(mock_print.mock_calls, indent=2, default=str))
         self.assertEqual(mock_print.mock_calls, calls)
 
-  def test_validArchitectures(self):
+  def test_validArchitectures(self) -> None:
     for arch in VALID_ARCHS:
       self.assertTrue(matchValidArch(arch))
     for arch in INVALID_ARCHS:
diff --git a/tests/test_build.py b/tests/test_build.py
index a0f4ed72..ee42506f 100644
--- a/tests/test_build.py
+++ b/tests/test_build.py
@@ -9,6 +9,7 @@
 from unittest.mock import call, patch, MagicMock, DEFAULT
 from io import StringIO
 from collections import OrderedDict
+from typing import Dict
 
 from alibuild_helpers.utilities import parseRecipe, resolve_tag
 from alibuild_helpers.build import doBuild, storeHashes, generate_initdotsh
@@ -130,7 +131,7 @@ def dummy_git(args, directory=".", check=True, prompt=True):
     }[(tuple(args), directory, check)]
 
 
-TIMES_ASKED = {}
+TIMES_ASKED : Dict[str, int] = {}
 
 
 def dummy_open(x, mode="r", encoding=None, errors=None):
@@ -240,7 +241,7 @@ class BuildTestCase(unittest.TestCase):
     @patch("alibuild_helpers.workarea.is_writeable", new=MagicMock(return_value=True))
     @patch("alibuild_helpers.build.basename", new=MagicMock(return_value="aliBuild"))
     @patch("alibuild_helpers.build.install_wrapper_script", new=MagicMock())
-    def test_coverDoBuild(self, mock_debug, mock_listdir, mock_warning, mock_sys, mock_git_git):
+    def test_coverDoBuild(self, mock_debug, mock_listdir, mock_warning, mock_sys, mock_git_git) -> None:
         mock_git_git.side_effect = dummy_git
         mock_debug.side_effect = lambda *args: None
         mock_warning.side_effect = lambda *args: None
@@ -339,7 +340,7 @@ def setup_spec(self, script):
         spec["tag"] = resolve_tag(spec)
         return spec
 
-    def test_hashing(self):
+    def test_hashing(self) -> None:
         """Check that the hashes assigned to packages remain constant."""
         default = self.setup_spec(TEST_DEFAULT_RELEASE)
         zlib = self.setup_spec(TEST_ZLIB_RECIPE)
@@ -387,7 +388,7 @@ def test_hashing(self):
         self.assertEqual(len(extra["remote_hashes"]), 3)
         self.assertEqual(extra["local_hashes"][0], TEST_EXTRA_BUILD_HASH)
 
-    def test_initdotsh(self):
+    def test_initdotsh(self) -> None:
         """Sanity-check the generated init.sh for a few variables."""
         specs = {
             # Add some attributes that are normally set by doBuild(), but
diff --git a/tests/test_git.py b/tests/test_git.py
index f6af40be..3af937ee 100644
--- a/tests/test_git.py
+++ b/tests/test_git.py
@@ -15,33 +15,33 @@
 class GitWrapperTestCase(unittest.TestCase):
     """Make sure the git() wrapper function is working."""
 
-    def setUp(self):
+    def setUp(self) -> None:
         # Disable reading all git configuration files, including the user's, in
         # case they have access to PRIVATE_REPO.
         self._prev_git_config_global = os.environ.get("GIT_CONFIG_GLOBAL")
         os.environ["GIT_CONFIG_GLOBAL"] = os.devnull
 
-    def tearDown(self):
+    def tearDown(self) -> None:
         # Restore the original value of GIT_CONFIG_GLOBAL, if any.
         if self._prev_git_config_global is None:
             del os.environ["GIT_CONFIG_GLOBAL"]
         else:
             os.environ["GIT_CONFIG_GLOBAL"] = self._prev_git_config_global
 
-    def test_git_existing_repo(self):
+    def test_git_existing_repo(self) -> None:
         """Check git can read an existing repo."""
         err, out = git(("ls-remote", "-ht", EXISTING_REPO),
                        check=False, prompt=False)
         self.assertEqual(err, 0, "git output:\n" + out)
         self.assertTrue(out, "expected non-empty output from git")
 
-    def test_git_missing_repo(self):
+    def test_git_missing_repo(self) -> None:
         """Check we get the right exception when a repo doesn't exist."""
         self.assertRaises(SCMError, git, (
             "ls-remote", "-ht", MISSING_REPO,
         ), prompt=False)
 
-    def test_git_private_repo(self):
+    def test_git_private_repo(self) -> None:
         """Check we get the right exception when credentials are required."""
         self.assertRaises(SCMError, git, (
             "-c", "credential.helper=", "ls-remote", "-ht", PRIVATE_REPO,
diff --git a/tests/test_hashing.py b/tests/test_hashing.py
index e0e09573..56283c4c 100644
--- a/tests/test_hashing.py
+++ b/tests/test_hashing.py
@@ -28,7 +28,7 @@ class KnownGoodHashesTestCase(unittest.TestCase):
 
     @unittest.skipIf(not os.path.exists(LOGFILE),
                      "Need a reference build log at path " + LOGFILE)
-    def test_hashes_match_build_log(self):
+    def test_hashes_match_build_log(self) -> None:
         checked = set()
         specs = {}
         with codecs.open(LOGFILE, encoding="utf-8") as logf:
diff --git a/tests/test_init.py b/tests/test_init.py
index 97b968c4..42e477d2 100644
--- a/tests/test_init.py
+++ b/tests/test_init.py
@@ -25,7 +25,7 @@ def dummy_exists(x):
 
 
 class InitTestCase(unittest.TestCase):
-    def test_packageDefinition(self):
+    def test_packageDefinition(self) -> None:
       self.assertEqual(parsePackagesDefinition("AliRoot@v5-08-16,AliPhysics@v5-08-16-01"),
                        [{'ver': 'v5-08-16', 'name': 'AliRoot'},
                         {'ver': 'v5-08-16-01', 'name': 'AliPhysics'}])
@@ -36,7 +36,7 @@ def test_packageDefinition(self):
     @patch("alibuild_helpers.init.info")
     @patch("alibuild_helpers.init.path")
     @patch("alibuild_helpers.init.os")
-    def test_doDryRunInit(self, mock_os, mock_path,  mock_info):
+    def test_doDryRunInit(self, mock_os, mock_path,  mock_info) -> None:
       fake_dist = {"repo": "alisw/alidist", "ver": "master"}
       args = Namespace(
         develPrefix = ".",
@@ -60,7 +60,7 @@ def test_doDryRunInit(self, mock_os, mock_path,  mock_info):
     @patch("alibuild_helpers.init.updateReferenceRepoSpec")
     @patch("alibuild_helpers.utilities.open")
     @patch("alibuild_helpers.init.readDefaults")
-    def test_doRealInit(self, mock_read_defaults, mock_open, mock_update_reference, mock_git, mock_os, mock_path,  mock_info, mock_banner):
+    def test_doRealInit(self, mock_read_defaults, mock_open, mock_update_reference, mock_git, mock_os, mock_path,  mock_info, mock_banner) -> None:
       fake_dist = {"repo": "alisw/alidist", "ver": "master"}
       mock_open.side_effect = lambda x: {
         "/alidist/defaults-release.sh": StringIO("package: defaults-release\nversion: v1\n---"),
diff --git a/tests/test_log.py b/tests/test_log.py
index 06553f1b..3831bf80 100644
--- a/tests/test_log.py
+++ b/tests/test_log.py
@@ -20,7 +20,7 @@ def test_dieOnError(self, mock_sys, mock_error):
 
     @patch("sys.stdout.isatty", new=MagicMock(return_value=True))
     @patch("sys.stderr", new=MagicMock(return_value=True))
-    def test_ProgressPrint(self):
+    def test_ProgressPrint(self) -> None:
         """Make sure ProgressPrint updates correctly."""
         # ProgressPrint only parses messages every 0.5s. Trick it into thinking
         # the last message was <interval> seconds ago.
diff --git a/tests/test_packagelist.py b/tests/test_packagelist.py
index 4f9cd93f..4813446c 100644
--- a/tests/test_packagelist.py
+++ b/tests/test_packagelist.py
@@ -71,7 +71,7 @@
 
 
 class MockReader:
-    def __init__(self, url, dist=None):
+    def __init__(self, url, dist=None) -> None:
         self._contents = RECIPES[url]
         self.url = "mock://" + url
 
@@ -109,7 +109,7 @@ def getPackageListWithDefaults(packages, force_rebuild=()):
 class ReplacementTestCase(unittest.TestCase):
     """Test that system package replacements are working."""
 
-    def test_disable(self):
+    def test_disable(self) -> None:
         """Check that not specifying any replacement disables the package.
 
         This is was the only available behaviour in previous aliBuild versions
@@ -121,7 +121,7 @@ def test_disable(self):
         self.assertNotIn("disable", ownPkgs)
         self.assertNotIn("disable", specs)
 
-    def test_replacement_given(self):
+    def test_replacement_given(self) -> None:
         """Check that specifying a replacement spec means it is used.
 
         This also checks that if no recipe is given, we report the package as
@@ -138,7 +138,7 @@ def test_replacement_given(self):
         self.assertIn("with-replacement", systemPkgs)
         self.assertNotIn("with-replacement", ownPkgs)
 
-    def test_replacement_recipe_given(self):
+    def test_replacement_recipe_given(self) -> None:
         """Check that specifying a replacement recipe means it is used.
 
         Also check that we report to the user that a package will be compiled
@@ -174,13 +174,13 @@ def side_effect(msg, *args, **kwargs):
 class ForceRebuildTestCase(unittest.TestCase):
     """Test that force_rebuild keys are applied properly."""
 
-    def test_force_rebuild_recipe(self):
+    def test_force_rebuild_recipe(self) -> None:
         """If the recipe specifies force_rebuild, it must be applied."""
         specs, _, _, _, _ = getPackageListWithDefaults(["force-rebuild"])
         self.assertTrue(specs["force-rebuild"]["force_rebuild"])
         self.assertFalse(specs["defaults-release"]["force_rebuild"])
 
-    def test_force_rebuild_command_line(self):
+    def test_force_rebuild_command_line(self) -> None:
         """The --force-rebuild option must take precedence, if given."""
         specs, _, _, _, _ = getPackageListWithDefaults(
             ["force-rebuild"], force_rebuild=["defaults-release", "force-rebuild"],
diff --git a/tests/test_parseRecipe.py b/tests/test_parseRecipe.py
index 82edbb0c..b6785737 100644
--- a/tests/test_parseRecipe.py
+++ b/tests/test_parseRecipe.py
@@ -51,7 +51,7 @@
     ^"""
 
 class Recoder(object):
-  def __init__(self):
+  def __init__(self) -> None:
     self.buffer = ""
   def __call__(self, s, *a):
     self.buffer += s % a
@@ -67,7 +67,7 @@ def __call__(self):
       return self.buffer
 
 class TestRecipes(unittest.TestCase):
-  def test_recipes(self):
+  def test_recipes(self) -> None:
     err, meta, body = parseRecipe(BufferReader("test1.sh", TEST1))
     self.assertEqual(err, None)
     self.assertEqual(meta["package"], "foo")
@@ -88,7 +88,7 @@ def test_recipes(self):
     err, meta, body = parseRecipe(BufferReader("test_broken_6.sh", TEST_BROKEN_6))
     self.assertEqual(err.strip(), ERROR_MSG_6.strip())
 
-  def test_getRecipeReader(self):
+  def test_getRecipeReader(self) -> None:
     f = getRecipeReader("foo")
     self.assertEqual(type(f), FileReader)
     f = getRecipeReader("dist:foo@master")
@@ -107,7 +107,7 @@ def test_parseDefaults(self):
     self.assertEqual(overrides, {'defaults-release': {}, 'root': {'requires': 'GCC'}})
     self.assertEqual(taps, {'root': 'dist:ROOT@master'})
 
-  def test_validateDefault(self):
+  def test_validateDefault(self) -> None:
     ok, out, validDefaults = validateDefaults({"something": True}, "release")
     self.assertEqual(ok, True)
     ok, out, validDefaults = validateDefaults({"package": "foo","valid_defaults": ["o2", "o2-dataflow"]}, "release")
diff --git a/tests/test_sync.py b/tests/test_sync.py
index 76103742..c55f3bf3 100644
--- a/tests/test_sync.py
+++ b/tests/test_sync.py
@@ -44,7 +44,7 @@ def tarball_name(spec):
 
 
 class MockRequest:
-    def __init__(self, j, simulate_err=False):
+    def __init__(self, j, simulate_err=False) -> None:
         self.j = j
         self.simulate_err = simulate_err
         self.status_code = 200 if j else 404
@@ -233,7 +233,7 @@ def head_object(Bucket, Key):
                 raise ClientError({"Error": {"Code": "404"}}, "head_object")
             return {}
 
-        def download_file(Bucket, Key, Filename, Callback=None):
+        def download_file(Bucket, Key, Filename, Callback=None) -> None:
             self.assertNotIn(NONEXISTENT_HASH, Key, "tried to fetch missing tarball")
             self.assertNotIn(BAD_HASH, Key, "tried to follow bad symlink")
 
@@ -266,7 +266,7 @@ def get_paginator(method):
     @patch("os.path.isfile", new=MagicMock(return_value=False))
     @patch("os.path.islink", new=MagicMock(return_value=False))
     @patch("alibuild_helpers.sync.execute", new=MagicMock(return_value=0))
-    def test_tarball_download(self):
+    def test_tarball_download(self) -> None:
         """Test boto3 behaviour when downloading tarballs from the remote."""
         b3sync = sync.Boto3RemoteSync(
             remoteStore="b3://localhost", writeStore="b3://localhost",
@@ -296,7 +296,7 @@ def test_tarball_download(self):
     ))
     @patch("os.readlink", new=MagicMock(return_value="dummy path"))
     @patch("os.path.islink", new=MagicMock(return_value=True))
-    def test_tarball_upload(self):
+    def test_tarball_upload(self) -> None:
         """Test boto3 behaviour when building packages for upload locally."""
         b3sync = sync.Boto3RemoteSync(
             remoteStore="b3://localhost", writeStore="b3://localhost",
diff --git a/tests/test_utilities.py b/tests/test_utilities.py
index 3fc2beef..dd1497ac 100644
--- a/tests/test_utilities.py
+++ b/tests/test_utilities.py
@@ -136,18 +136,18 @@
 ]
 
 class TestUtilities(unittest.TestCase):
-  def test_osx(self):
+  def test_osx(self) -> None:
     for payload in architecturePayloads:
       result, hasOsRelease, osReleaseLines, platformTuple, platformSystem, platformProcessor = payload
       self.assertEqual(result, doDetectArch(hasOsRelease, osReleaseLines, platformTuple, platformSystem, platformProcessor))
   # Test by mocking platform.processor
-  def test_osx_mock(self):
+  def test_osx_mock(self) -> None:
     for payload in macOSArchitecturePayloads:
       result, hasOsRelease, osReleaseLines, platformTuple, platformSystem, platformProcessor = payload
       with patch('platform.machine', return_value=platformProcessor):
         platformProcessor = None
         self.assertEqual(result, doDetectArch(hasOsRelease, osReleaseLines, platformTuple, platformSystem, None))
-  def test_Hasher(self):
+  def test_Hasher(self) -> None:
     h = Hasher()
     h("foo")
     self.assertEqual("0beec7b5ea3f0fdbc95d0dd47f3c5bc275da8a33", h.hexdigest())
@@ -157,7 +157,7 @@ def test_Hasher(self):
     h("bar")
     self.assertEqual("8843d7f92416211de9ebb963ff4ce28125932878", h.hexdigest())
 
-  def test_UTF8_Hasher(self):
+  def test_UTF8_Hasher(self) -> None:
     h1 = Hasher()
     h2 = Hasher()
     h3 = Hasher()
@@ -169,12 +169,12 @@ def test_UTF8_Hasher(self):
     self.assertEqual(h3.hexdigest(), "0beec7b5ea3f0fdbc95d0dd47f3c5bc275da8a33")
     self.assertNotEqual(h1.hexdigest(), h2.hexdigest())
 
-  def test_asList(self):
+  def test_asList(self) -> None:
     self.assertEqual(asList("a"), ["a"])
     self.assertEqual(asList(["a"]), ["a"])
     self.assertEqual(asList(None), [None])
 
-  def test_filterByArchitecture(self):
+  def test_filterByArchitecture(self) -> None:
     self.assertEqual(["AliRoot"], list(filterByArchitecture("osx_x86-64", ["AliRoot"])))
     self.assertEqual([], list(filterByArchitecture("osx_x86-64", ["AliRoot:(?!osx)"])))
     self.assertEqual(["GCC"], list(filterByArchitecture("osx_x86-64", ["AliRoot:(?!osx)", "GCC"])))
@@ -213,7 +213,7 @@ def test_prunePaths(self):
       self.assertTrue(fake_env_copy["DYLD_LIBRARY_PATH"] == "/sw/lib")
       self.assertTrue(fake_env_copy["ALIBUILD_VERSION"] == "v1.0.0")
 
-  def test_resolver(self):
+  def test_resolver(self) -> None:
     spec = {"package": "test-pkg",
       "version": "%(tag_basename)s",
       "tag": "foo/bar",
@@ -231,7 +231,7 @@ def test_resolver(self):
 class TestTopologicalSort(unittest.TestCase):
     """Check that various properties of topological sorting hold."""
 
-    def test_resolve_dependency_chain(self):
+    def test_resolve_dependency_chain(self) -> None:
         """Test that topological sorting correctly sorts packages in a dependency chain."""
         # Topological sorting only takes "requires" into account, since the
         # build/runtime distinction does not matter for resolving build order.
@@ -241,7 +241,7 @@ def test_resolve_dependency_chain(self):
             "c": {"package": "c", "requires": []},
         })))
 
-    def test_diamond_dependency(self):
+    def test_diamond_dependency(self) -> None:
         """Test that a diamond dependency relationship is handled correctly."""
         self.assertEqual(["base", "mid2", "mid1", "top"], list(topological_sort({
             "top": {"package": "top", "requires": ["mid1", "mid2"]},
@@ -251,7 +251,7 @@ def test_diamond_dependency(self):
             "base": {"package": "base", "requires": []},
         })))
 
-    def test_dont_drop_packages(self):
+    def test_dont_drop_packages(self) -> None:
         """Check that topological sorting doesn't drop any packages."""
         # For half the packages, depend on the first package, to make this a
         # little more than trivial.