diff --git a/src/cwhy/cwhy.py b/src/cwhy/cwhy.py
index aa35002..3457841 100755
--- a/src/cwhy/cwhy.py
+++ b/src/cwhy/cwhy.py
@@ -21,7 +21,7 @@
 from .print_debug import dprint, enable_debug_printing
 
 
-def print_key_info():
+def print_key_info() -> None:
     dprint("You need a key (or keys) from an AI service to use CWhy.")
     dprint()
     dprint("OpenAI:")
diff --git a/src/cwhy/print_debug.py b/src/cwhy/print_debug.py
index 58a557b..7056437 100644
--- a/src/cwhy/print_debug.py
+++ b/src/cwhy/print_debug.py
@@ -5,7 +5,10 @@
 _INFO_WIDTH = 30
 
 
-def dprint(*objects, sep=" ", end="\n", file=None, flush=False):
+def dprint(
+    *objects, sep: str = " ", end: str = "\n", file=None, flush: bool = False
+) -> None:
+    global _debug
     if not _debug:
         return print(*objects, sep=sep, end=end, file=file, flush=flush)
 
@@ -25,6 +28,6 @@ def dprint(*objects, sep=" ", end="\n", file=None, flush=False):
         print(info, line, file=file, flush=flush)
 
 
-def enable_debug_printing():
+def enable_debug_printing() -> None:
     global _debug
     _debug = True
diff --git a/tests/anonymizer.py b/tests/anonymizer.py
index 6467c95..9530714 100755
--- a/tests/anonymizer.py
+++ b/tests/anonymizer.py
@@ -2,9 +2,10 @@
 
 import argparse
 import tempfile
+from typing import Tuple
 
 
-def anonymize(path):
+def anonymize(path: str) -> Tuple[str, str]:
     with open(path) as file:
         content = file.read()
 
diff --git a/tests/apply_diff.py b/tests/apply_diff.py
index 5613463..4241a50 100755
--- a/tests/apply_diff.py
+++ b/tests/apply_diff.py
@@ -4,7 +4,7 @@
 import sys
 
 
-def apply(data):
+def apply(data) -> None:
     # Sort modifications by reverse start line number to apply them in that order.
     data["modifications"].sort(key=lambda m: m["start-line-number"], reverse=True)
 
diff --git a/tests/prepare.py b/tests/prepare.py
index 440c373..e0a5c75 100644
--- a/tests/prepare.py
+++ b/tests/prepare.py
@@ -7,7 +7,7 @@
 ROOT = os.path.dirname(os.path.abspath(__file__))
 
 
-def prepare(language):
+def prepare(language) -> None:
     try:
         with open(os.path.join(ROOT, language, "manifest.yml"), "r") as stream:
             manifest = yaml.load(stream, yaml.Loader)
@@ -26,11 +26,11 @@ def prepare(language):
             subprocess.run(command, shell=True).check_returncode()
 
 
-def prepare_all():
+def prepare_all() -> None:
     for directory in os.listdir(ROOT):
         if os.path.isdir(os.path.join(ROOT, directory)):
             prepare(directory)
 
 
-def clean():
+def clean() -> None:
     shutil.rmtree(os.path.join(ROOT, "_deps"), ignore_errors=True)
diff --git a/tests/regression.py b/tests/regression.py
index b26d02f..3e434e0 100644
--- a/tests/regression.py
+++ b/tests/regression.py
@@ -10,7 +10,7 @@
 ROOT = os.path.dirname(os.path.abspath(__file__))
 
 
-def get_cwhy_prompt(invocation):
+def get_cwhy_prompt(invocation) -> str:
     process = subprocess.run(
         f"cwhy --show-prompt --- {invocation}",
         text=True,
@@ -28,7 +28,7 @@ def get_cwhy_prompt(invocation):
     return stdout
 
 
-def main(args):
+def main(args) -> None:
     prepare.clean()
     prepare.prepare_all()