Add type annotations to tests

That was... a lot.

There are still errors in the os_sorted tests, but that requires me to
rethink some typing decisions in the main code so that will be for a
future commit.

20 files changed, 328 insertions, 211 deletions

diff --git a/tests/conftest.py b/tests/conftest.py
index 74d7f4f..c63e149 100644
--- a/tests/conftest.py
+++ b/tests/conftest.py
@@ -3,6 +3,7 @@ Fixtures for pytest.
 """
 
 import locale
+from typing import Iterator
 
 import hypothesis
 import pytest
@@ -16,7 +17,7 @@ hypothesis.settings.register_profile(
 )
 
 
-def load_locale(x):
+def load_locale(x: str) -> None:
     """Convenience to load a locale, trying ISO8859-1 first."""
     try:
         locale.setlocale(locale.LC_ALL, str("{}.ISO8859-1".format(x)))
@@ -25,15 +26,16 @@ def load_locale(x):
 
 
 @pytest.fixture()
-def with_locale_en_us():
+def with_locale_en_us() -> Iterator[None]:
     """Convenience to load the en_US locale - reset when complete."""
     orig = locale.getlocale()
-    yield load_locale("en_US")
+    load_locale("en_US")
+    yield
     locale.setlocale(locale.LC_ALL, orig)
 
 
 @pytest.fixture()
-def with_locale_de_de():
+def with_locale_de_de() -> Iterator[None]:
     """
     Convenience to load the de_DE locale - reset when complete - skip if missing.
     """
diff --git a/tests/profile_natsorted.py b/tests/profile_natsorted.py
index a2b1a5c..f6580a3 100644
--- a/tests/profile_natsorted.py
+++ b/tests/profile_natsorted.py
@@ -7,6 +7,7 @@ inputs and different settings.
 import cProfile
 import locale
 import sys
+from typing import List, Union
 
 try:
     from natsort import ns, natsort_keygen
@@ -14,6 +15,8 @@ except ImportError:
     sys.path.insert(0, ".")
     from natsort import ns, natsort_keygen
 
+from natsort.natsort import NatsortKeyType
+
 locale.setlocale(locale.LC_ALL, "en_US.UTF-8")
 
 # Samples to parse
@@ -32,7 +35,7 @@ path_key = natsort_keygen(alg=ns.PATH)
 locale_key = natsort_keygen(alg=ns.LOCALE)
 
 
-def prof_time_to_generate():
+def prof_time_to_generate() -> None:
     print("*** Generate Plain Key ***")
     for _ in range(100000):
         natsort_keygen()
@@ -41,7 +44,9 @@ def prof_time_to_generate():
 cProfile.run("prof_time_to_generate()", sort="time")
 
 
-def prof_parsing(a, msg, key=basic_key):
+def prof_parsing(
+    a: Union[str, int, bytes, List[str]], msg: str, key: NatsortKeyType = basic_key
+) -> None:
     print(msg)
     for _ in range(100000):
         key(a)
diff --git a/tests/test_fake_fastnumbers.py b/tests/test_fake_fastnumbers.py
index c75bb11..574f7cf 100644
--- a/tests/test_fake_fastnumbers.py
+++ b/tests/test_fake_fastnumbers.py
@@ -5,13 +5,14 @@ Test the fake fastnumbers module.
 
 import unicodedata
 from math import isnan
+from typing import Union, cast
 
 from hypothesis import given
 from hypothesis.strategies import floats, integers, text
 from natsort.compat.fake_fastnumbers import fast_float, fast_int
 
 
-def is_float(x):
+def is_float(x: str) -> bool:
     try:
         float(x)
     except ValueError:
@@ -25,19 +26,19 @@ def is_float(x):
         return True
 
 
-def not_a_float(x):
+def not_a_float(x: str) -> bool:
     return not is_float(x)
 
 
-def is_int(x):
+def is_int(x: Union[str, float]) -> bool:
     try:
-        return x.is_integer()
+        return cast(float, x).is_integer()
     except AttributeError:
         try:
             int(x)
         except ValueError:
             try:
-                unicodedata.digit(x)
+                unicodedata.digit(cast(str, x))
             except (ValueError, TypeError):
                 return False
             else:
@@ -46,7 +47,7 @@ def is_int(x):
             return True
 
 
-def not_an_int(x):
+def not_an_int(x: Union[str, float]) -> bool:
     return not is_int(x)
 
 
@@ -54,56 +55,56 @@ def not_an_int(x):
 # and a test that uses the hypothesis module.
 
 
-def test_fast_float_returns_nan_alternate_if_nan_option_is_given():
+def test_fast_float_returns_nan_alternate_if_nan_option_is_given() -> None:
     assert fast_float("nan", nan=7) == 7
 
 
-def test_fast_float_converts_float_string_to_float_example():
+def test_fast_float_converts_float_string_to_float_example() -> None:
     assert fast_float("45.8") == 45.8
     assert fast_float("-45") == -45.0
     assert fast_float("45.8e-2", key=len) == 45.8e-2
-    assert isnan(fast_float("nan"))
-    assert isnan(fast_float("+nan"))
-    assert isnan(fast_float("-NaN"))
+    assert isnan(cast(float, fast_float("nan")))
+    assert isnan(cast(float, fast_float("+nan")))
+    assert isnan(cast(float, fast_float("-NaN")))
     assert fast_float("۱۲.۱۲") == 12.12
     assert fast_float("-۱۲.۱۲") == -12.12
 
 
 @given(floats(allow_nan=False))
-def test_fast_float_converts_float_string_to_float(x):
+def test_fast_float_converts_float_string_to_float(x: float) -> None:
     assert fast_float(repr(x)) == x
 
 
-def test_fast_float_leaves_string_as_is_example():
+def test_fast_float_leaves_string_as_is_example() -> None:
     assert fast_float("invalid") == "invalid"
 
 
 @given(text().filter(not_a_float).filter(bool))
-def test_fast_float_leaves_string_as_is(x):
+def test_fast_float_leaves_string_as_is(x: str) -> None:
     assert fast_float(x) == x
 
 
-def test_fast_float_with_key_applies_to_string_example():
+def test_fast_float_with_key_applies_to_string_example() -> None:
     assert fast_float("invalid", key=len) == len("invalid")
 
 
 @given(text().filter(not_a_float).filter(bool))
-def test_fast_float_with_key_applies_to_string(x):
+def test_fast_float_with_key_applies_to_string(x: str) -> None:
     assert fast_float(x, key=len) == len(x)
 
 
-def test_fast_int_leaves_float_string_as_is_example():
+def test_fast_int_leaves_float_string_as_is_example() -> None:
     assert fast_int("45.8") == "45.8"
     assert fast_int("nan") == "nan"
     assert fast_int("inf") == "inf"
 
 
 @given(floats().filter(not_an_int))
-def test_fast_int_leaves_float_string_as_is(x):
+def test_fast_int_leaves_float_string_as_is(x: float) -> None:
     assert fast_int(repr(x)) == repr(x)
 
 
-def test_fast_int_converts_int_string_to_int_example():
+def test_fast_int_converts_int_string_to_int_example() -> None:
     assert fast_int("-45") == -45
     assert fast_int("+45") == 45
     assert fast_int("۱۲") == 12
@@ -111,23 +112,23 @@ def test_fast_int_converts_int_string_to_int_example():
 
 
 @given(integers())
-def test_fast_int_converts_int_string_to_int(x):
+def test_fast_int_converts_int_string_to_int(x: int) -> None:
     assert fast_int(repr(x)) == x
 
 
-def test_fast_int_leaves_string_as_is_example():
+def test_fast_int_leaves_string_as_is_example() -> None:
     assert fast_int("invalid") == "invalid"
 
 
 @given(text().filter(not_an_int).filter(bool))
-def test_fast_int_leaves_string_as_is(x):
+def test_fast_int_leaves_string_as_is(x: str) -> None:
     assert fast_int(x) == x
 
 
-def test_fast_int_with_key_applies_to_string_example():
+def test_fast_int_with_key_applies_to_string_example() -> None:
     assert fast_int("invalid", key=len) == len("invalid")
 
 
 @given(text().filter(not_an_int).filter(bool))
-def test_fast_int_with_key_applies_to_string(x):
+def test_fast_int_with_key_applies_to_string(x: str) -> None:
     assert fast_int(x, key=len) == len(x)
diff --git a/tests/test_final_data_transform_factory.py b/tests/test_final_data_transform_factory.py
index f6bf636..5437d53 100644
--- a/tests/test_final_data_transform_factory.py
+++ b/tests/test_final_data_transform_factory.py
@@ -1,17 +1,20 @@
 # -*- coding: utf-8 -*-
 """These test the utils.py functions."""
+from typing import Callable, Union
 
 import pytest
 from hypothesis import example, given
 from hypothesis.strategies import floats, integers, text
-from natsort.ns_enum import NS_DUMB, ns
+from natsort.ns_enum import NS_DUMB, NS_t, ns
 from natsort.utils import final_data_transform_factory
 
 
 @pytest.mark.parametrize("alg", [ns.DEFAULT, ns.UNGROUPLETTERS, ns.LOCALE])
 @given(x=text(), y=floats(allow_nan=False, allow_infinity=False) | integers())
 @pytest.mark.usefixtures("with_locale_en_us")
-def test_final_data_transform_factory_default(x, y, alg):
+def test_final_data_transform_factory_default(
+    x: str, y: Union[int, float], alg: NS_t
+) -> None:
     final_data_transform_func = final_data_transform_factory(alg, "", "::")
     value = (x, y)
     original_value = "".join(map(str, value))
@@ -34,7 +37,9 @@ def test_final_data_transform_factory_default(x, y, alg):
 @given(x=text(), y=floats(allow_nan=False, allow_infinity=False) | integers())
 @example(x="İ", y=0)
 @pytest.mark.usefixtures("with_locale_en_us")
-def test_final_data_transform_factory_ungroup_and_locale(x, y, alg, func):
+def test_final_data_transform_factory_ungroup_and_locale(
+    x: str, y: Union[int, float], alg: NS_t, func: Callable[[str], str]
+) -> None:
     final_data_transform_func = final_data_transform_factory(alg, "", "::")
     value = (x, y)
     original_value = "".join(map(str, value))
@@ -46,6 +51,6 @@ def test_final_data_transform_factory_ungroup_and_locale(x, y, alg, func):
     assert result == expected
 
 
-def test_final_data_transform_factory_ungroup_and_locale_empty_tuple():
+def test_final_data_transform_factory_ungroup_and_locale_empty_tuple() -> None:
     final_data_transform_func = final_data_transform_factory(ns.UG | ns.L, "", "::")
     assert final_data_transform_func((), "") == ((), ())
diff --git a/tests/test_input_string_transform_factory.py b/tests/test_input_string_transform_factory.py
index 7d54afd..d265492 100644
--- a/tests/test_input_string_transform_factory.py
+++ b/tests/test_input_string_transform_factory.py
@@ -1,14 +1,15 @@
 # -*- coding: utf-8 -*-
 """These test the utils.py functions."""
+from typing import Callable
 
 import pytest
 from hypothesis import example, given
 from hypothesis.strategies import integers, text
-from natsort.ns_enum import NS_DUMB, ns
+from natsort.ns_enum import NS_DUMB, NS_t, ns
 from natsort.utils import input_string_transform_factory
 
 
-def thousands_separated_int(n):
+def thousands_separated_int(n: str) -> str:
     """Insert thousands separators in an int."""
     new_int = ""
     for i, y in enumerate(reversed(n), 1):
@@ -20,7 +21,7 @@ def thousands_separated_int(n):
 
 
 @given(text())
-def test_input_string_transform_factory_is_no_op_for_no_alg_options(x):
+def test_input_string_transform_factory_is_no_op_for_no_alg_options(x: str) -> None:
     input_string_transform_func = input_string_transform_factory(ns.DEFAULT)
     assert input_string_transform_func(x) is x
 
@@ -36,7 +37,9 @@ def test_input_string_transform_factory_is_no_op_for_no_alg_options(x):
     ],
 )
 @given(x=text())
-def test_input_string_transform_factory(x, alg, example_func):
+def test_input_string_transform_factory(
+    x: str, alg: NS_t, example_func: Callable[[str], str]
+) -> None:
     input_string_transform_func = input_string_transform_factory(alg)
     assert input_string_transform_func(x) == example_func(x)
 
@@ -44,7 +47,7 @@ def test_input_string_transform_factory(x, alg, example_func):
 @example(12543642642534980)  # 12,543,642,642,534,980 => 12543642642534980
 @given(x=integers(min_value=1000))
 @pytest.mark.usefixtures("with_locale_en_us")
-def test_input_string_transform_factory_cleans_thousands(x):
+def test_input_string_transform_factory_cleans_thousands(x: int) -> None:
     int_str = str(x).rstrip("lL")
     thousands_int_str = thousands_separated_int(int_str)
     assert thousands_int_str.replace(",", "") != thousands_int_str
@@ -69,7 +72,9 @@ def test_input_string_transform_factory_cleans_thousands(x):
     ],
 )
 @pytest.mark.usefixtures("with_locale_en_us")
-def test_input_string_transform_factory_handles_us_locale(x, expected):
+def test_input_string_transform_factory_handles_us_locale(
+    x: str, expected: str
+) -> None:
     input_string_transform_func = input_string_transform_factory(ns.LOCALE)
     assert input_string_transform_func(x) == expected
 
@@ -83,7 +88,9 @@ def test_input_string_transform_factory_handles_us_locale(x, expected):
     ],
 )
 @pytest.mark.usefixtures("with_locale_de_de")
-def test_input_string_transform_factory_handles_de_locale(x, expected):
+def test_input_string_transform_factory_handles_de_locale(
+    x: str, expected: str
+) -> None:
     input_string_transform_func = input_string_transform_factory(ns.LOCALE)
     assert input_string_transform_func(x) == expected
 
@@ -97,13 +104,15 @@ def test_input_string_transform_factory_handles_de_locale(x, expected):
     ],
 )
 @pytest.mark.usefixtures("with_locale_de_de")
-def test_input_string_transform_factory_handles_german_locale(alg, expected):
+def test_input_string_transform_factory_handles_german_locale(
+    alg: NS_t, expected: str
+) -> None:
     input_string_transform_func = input_string_transform_factory(alg)
     assert input_string_transform_func("1543,753") == expected
 
 
 @pytest.mark.usefixtures("with_locale_de_de")
-def test_input_string_transform_factory_does_nothing_with_non_num_input():
+def test_input_string_transform_factory_does_nothing_with_non_num_input() -> None:
     input_string_transform_func = input_string_transform_factory(ns.LOCALE | ns.FLOAT)
     expected = "154s,t53"
     assert input_string_transform_func("154s,t53") == expected
diff --git a/tests/test_main.py b/tests/test_main.py
index da91fdd..d2a3e28 100644
--- a/tests/test_main.py
+++ b/tests/test_main.py
@@ -5,11 +5,13 @@ Test the natsort command-line tool functions.
 
 import re
 import sys
+from typing import Any as Any_t, List, Union
 
 import pytest
 from hypothesis import given
-from hypothesis.strategies import data, floats, integers, lists
+from hypothesis.strategies import DataObject, data, floats, integers, lists
 from natsort.__main__ import (
+    TypedArgs,
     check_filters,
     keep_entry_range,
     keep_entry_value,
@@ -17,9 +19,12 @@ from natsort.__main__ import (
     range_check,
     sort_and_print_entries,
 )
+from pytest_mock import MockerFixture
 
 
-def test_main_passes_default_arguments_with_no_command_line_options(mocker):
+def test_main_passes_default_arguments_with_no_command_line_options(
+    mocker: MockerFixture,
+) -> None:
     p = mocker.patch("natsort.__main__.sort_and_print_entries")
     main("num-2", "num-6", "num-1")
     args = p.call_args[0][1]
@@ -34,7 +39,9 @@ def test_main_passes_default_arguments_with_no_command_line_options(mocker):
     assert not args.locale
 
 
-def test_main_passes_arguments_with_all_command_line_options(mocker):
+def test_main_passes_arguments_with_all_command_line_options(
+    mocker: MockerFixture,
+) -> None:
     arguments = ["--paths", "--reverse", "--locale"]
     arguments.extend(["--filter", "4", "10"])
     arguments.extend(["--reverse-filter", "100", "110"])
@@ -57,21 +64,6 @@ def test_main_passes_arguments_with_all_command_line_options(mocker):
     assert args.locale
 
 
-class Args:
-    """A dummy class to simulate the argparse Namespace object"""
-
-    def __init__(self, filt, reverse_filter, exclude, as_path, reverse):
-        self.filter = filt
-        self.reverse_filter = reverse_filter
-        self.exclude = exclude
-        self.reverse = reverse
-        self.number_type = "float"
-        self.signed = True
-        self.exp = True
-        self.paths = as_path
-        self.locale = 0
-
-
 mock_print = "__builtin__.print" if sys.version[0] == "2" else "builtins.print"
 
 entries = [
@@ -135,9 +127,11 @@ entries = [
         ([None, None, False, True, True], reversed([2, 3, 1, 0, 5, 6, 4])),
     ],
 )
-def test_sort_and_print_entries(options, order, mocker):
+def test_sort_and_print_entries(
+    options: List[Any_t], order: List[int], mocker: MockerFixture
+) -> None:
     p = mocker.patch(mock_print)
-    sort_and_print_entries(entries, Args(*options))
+    sort_and_print_entries(entries, TypedArgs(*options))
     e = [mocker.call(entries[i]) for i in order]
     p.assert_has_calls(e)
 
@@ -146,13 +140,15 @@ def test_sort_and_print_entries(options, order, mocker):
 # and a test that uses the hypothesis module.
 
 
-def test_range_check_returns_range_as_is_but_with_floats_example():
+def test_range_check_returns_range_as_is_but_with_floats_example() -> None:
     assert range_check(10, 11) == (10.0, 11.0)
     assert range_check(6.4, 30) == (6.4, 30.0)
 
 
 @given(x=floats(allow_nan=False, min_value=-1e8, max_value=1e8) | integers(), d=data())
-def test_range_check_returns_range_as_is_if_first_is_less_than_second(x, d):
+def test_range_check_returns_range_as_is_if_first_is_less_than_second(
+    x: Union[int, float], d: DataObject
+) -> None:
     # Pull data such that the first is less than the second.
     if isinstance(x, float):
         y = d.draw(floats(min_value=x + 1.0, max_value=1e9, allow_nan=False))
@@ -161,44 +157,48 @@ def test_range_check_returns_range_as_is_if_first_is_less_than_second(x, d):
     assert range_check(x, y) == (x, y)
 
 
-def test_range_check_raises_value_error_if_second_is_less_than_first_example():
+def test_range_check_raises_value_error_if_second_is_less_than_first_example() -> None:
     with pytest.raises(ValueError, match="low >= high"):
         range_check(7, 2)
 
 
 @given(x=floats(allow_nan=False), d=data())
-def test_range_check_raises_value_error_if_second_is_less_than_first(x, d):
+def test_range_check_raises_value_error_if_second_is_less_than_first(
+    x: float, d: DataObject
+) -> None:
     # Pull data such that the first is greater than or equal to the second.
     y = d.draw(floats(max_value=x, allow_nan=False))
     with pytest.raises(ValueError, match="low >= high"):
         range_check(x, y)
 
 
-def test_check_filters_returns_none_if_filter_evaluates_to_false():
+def test_check_filters_returns_none_if_filter_evaluates_to_false() -> None:
     assert check_filters(()) is None
-    assert check_filters(False) is None
-    assert check_filters(None) is None
 
 
-def test_check_filters_returns_input_as_is_if_filter_is_valid_example():
+def test_check_filters_returns_input_as_is_if_filter_is_valid_example() -> None:
     assert check_filters([(6, 7)]) == [(6, 7)]
     assert check_filters([(6, 7), (2, 8)]) == [(6, 7), (2, 8)]
 
 
 @given(x=lists(integers(), min_size=1), d=data())
-def test_check_filters_returns_input_as_is_if_filter_is_valid(x, d):
+def test_check_filters_returns_input_as_is_if_filter_is_valid(
+    x: List[int], d: DataObject
+) -> None:
     # ensure y is element-wise greater than x
     y = [d.draw(integers(min_value=val + 1)) for val in x]
     assert check_filters(list(zip(x, y))) == [(i, j) for i, j in zip(x, y)]
 
 
-def test_check_filters_raises_value_error_if_filter_is_invalid_example():
+def test_check_filters_raises_value_error_if_filter_is_invalid_example() -> None:
     with pytest.raises(ValueError, match="Error in --filter: low >= high"):
         check_filters([(7, 2)])
 
 
 @given(x=lists(integers(), min_size=1), d=data())
-def test_check_filters_raises_value_error_if_filter_is_invalid(x, d):
+def test_check_filters_raises_value_error_if_filter_is_invalid(
+    x: List[int], d: DataObject
+) -> None:
     # ensure y is element-wise less than or equal to x
     y = [d.draw(integers(max_value=val)) for val in x]
     with pytest.raises(ValueError, match="Error in --filter: low >= high"):
@@ -212,11 +212,11 @@ def test_check_filters_raises_value_error_if_filter_is_invalid(x, d):
     # 3. No portion is between the bounds => False.
     [([0], [100], True), ([1, 88], [20, 90], True), ([1], [20], False)],
 )
-def test_keep_entry_range(lows, highs, truth):
+def test_keep_entry_range(lows: List[int], highs: List[int], truth: bool) -> None:
     assert keep_entry_range("a56b23c89", lows, highs, int, re.compile(r"\d+")) is truth
 
 
 # 1. Values not in entry => True. 2. Values in entry => False.
 @pytest.mark.parametrize("values, truth", [([100, 45], True), ([23], False)])
-def test_keep_entry_value(values, truth):
+def test_keep_entry_value(values: List[int], truth: bool) -> None:
     assert keep_entry_value("a56b23c89", values, int, re.compile(r"\d+")) is truth
diff --git a/tests/test_natsort_key.py b/tests/test_natsort_key.py
index 6b56bb1..8034a8c 100644
--- a/tests/test_natsort_key.py
+++ b/tests/test_natsort_key.py
@@ -1,42 +1,43 @@
 # -*- coding: utf-8 -*-
 """These test the utils.py functions."""
+from typing import Any as Any_t, List, NoReturn, Tuple, Union
 
 from hypothesis import given
 from hypothesis.strategies import binary, floats, integers, lists, text
 from natsort.utils import natsort_key
 
 
-def str_func(x):
+def str_func(x: Any_t) -> Tuple[str]:
     if isinstance(x, str):
-        return x
+        return (x,)
     else:
         raise TypeError("Not a str!")
 
 
-def fail(_):
+def fail(_: Any_t) -> NoReturn:
     raise AssertionError("This should never be reached!")
 
 
 @given(floats(allow_nan=False) | integers())
-def test_natsort_key_with_numeric_input_takes_number_path(x):
-    assert natsort_key(x, None, str_func, fail, lambda y: y) is x
+def test_natsort_key_with_numeric_input_takes_number_path(x: Union[float, int]) -> None:
+    assert natsort_key(x, None, str_func, fail, lambda y: ("", y)) is x
 
 
 @given(binary().filter(bool))
-def test_natsort_key_with_bytes_input_takes_bytes_path(x):
-    assert natsort_key(x, None, str_func, lambda y: y, fail) is x
+def test_natsort_key_with_bytes_input_takes_bytes_path(x: bytes) -> None:
+    assert natsort_key(x, None, str_func, lambda y: (y,), fail) is x
 
 
 @given(text())
-def test_natsort_key_with_text_input_takes_string_path(x):
+def test_natsort_key_with_text_input_takes_string_path(x: str) -> None:
     assert natsort_key(x, None, str_func, fail, fail) is x
 
 
 @given(lists(elements=text(), min_size=1, max_size=10))
-def test_natsort_key_with_nested_input_takes_nested_path(x):
+def test_natsort_key_with_nested_input_takes_nested_path(x: List[str]) -> None:
     assert natsort_key(x, None, str_func, fail, fail) == tuple(x)
 
 
 @given(text())
-def test_natsort_key_with_key_argument_applies_key_before_processing(x):
-    assert natsort_key(x, len, str_func, fail, lambda y: y) == len(x)
+def test_natsort_key_with_key_argument_applies_key_before_processing(x: str) -> None:
+    assert natsort_key(x, len, str_func, fail, lambda y: ("", y)) == len(x)
diff --git a/tests/test_natsort_keygen.py b/tests/test_natsort_keygen.py
index 5fb8cf3..0abdefb 100644
--- a/tests/test_natsort_keygen.py
+++ b/tests/test_natsort_keygen.py
@@ -5,23 +5,27 @@ See the README or the natsort homepage for more details.
 """
 
 import os
+from typing import List, Tuple, Union
 
 import pytest
 from natsort import natsort_key, natsort_keygen, natsorted, ns
 from natsort.compat.locale import get_strxfrm, null_string_locale
+from natsort.ns_enum import NS_t
+from natsort.utils import BytesTransform, FinalTransform
+from pytest_mock import MockerFixture
 
 
 @pytest.fixture
-def arbitrary_input():
+def arbitrary_input() -> List[Union[str, float]]:
     return ["6A-5.034e+1", "/Folder (1)/Foo", 56.7]
 
 
 @pytest.fixture
-def bytes_input():
+def bytes_input() -> bytes:
     return b"6A-5.034e+1"
 
 
-def test_natsort_keygen_demonstration():
+def test_natsort_keygen_demonstration() -> None:
     original_list = ["a50", "a51.", "a50.31", "a50.4", "a5.034e1", "a50.300"]
     copy_of_list = original_list[:]
     original_list.sort(key=natsort_keygen(alg=ns.F))
@@ -29,21 +33,23 @@ def test_natsort_keygen_demonstration():
     assert original_list == natsorted(copy_of_list, alg=ns.F)
 
 
-def test_natsort_key_public():
+def test_natsort_key_public() -> None:
     assert natsort_key("a-5.034e2") == ("a-", 5, ".", 34, "e", 2)
 
 
-def test_natsort_keygen_with_invalid_alg_input_raises_value_error():
+def test_natsort_keygen_with_invalid_alg_input_raises_value_error() -> None:
     # Invalid arguments give the correct response
     with pytest.raises(ValueError, match="'alg' argument"):
-        natsort_keygen(None, "1")
+        natsort_keygen(None, "1")  # type: ignore
 
 
 @pytest.mark.parametrize(
     "alg, expected",
     [(ns.DEFAULT, ("a-", 5, ".", 34, "e", 1)), (ns.FLOAT | ns.SIGNED, ("a", -50.34))],
 )
-def test_natsort_keygen_returns_natsort_key_that_parses_input(alg, expected):
+def test_natsort_keygen_returns_natsort_key_that_parses_input(
+    alg: NS_t, expected: Tuple[Union[str, int, float], ...]
+) -> None:
     ns_key = natsort_keygen(alg=alg)
     assert ns_key("a-5.034e1") == expected
 
@@ -78,7 +84,9 @@ def test_natsort_keygen_returns_natsort_key_that_parses_input(alg, expected):
         ),
     ],
 )
-def test_natsort_keygen_handles_arbitrary_input(arbitrary_input, alg, expected):
+def test_natsort_keygen_handles_arbitrary_input(
+    arbitrary_input: List[Union[str, float]], alg: NS_t, expected: FinalTransform
+) -> None:
     ns_key = natsort_keygen(alg=alg)
     assert ns_key(arbitrary_input) == expected
 
@@ -93,13 +101,15 @@ def test_natsort_keygen_handles_arbitrary_input(arbitrary_input, alg, expected):
         (ns.PATH | ns.GROUPLETTERS, ((b"6A-5.034e+1",),)),
     ],
 )
-def test_natsort_keygen_handles_bytes_input(bytes_input, alg, expected):
+def test_natsort_keygen_handles_bytes_input(
+    bytes_input: bytes, alg: NS_t, expected: BytesTransform
+) -> None:
     ns_key = natsort_keygen(alg=alg)
     assert ns_key(bytes_input) == expected
 
 
 @pytest.mark.parametrize(
-    "alg, expected, is_dumb",
+    "alg, expected_in, is_dumb",
     [
         (
             ns.LOCALE,
@@ -131,23 +141,29 @@ def test_natsort_keygen_handles_bytes_input(bytes_input, alg, expected):
     ],
 )
 @pytest.mark.usefixtures("with_locale_en_us")
-def test_natsort_keygen_with_locale(mocker, arbitrary_input, alg, expected, is_dumb):
+def test_natsort_keygen_with_locale(
+    mocker: MockerFixture,
+    arbitrary_input: List[Union[str, float]],
+    alg: NS_t,
+    expected: FinalTransform,
+    is_dumb: bool,
+) -> None:
     # First, apply the correct strxfrm function to the string values.
     strxfrm = get_strxfrm()
-    expected = [list(sub) for sub in expected]
+    expected_tmp = [list(sub) for sub in expected]
     try:
         for i in (2, 4, 6):
-            expected[0][i] = strxfrm(expected[0][i])
+            expected_tmp[0][i] = strxfrm(expected_tmp[0][i])
         for i in (0, 2):
-            expected[1][i] = strxfrm(expected[1][i])
-        expected = tuple(tuple(sub) for sub in expected)
+            expected_tmp[1][i] = strxfrm(expected_tmp[1][i])
+        expected = tuple(tuple(sub) for sub in expected_tmp)
     except IndexError:  # ns.LOCALE | ns.CAPITALFIRST
-        expected = [[list(subsub) for subsub in sub] for sub in expected]
+        expected_tmp = [[list(subsub) for subsub in sub] for sub in expected_tmp]
         for i in (2, 4, 6):
-            expected[0][1][i] = strxfrm(expected[0][1][i])
+            expected_tmp[0][1][i] = strxfrm(expected_tmp[0][1][i])
         for i in (0, 2):
-            expected[1][1][i] = strxfrm(expected[1][1][i])
-        expected = tuple(tuple(tuple(subsub) for subsub in sub) for sub in expected)
+            expected_tmp[1][1][i] = strxfrm(expected_tmp[1][1][i])
+        expected = tuple(tuple(tuple(subsub) for subsub in sub) for sub in expected_tmp)
 
     mocker.patch("natsort.compat.locale.dumb_sort", return_value=is_dumb)
     ns_key = natsort_keygen(alg=alg)
@@ -159,7 +175,9 @@ def test_natsort_keygen_with_locale(mocker, arbitrary_input, alg, expected, is_d
     [(ns.LOCALE, False), (ns.LOCALE, True), (ns.LOCALE | ns.CAPITALFIRST, False)],
 )
 @pytest.mark.usefixtures("with_locale_en_us")
-def test_natsort_keygen_with_locale_bytes(mocker, bytes_input, alg, is_dumb):
+def test_natsort_keygen_with_locale_bytes(
+    mocker: MockerFixture, bytes_input: bytes, alg: NS_t, is_dumb: bool
+) -> None:
     expected = (b"6A-5.034e+1",)
     mocker.patch("natsort.compat.locale.dumb_sort", return_value=is_dumb)
     ns_key = natsort_keygen(alg=alg)
diff --git a/tests/test_natsorted.py b/tests/test_natsorted.py
index 4254e6c..e71678f 100644
--- a/tests/test_natsorted.py
+++ b/tests/test_natsorted.py
@@ -5,6 +5,8 @@ See the README or the natsort homepage for more details.
 """
 
 from operator import itemgetter
+from typing import List, Tuple, Union
+from natsort.ns_enum import NS_t
 
 import pytest
 from natsort import as_utf8, natsorted, ns
@@ -12,27 +14,29 @@ from pytest import raises
 
 
 @pytest.fixture
-def float_list():
+def float_list() -> List[str]:
     return ["a50", "a51.", "a50.31", "a-50", "a50.4", "a5.034e1", "a50.300"]
 
 
 @pytest.fixture
-def fruit_list():
+def fruit_list() -> List[str]:
     return ["Apple", "corn", "Corn", "Banana", "apple", "banana"]
 
 
 @pytest.fixture
-def mixed_list():
+def mixed_list() -> List[Union[str, int, float]]:
     return ["Ä", "0", "ä", 3, "b", 1.5, "2", "Z"]
 
 
-def test_natsorted_numbers_in_ascending_order():
+def test_natsorted_numbers_in_ascending_order() -> None:
     given = ["a2", "a5", "a9", "a1", "a4", "a10", "a6"]
     expected = ["a1", "a2", "a4", "a5", "a6", "a9", "a10"]
     assert natsorted(given) == expected
 
 
-def test_natsorted_can_sort_as_signed_floats_with_exponents(float_list):
+def test_natsorted_can_sort_as_signed_floats_with_exponents(
+    float_list: List[str],
+) -> None:
     expected = ["a-50", "a50", "a50.300", "a50.31", "a5.034e1", "a50.4", "a51."]
     assert natsorted(float_list, alg=ns.REAL) == expected
 
@@ -42,19 +46,23 @@ def test_natsorted_can_sort_as_signed_floats_with_exponents(float_list):
     "alg",
     [ns.NOEXP | ns.FLOAT | ns.UNSIGNED, ns.NOEXP | ns.FLOAT],
 )
-def test_natsorted_can_sort_as_unsigned_and_ignore_exponents(float_list, alg):
+def test_natsorted_can_sort_as_unsigned_and_ignore_exponents(
+    float_list: List[str], alg: NS_t
+) -> None:
     expected = ["a5.034e1", "a50", "a50.300", "a50.31", "a50.4", "a51.", "a-50"]
     assert natsorted(float_list, alg=alg) == expected
 
 
 # DEFAULT and INT are all equivalent.
 @pytest.mark.parametrize("alg", [ns.DEFAULT, ns.INT])
-def test_natsorted_can_sort_as_unsigned_ints_which_is_default(float_list, alg):
+def test_natsorted_can_sort_as_unsigned_ints_which_is_default(
+    float_list: List[str], alg: NS_t
+) -> None:
     expected = ["a5.034e1", "a50", "a50.4", "a50.31", "a50.300", "a51.", "a-50"]
     assert natsorted(float_list, alg=alg) == expected
 
 
-def test_natsorted_can_sort_as_signed_ints(float_list):
+def test_natsorted_can_sort_as_signed_ints(float_list: List[str]) -> None:
     expected = ["a-50", "a5.034e1", "a50", "a50.4", "a50.31", "a50.300", "a51."]
     assert natsorted(float_list, alg=ns.SIGNED) == expected
 
@@ -63,12 +71,14 @@ def test_natsorted_can_sort_as_signed_ints(float_list):
     "alg, expected",
     [(ns.UNSIGNED, ["a7", "a+2", "a-5"]), (ns.SIGNED, ["a-5", "a+2", "a7"])],
 )
-def test_natsorted_can_sort_with_or_without_accounting_for_sign(alg, expected):
+def test_natsorted_can_sort_with_or_without_accounting_for_sign(
+    alg: NS_t, expected: List[str]
+) -> None:
     given = ["a-5", "a7", "a+2"]
     assert natsorted(given, alg=alg) == expected
 
 
-def test_natsorted_can_sort_as_version_numbers():
+def test_natsorted_can_sort_as_version_numbers() -> None:
     given = ["1.9.9a", "1.11", "1.9.9b", "1.11.4", "1.10.1"]
     expected = ["1.9.9a", "1.9.9b", "1.10.1", "1.11", "1.11.4"]
     assert natsorted(given) == expected
@@ -81,7 +91,11 @@ def test_natsorted_can_sort_as_version_numbers():
         (ns.NUMAFTER, ["Ä", "Z", "ä", "b", "0", 1.5, "2", 3]),
     ],
 )
-def test_natsorted_handles_mixed_types(mixed_list, alg, expected):
+def test_natsorted_handles_mixed_types(
+    mixed_list: List[Union[str, int, float]],
+    alg: NS_t,
+    expected: List[Union[str, int, float]],
+) -> None:
     assert natsorted(mixed_list, alg=alg) == expected
 
 
@@ -92,14 +106,16 @@ def test_natsorted_handles_mixed_types(mixed_list, alg, expected):
         (ns.NANLAST, [5, "25", 1e40, float("nan")], slice(None, 3)),
     ],
 )
-def test_natsorted_handles_nan(alg, expected, slc):
-    given = ["25", 5, float("nan"), 1e40]
+def test_natsorted_handles_nan(
+    alg: NS_t, expected: List[Union[str, float, int]], slc: slice
+) -> None:
+    given: List[Union[str, float, int]] = ["25", 5, float("nan"), 1e40]
     # The slice is because NaN != NaN
     # noinspection PyUnresolvedReferences
     assert natsorted(given, alg=alg)[slc] == expected[slc]
 
 
-def test_natsorted_with_mixed_bytes_and_str_input_raises_type_error():
+def test_natsorted_with_mixed_bytes_and_str_input_raises_type_error() -> None:
     with raises(TypeError, match="bytes"):
         natsorted(["ä", b"b"])
 
@@ -107,29 +123,31 @@ def test_natsorted_with_mixed_bytes_and_str_input_raises_type_error():
     assert natsorted(["ä", b"b"], key=as_utf8) == ["ä", b"b"]
 
 
-def test_natsorted_raises_type_error_for_non_iterable_input():
+def test_natsorted_raises_type_error_for_non_iterable_input() -> None:
     with raises(TypeError, match="'int' object is not iterable"):
-        natsorted(100)
+        natsorted(100)  # type: ignore
 
 
-def test_natsorted_recurses_into_nested_lists():
+def test_natsorted_recurses_into_nested_lists() -> None:
     given = [["a1", "a5"], ["a1", "a40"], ["a10", "a1"], ["a2", "a5"]]
     expected = [["a1", "a5"], ["a1", "a40"], ["a2", "a5"], ["a10", "a1"]]
     assert natsorted(given) == expected
 
 
-def test_natsorted_applies_key_to_each_list_element_before_sorting_list():
+def test_natsorted_applies_key_to_each_list_element_before_sorting_list() -> None:
     given = [("a", "num3"), ("b", "num5"), ("c", "num2")]
     expected = [("c", "num2"), ("a", "num3"), ("b", "num5")]
     assert natsorted(given, key=itemgetter(1)) == expected
 
 
-def test_natsorted_returns_list_in_reversed_order_with_reverse_option(float_list):
+def test_natsorted_returns_list_in_reversed_order_with_reverse_option(
+    float_list: List[str],
+) -> None:
     expected = natsorted(float_list)[::-1]
     assert natsorted(float_list, reverse=True) == expected
 
 
-def test_natsorted_handles_filesystem_paths():
+def test_natsorted_handles_filesystem_paths() -> None:
     given = [
         "/p/Folder (10)/file.tar.gz",
         "/p/Folder (1)/file (1).tar.gz",
@@ -157,10 +175,10 @@ def test_natsorted_handles_filesystem_paths():
     assert natsorted(given, alg=ns.FLOAT | ns.PATH) == expected_correct
 
 
-def test_natsorted_handles_numbers_and_filesystem_paths_simultaneously():
+def test_natsorted_handles_numbers_and_filesystem_paths_simultaneously() -> None:
     # You can sort paths and numbers, not that you'd want to
-    given = ["/Folder (9)/file.exe", 43]
-    expected = [43, "/Folder (9)/file.exe"]
+    given: List[Union[str, int]] = ["/Folder (9)/file.exe", 43]
+    expected: List[Union[str, int]] = [43, "/Folder (9)/file.exe"]
     assert natsorted(given, alg=ns.PATH) == expected
 
 
@@ -174,7 +192,9 @@ def test_natsorted_handles_numbers_and_filesystem_paths_simultaneously():
         (ns.G | ns.LF, ["apple", "Apple", "banana", "Banana", "corn", "Corn"]),
     ],
 )
-def test_natsorted_supports_case_handling(alg, expected, fruit_list):
+def test_natsorted_supports_case_handling(
+    alg: NS_t, expected: List[str], fruit_list: List[str]
+) -> None:
     assert natsorted(fruit_list, alg=alg) == expected
 
 
@@ -186,7 +206,9 @@ def test_natsorted_supports_case_handling(alg, expected, fruit_list):
         (ns.IGNORECASE, [("a3", "a1"), ("A5", "a6")]),
     ],
 )
-def test_natsorted_supports_nested_case_handling(alg, expected):
+def test_natsorted_supports_nested_case_handling(
+    alg: NS_t, expected: List[Tuple[str, str]]
+) -> None:
     given = [("A5", "a6"), ("a3", "a1")]
     assert natsorted(given, alg=alg) == expected
 
@@ -201,26 +223,28 @@ def test_natsorted_supports_nested_case_handling(alg, expected):
     ],
 )
 @pytest.mark.usefixtures("with_locale_en_us")
-def test_natsorted_can_sort_using_locale(fruit_list, alg, expected):
+def test_natsorted_can_sort_using_locale(
+    fruit_list: List[str], alg: NS_t, expected: List[str]
+) -> None:
     assert natsorted(fruit_list, alg=ns.LOCALE | alg) == expected
 
 
 @pytest.mark.usefixtures("with_locale_en_us")
-def test_natsorted_can_sort_locale_specific_numbers_en():
+def test_natsorted_can_sort_locale_specific_numbers_en() -> None:
     given = ["c", "a5,467.86", "ä", "b", "a5367.86", "a5,6", "a5,50"]
     expected = ["a5,6", "a5,50", "a5367.86", "a5,467.86", "ä", "b", "c"]
     assert natsorted(given, alg=ns.LOCALE | ns.F) == expected
 
 
 @pytest.mark.usefixtures("with_locale_de_de")
-def test_natsorted_can_sort_locale_specific_numbers_de():
+def test_natsorted_can_sort_locale_specific_numbers_de() -> None:
     given = ["c", "a5.467,86", "ä", "b", "a5367.86", "a5,6", "a5,50"]
     expected = ["a5,50", "a5,6", "a5367.86", "a5.467,86", "ä", "b", "c"]
     assert natsorted(given, alg=ns.LOCALE | ns.F) == expected
 
 
 @pytest.mark.usefixtures("with_locale_de_de")
-def test_natsorted_locale_bug_regression_test_109():
+def test_natsorted_locale_bug_regression_test_109() -> None:
     # https://github.com/SethMMorton/natsort/issues/109
     given = ["462166", "461761"]
     expected = ["461761", "462166"]
@@ -242,7 +266,11 @@ def test_natsorted_locale_bug_regression_test_109():
     ],
 )
 @pytest.mark.usefixtures("with_locale_en_us")
-def test_natsorted_handles_mixed_types_with_locale(mixed_list, alg, expected):
+def test_natsorted_handles_mixed_types_with_locale(
+    mixed_list: List[Union[str, int, float]],
+    alg: NS_t,
+    expected: List[Union[str, int, float]],
+) -> None:
     assert natsorted(mixed_list, alg=ns.LOCALE | alg) == expected
 
 
@@ -253,12 +281,14 @@ def test_natsorted_handles_mixed_types_with_locale(mixed_list, alg, expected):
         (ns.NUMAFTER, ["Banana", "apple", "corn", "~~~~~~", "73", "5039"]),
     ],
 )
-def test_natsorted_sorts_an_odd_collection_of_strings(alg, expected):
+def test_natsorted_sorts_an_odd_collection_of_strings(
+    alg: NS_t, expected: List[str]
+) -> None:
     given = ["apple", "Banana", "73", "5039", "corn", "~~~~~~"]
     assert natsorted(given, alg=alg) == expected
 
 
-def test_natsorted_sorts_mixed_ascii_and_non_ascii_numbers():
+def test_natsorted_sorts_mixed_ascii_and_non_ascii_numbers() -> None:
     given = [
         "1st street",
         "10th street",
diff --git a/tests/test_natsorted_convenience.py b/tests/test_natsorted_convenience.py
index cdc2c50..599c4ba 100644
--- a/tests/test_natsorted_convenience.py
+++ b/tests/test_natsorted_convenience.py
@@ -5,6 +5,7 @@ See the README or the natsort homepage for more details.
 """
 
 from operator import itemgetter
+from typing import List
 
 import pytest
 from natsort import (
@@ -23,21 +24,21 @@ from natsort import (
 
 
 @pytest.fixture
-def version_list():
+def version_list() -> List[str]:
     return ["1.9.9a", "1.11", "1.9.9b", "1.11.4", "1.10.1"]
 
 
 @pytest.fixture
-def float_list():
+def float_list() -> List[str]:
     return ["a50", "a51.", "a50.31", "a-50", "a50.4", "a5.034e1", "a50.300"]
 
 
 @pytest.fixture
-def fruit_list():
+def fruit_list() -> List[str]:
     return ["Apple", "corn", "Corn", "Banana", "apple", "banana"]
 
 
-def test_decoder_returns_function_that_can_decode_bytes_but_return_non_bytes_as_is():
+def test_decoder_returns_function_that_can_decode_bytes_but_return_non_bytes_as_is() -> None:
     func = decoder("latin1")
     str_obj = "bytes"
     int_obj = 14
@@ -46,24 +47,28 @@ def test_decoder_returns_function_that_can_decode_bytes_but_return_non_bytes_as_
     assert func(str_obj) is str_obj  # same object returned b/c only bytes has decode
 
 
-def test_as_ascii_converts_bytes_to_ascii():
+def test_as_ascii_converts_bytes_to_ascii() -> None:
     assert decoder("ascii")(b"bytes") == as_ascii(b"bytes")
 
 
-def test_as_utf8_converts_bytes_to_utf8():
+def test_as_utf8_converts_bytes_to_utf8() -> None:
     assert decoder("utf8")(b"bytes") == as_utf8(b"bytes")
 
 
-def test_realsorted_is_identical_to_natsorted_with_real_alg(float_list):
+def test_realsorted_is_identical_to_natsorted_with_real_alg(
+    float_list: List[str],
+) -> None:
     assert realsorted(float_list) == natsorted(float_list, alg=ns.REAL)
 
 
 @pytest.mark.usefixtures("with_locale_en_us")
-def test_humansorted_is_identical_to_natsorted_with_locale_alg(fruit_list):
+def test_humansorted_is_identical_to_natsorted_with_locale_alg(
+    fruit_list: List[str],
+) -> None:
     assert humansorted(fruit_list) == natsorted(fruit_list, alg=ns.LOCALE)
 
 
-def test_index_natsorted_returns_integer_list_of_sort_order_for_input_list():
+def test_index_natsorted_returns_integer_list_of_sort_order_for_input_list() -> None:
     given = ["num3", "num5", "num2"]
     other = ["foo", "bar", "baz"]
     index = index_natsorted(given)
@@ -72,27 +77,31 @@ def test_index_natsorted_returns_integer_list_of_sort_order_for_input_list():
     assert [other[i] for i in index] == ["baz", "foo", "bar"]
 
 
-def test_index_natsorted_reverse():
+def test_index_natsorted_reverse() -> None:
     given = ["num3", "num5", "num2"]
     assert index_natsorted(given, reverse=True) == index_natsorted(given)[::-1]
 
 
-def test_index_natsorted_applies_key_function_before_sorting():
+def test_index_natsorted_applies_key_function_before_sorting() -> None:
     given = [("a", "num3"), ("b", "num5"), ("c", "num2")]
     expected = [2, 0, 1]
     assert index_natsorted(given, key=itemgetter(1)) == expected
 
 
-def test_index_realsorted_is_identical_to_index_natsorted_with_real_alg(float_list):
+def test_index_realsorted_is_identical_to_index_natsorted_with_real_alg(
+    float_list: List[str],
+) -> None:
     assert index_realsorted(float_list) == index_natsorted(float_list, alg=ns.REAL)
 
 
 @pytest.mark.usefixtures("with_locale_en_us")
-def test_index_humansorted_is_identical_to_index_natsorted_with_locale_alg(fruit_list):
+def test_index_humansorted_is_identical_to_index_natsorted_with_locale_alg(
+    fruit_list: List[str],
+) -> None:
     assert index_humansorted(fruit_list) == index_natsorted(fruit_list, alg=ns.LOCALE)
 
 
-def test_order_by_index_sorts_list_according_to_order_of_integer_list():
+def test_order_by_index_sorts_list_according_to_order_of_integer_list() -> None:
     given = ["num3", "num5", "num2"]
     index = [2, 0, 1]
     expected = [given[i] for i in index]
@@ -100,7 +109,7 @@ def test_order_by_index_sorts_list_according_to_order_of_integer_list():
     assert order_by_index(given, index) == expected
 
 
-def test_order_by_index_returns_generator_with_iter_true():
+def test_order_by_index_returns_generator_with_iter_true() -> None:
     given = ["num3", "num5", "num2"]
     index = [2, 0, 1]
     assert order_by_index(given, index, True) != [given[i] for i in index]
diff --git a/tests/test_ns_enum.py b/tests/test_ns_enum.py
index 312c454..7a30718 100644
--- a/tests/test_ns_enum.py
+++ b/tests/test_ns_enum.py
@@ -42,7 +42,7 @@ from natsort import ns
         ("NL", 0x0400),
         ("CN", 0x0800),
         ("NA", 0x1000),
-    ]
+    ],
 )
-def test_ns_enum(given, expected):
+def test_ns_enum(given: str, expected: int) -> None:
     assert ns[given] == expected
diff --git a/tests/test_os_sorted.py b/tests/test_os_sorted.py
index afb15cf..d0ecc79 100644
--- a/tests/test_os_sorted.py
+++ b/tests/test_os_sorted.py
@@ -3,6 +3,7 @@
 Testing for the OS sorting
 """
 import platform
+from typing import cast
 
 import natsort
 import pytest
@@ -15,7 +16,7 @@ else:
     has_icu = True
 
 
-def test_os_sorted_compound():
+def test_os_sorted_compound() -> None:
     given = [
         "/p/Folder (10)/file.tar.gz",
         "/p/Folder (1)/file (1).tar.gz",
@@ -36,14 +37,14 @@ def test_os_sorted_compound():
     assert result == expected
 
 
-def test_os_sorted_misc_no_fail():
+def test_os_sorted_misc_no_fail() -> None:
     natsort.os_sorted([9, 4.3, None, float("nan")])
 
 
-def test_os_sorted_key():
+def test_os_sorted_key() -> None:
     given = ["foo0", "foo2", "goo1"]
     expected = ["foo0", "goo1", "foo2"]
-    result = natsort.os_sorted(given, key=lambda x: x.replace("g", "f"))
+    result = natsort.os_sorted(given, key=lambda x: cast(str, x).replace("g", "f"))
     assert result == expected
 
 
@@ -199,7 +200,7 @@ else:
 
 
 @pytest.mark.usefixtures("with_locale_en_us")
-def test_os_sorted_corpus():
+def test_os_sorted_corpus() -> None:
     result = natsort.os_sorted(given)
     print(result)
     assert result == expected
diff --git a/tests/test_parse_bytes_function.py b/tests/test_parse_bytes_function.py
index 6637cbd..fd358ba 100644
--- a/tests/test_parse_bytes_function.py
+++ b/tests/test_parse_bytes_function.py
@@ -4,8 +4,8 @@
 import pytest
 from hypothesis import given
 from hypothesis.strategies import binary
-from natsort.ns_enum import ns
-from natsort.utils import parse_bytes_factory
+from natsort.ns_enum import NS_t, ns
+from natsort.utils import BytesTransformer, parse_bytes_factory
 
 
 @pytest.mark.parametrize(
@@ -19,6 +19,8 @@ from natsort.utils import parse_bytes_factory
     ],
 )
 @given(x=binary())
-def test_parse_bytest_factory_makes_function_that_returns_tuple(x, alg, example_func):
+def test_parse_bytest_factory_makes_function_that_returns_tuple(
+    x: bytes, alg: NS_t, example_func: BytesTransformer
+) -> None:
     parse_bytes_func = parse_bytes_factory(alg)
     assert parse_bytes_func(x) == example_func(x)
diff --git a/tests/test_parse_number_function.py b/tests/test_parse_number_function.py
index 29ee5a3..3ce6d97 100644
--- a/tests/test_parse_number_function.py
+++ b/tests/test_parse_number_function.py
@@ -1,11 +1,13 @@
 # -*- coding: utf-8 -*-
 """These test the utils.py functions."""
 
+from typing import Optional, Tuple, Union
+
 import pytest
 from hypothesis import given
 from hypothesis.strategies import floats, integers
-from natsort.ns_enum import ns
-from natsort.utils import parse_number_or_none_factory
+from natsort.ns_enum import NS_t, ns
+from natsort.utils import MaybeNumTransformer, parse_number_or_none_factory
 
 
 @pytest.mark.usefixtures("with_locale_en_us")
@@ -19,7 +21,9 @@ from natsort.utils import parse_number_or_none_factory
     ],
 )
 @given(x=floats(allow_nan=False) | integers())
-def test_parse_number_factory_makes_function_that_returns_tuple(x, alg, example_func):
+def test_parse_number_factory_makes_function_that_returns_tuple(
+    x: Union[float, int], alg: NS_t, example_func: MaybeNumTransformer
+) -> None:
     parse_number_func = parse_number_or_none_factory(alg, "", "xx")
     assert parse_number_func(x) == example_func(x)
 
@@ -34,6 +38,8 @@ def test_parse_number_factory_makes_function_that_returns_tuple(x, alg, example_
         (ns.NANLAST, None, ("", float("+inf"))),  # NANLAST makes it +infinity
     ],
 )
-def test_parse_number_factory_treats_nan_and_none_special(alg, x, result):
+def test_parse_number_factory_treats_nan_and_none_special(
+    alg: NS_t, x: Optional[Union[float, int]], result: Tuple[str, Union[float, int]]
+) -> None:
     parse_number_func = parse_number_or_none_factory(alg, "", "xx")
     assert parse_number_func(x) == result
diff --git a/tests/test_parse_string_function.py b/tests/test_parse_string_function.py
index 46347f1..c199964 100644
--- a/tests/test_parse_string_function.py
+++ b/tests/test_parse_string_function.py
@@ -2,23 +2,28 @@
 """These test the utils.py functions."""
 
 import unicodedata
+from typing import Any as Any_t, Callable, Iterable, List, Tuple, Union
 
 import pytest
 from hypothesis import given
 from hypothesis.strategies import floats, integers, lists, text
 from natsort.compat.fastnumbers import fast_float
-from natsort.ns_enum import NS_DUMB, ns
-from natsort.utils import NumericalRegularExpressions as NumRegex
+from natsort.ns_enum import NS_DUMB, NS_t, ns
+from natsort.utils import (
+    FinalTransform,
+    NumericalRegularExpressions as NumRegex,
+    StrParser,
+)
 from natsort.utils import parse_string_factory
 
 
-class CustomTuple(tuple):
+class CustomTuple(Tuple[Any_t, ...]):
     """Used to ensure what is given during testing is what is returned."""
 
-    original = None
+    original: Any_t = None
 
 
-def input_transform(x):
+def input_transform(x: Any_t) -> Any_t:
     """Make uppercase."""
     try:
         return x.upper()
@@ -26,14 +31,14 @@ def input_transform(x):
         return x
 
 
-def final_transform(x, original):
+def final_transform(x: Iterable[Any_t], original: str) -> FinalTransform:
     """Make the input a CustomTuple."""
     t = CustomTuple(x)
     t.original = original
     return t
 
 
-def parse_string_func_factory(alg):
+def parse_string_func_factory(alg: NS_t) -> StrParser:
     """A parse_string_factory result with sample arguments."""
     sep = ""
     return parse_string_factory(
@@ -47,10 +52,12 @@ def parse_string_func_factory(alg):
 
 
 @given(x=floats() | integers())
-def test_parse_string_factory_raises_type_error_if_given_number(x):
+def test_parse_string_factory_raises_type_error_if_given_number(
+    x: Union[int, float]
+) -> None:
     parse_string_func = parse_string_func_factory(ns.DEFAULT)
     with pytest.raises(TypeError):
-        assert parse_string_func(x)
+        assert parse_string_func(x)  # type: ignore
 
 
 # noinspection PyCallingNonCallable
@@ -68,7 +75,9 @@ def test_parse_string_factory_raises_type_error_if_given_number(x):
     )
 )
 @pytest.mark.usefixtures("with_locale_en_us")
-def test_parse_string_factory_invariance(x, alg, orig_func):
+def test_parse_string_factory_invariance(
+    x: List[Union[float, str, int]], alg: NS_t, orig_func: Callable[[str], str]
+) -> None:
     parse_string_func = parse_string_func_factory(alg)
     # parse_string_factory is the high-level combination of several dedicated
     # functions involved in splitting and manipulating a string. The details of
diff --git a/tests/test_regex.py b/tests/test_regex.py
index f647f5f..67d66e0 100644
--- a/tests/test_regex.py
+++ b/tests/test_regex.py
@@ -1,8 +1,11 @@
 # -*- coding: utf-8 -*-
 """These test the splitting regular expressions."""
 
+from typing import List, Pattern
+
 import pytest
 from natsort import ns, numeric_regex_chooser
+from natsort.ns_enum import NS_t
 from natsort.utils import NumericalRegularExpressions as NumRegex
 
 
@@ -95,7 +98,9 @@ labels = ["{}-{}".format(given, regex_names[regex]) for given, _, regex in regex
 
 
 @pytest.mark.parametrize("x, expected, regex", regex_params, ids=labels)
-def test_regex_splits_correctly(x, expected, regex):
+def test_regex_splits_correctly(
+    x: str, expected: List[str], regex: Pattern[str]
+) -> None:
     # noinspection PyUnresolvedReferences
     assert regex.split(x) == expected
 
@@ -115,5 +120,5 @@ def test_regex_splits_correctly(x, expected, regex):
         (ns.FLOAT | ns.UNSIGNED | ns.NOEXP, NumRegex.float_nosign_noexp()),
     ],
 )
-def test_regex_chooser(given, expected):
+def test_regex_chooser(given: NS_t, expected: Pattern[str]) -> None:
     assert numeric_regex_chooser(given) == expected.pattern[1:-1]  # remove parens
diff --git a/tests/test_string_component_transform_factory.py b/tests/test_string_component_transform_factory.py
index 8b77d38..7af2036 100644
--- a/tests/test_string_component_transform_factory.py
+++ b/tests/test_string_component_transform_factory.py
@@ -2,13 +2,14 @@
 """These test the utils.py functions."""
 
 from functools import partial
+from typing import Any as Any_t, Callable, FrozenSet, Union
 
 import pytest
 from hypothesis import example, given
 from hypothesis.strategies import floats, integers, text
 from natsort.compat.fastnumbers import fast_float, fast_int
 from natsort.compat.locale import get_strxfrm
-from natsort.ns_enum import NS_DUMB, ns
+from natsort.ns_enum import NS_DUMB, NS_t, ns
 from natsort.utils import groupletters, string_component_transform_factory
 
 # There are some unicode values that are known failures with the builtin locale
@@ -21,12 +22,12 @@ except ValueError:
     bad_uni_chars = frozenset()
 
 
-def no_bad_uni_chars(x, _bad_chars=bad_uni_chars):
+def no_bad_uni_chars(x: str, _bad_chars: FrozenSet[str] = bad_uni_chars) -> bool:
     """Ensure text does not contain bad unicode characters"""
     return not any(y in _bad_chars for y in x)
 
 
-def no_null(x):
+def no_null(x: str) -> bool:
     """Ensure text does not contain a null character."""
     return "\0" not in x
 
@@ -65,7 +66,9 @@ def no_null(x):
     | text().filter(bool).filter(no_bad_uni_chars).filter(no_null)
 )
 @pytest.mark.usefixtures("with_locale_en_us")
-def test_string_component_transform_factory(x, alg, example_func):
+def test_string_component_transform_factory(
+    x: Union[str, float, int], alg: NS_t, example_func: Callable[[str], Any_t]
+) -> None:
     string_component_transform_func = string_component_transform_factory(alg)
     try:
         assert string_component_transform_func(str(x)) == example_func(str(x))
diff --git a/tests/test_unicode_numbers.py b/tests/test_unicode_numbers.py
index be0f4ee..eb71125 100644
--- a/tests/test_unicode_numbers.py
+++ b/tests/test_unicode_numbers.py
@@ -14,27 +14,27 @@ from natsort.unicode_numbers import (
     digits_no_decimals,
     numeric,
     numeric_chars,
-    numeric_hex,
     numeric_no_decimals,
 )
+from natsort.unicode_numeric_hex import numeric_hex
 
 
-def test_numeric_chars_contains_only_valid_unicode_numeric_characters():
+def test_numeric_chars_contains_only_valid_unicode_numeric_characters() -> None:
     for a in numeric_chars:
         assert unicodedata.numeric(a, None) is not None
 
 
-def test_digit_chars_contains_only_valid_unicode_digit_characters():
+def test_digit_chars_contains_only_valid_unicode_digit_characters() -> None:
     for a in digit_chars:
         assert unicodedata.digit(a, None) is not None
 
 
-def test_decimal_chars_contains_only_valid_unicode_decimal_characters():
+def test_decimal_chars_contains_only_valid_unicode_decimal_characters() -> None:
     for a in decimal_chars:
         assert unicodedata.decimal(a, None) is not None
 
 
-def test_numeric_chars_contains_all_valid_unicode_numeric_and_digit_characters():
+def test_numeric_chars_contains_all_valid_unicode_numeric_and_digit_characters() -> None:
     set_numeric_chars = set(numeric_chars)
     set_digit_chars = set(digit_chars)
     set_decimal_chars = set(decimal_chars)
@@ -46,7 +46,7 @@ def test_numeric_chars_contains_all_valid_unicode_numeric_and_digit_characters()
     assert set_numeric_chars.issuperset(numeric_no_decimals)
 
 
-def test_missing_unicode_number_in_collection():
+def test_missing_unicode_number_in_collection() -> None:
     ok = True
     set_numeric_hex = set(numeric_hex)
     for i in range(0x110000):
@@ -71,7 +71,7 @@ repository (https://github.com/SethMMorton/natsort) with the resulting change.
         )
 
 
-def test_combined_string_contains_all_characters_in_list():
+def test_combined_string_contains_all_characters_in_list() -> None:
     assert numeric == "".join(numeric_chars)
     assert digits == "".join(digit_chars)
     assert decimals == "".join(decimal_chars)
diff --git a/tests/test_utils.py b/tests/test_utils.py
index d559803..ff866b6 100644
--- a/tests/test_utils.py
+++ b/tests/test_utils.py
@@ -6,15 +6,16 @@ import pathlib
 import string
 from itertools import chain
 from operator import neg as op_neg
+from typing import List, Pattern, Union
 
 import pytest
 from hypothesis import given
 from hypothesis.strategies import integers, lists, sampled_from, text
 from natsort import utils
-from natsort.ns_enum import ns
+from natsort.ns_enum import NS_t, ns
 
 
-def test_do_decoding_decodes_bytes_string_to_unicode():
+def test_do_decoding_decodes_bytes_string_to_unicode() -> None:
     assert type(utils.do_decoding(b"bytes", "ascii")) is str
     assert utils.do_decoding(b"bytes", "ascii") == "bytes"
     assert utils.do_decoding(b"bytes", "ascii") == b"bytes".decode("ascii")
@@ -33,7 +34,9 @@ def test_do_decoding_decodes_bytes_string_to_unicode():
         (ns.F | ns.S | ns.N, utils.NumericalRegularExpressions.float_sign_noexp()),
     ],
 )
-def test_regex_chooser_returns_correct_regular_expression_object(alg, expected):
+def test_regex_chooser_returns_correct_regular_expression_object(
+    alg: NS_t, expected: Pattern[str]
+) -> None:
     assert utils.regex_chooser(alg).pattern == expected.pattern
 
 
@@ -68,21 +71,21 @@ def test_regex_chooser_returns_correct_regular_expression_object(alg, expected):
         (ns.REAL, ns.FLOAT | ns.SIGNED),
     ],
 )
-def test_ns_enum_values_and_aliases(alg, value_or_alias):
+def test_ns_enum_values_and_aliases(alg: NS_t, value_or_alias: NS_t) -> None:
     assert alg == value_or_alias
 
 
-def test_chain_functions_is_a_no_op_if_no_functions_are_given():
+def test_chain_functions_is_a_no_op_if_no_functions_are_given() -> None:
     x = 2345
     assert utils.chain_functions([])(x) is x
 
 
-def test_chain_functions_does_one_function_if_one_function_is_given():
+def test_chain_functions_does_one_function_if_one_function_is_given() -> None:
     x = "2345"
     assert utils.chain_functions([len])(x) == 4
 
 
-def test_chain_functions_combines_functions_in_given_order():
+def test_chain_functions_combines_functions_in_given_order() -> None:
     x = 2345
     assert utils.chain_functions([str, len, op_neg])(x) == -len(str(x))
 
@@ -91,33 +94,37 @@ def test_chain_functions_combines_functions_in_given_order():
 # and a test that uses the hypothesis module.
 
 
-def test_groupletters_returns_letters_with_lowercase_transform_of_letter_example():
+def test_groupletters_returns_letters_with_lowercase_transform_of_letter_example() -> None:
     assert utils.groupletters("HELLO") == "hHeElLlLoO"
     assert utils.groupletters("hello") == "hheelllloo"
 
 
 @given(text().filter(bool))
-def test_groupletters_returns_letters_with_lowercase_transform_of_letter(x):
+def test_groupletters_returns_letters_with_lowercase_transform_of_letter(
+    x: str,
+) -> None:
     assert utils.groupletters(x) == "".join(
         chain.from_iterable([y.casefold(), y] for y in x)
     )
 
 
-def test_sep_inserter_does_nothing_if_no_numbers_example():
+def test_sep_inserter_does_nothing_if_no_numbers_example() -> None:
     assert list(utils.sep_inserter(iter(["a", "b", "c"]), "")) == ["a", "b", "c"]
     assert list(utils.sep_inserter(iter(["a"]), "")) == ["a"]
 
 
-def test_sep_inserter_does_nothing_if_only_one_number_example():
+def test_sep_inserter_does_nothing_if_only_one_number_example() -> None:
     assert list(utils.sep_inserter(iter(["a", 5]), "")) == ["a", 5]
 
 
-def test_sep_inserter_inserts_separator_string_between_two_numbers_example():
+def test_sep_inserter_inserts_separator_string_between_two_numbers_example() -> None:
     assert list(utils.sep_inserter(iter([5, 9]), "")) == ["", 5, "", 9]
 
 
 @given(lists(elements=text().filter(bool) | integers(), min_size=3))
-def test_sep_inserter_inserts_separator_between_two_numbers(x):
+def test_sep_inserter_inserts_separator_between_two_numbers(
+    x: List[Union[str, int]]
+) -> None:
     # Rather than just replicating the results in a different algorithm,
     # validate that the "shape" of the output is as expected.
     result = list(utils.sep_inserter(iter(x), ""))
@@ -127,28 +134,29 @@ def test_sep_inserter_inserts_separator_between_two_numbers(x):
             assert isinstance(result[i + 1], int)
 
 
-def test_path_splitter_splits_path_string_by_separator_example():
+def test_path_splitter_splits_path_string_by_separator_example() -> None:
     given = "/this/is/a/path"
     expected = (os.sep, "this", "is", "a", "path")
     assert tuple(utils.path_splitter(given)) == tuple(expected)
-    given = pathlib.Path(given)
-    assert tuple(utils.path_splitter(given)) == tuple(expected)
+    assert tuple(utils.path_splitter(pathlib.Path(given))) == tuple(expected)
 
 
 @given(lists(sampled_from(string.ascii_letters), min_size=2).filter(all))
-def test_path_splitter_splits_path_string_by_separator(x):
+def test_path_splitter_splits_path_string_by_separator(x: List[str]) -> None:
     z = str(pathlib.Path(*x))
     assert tuple(utils.path_splitter(z)) == tuple(pathlib.Path(z).parts)
 
 
-def test_path_splitter_splits_path_string_by_separator_and_removes_extension_example():
+def test_path_splitter_splits_path_string_by_separator_and_removes_extension_example() -> None:
     given = "/this/is/a/path/file.x1.10.tar.gz"
     expected = (os.sep, "this", "is", "a", "path", "file.x1.10", ".tar", ".gz")
     assert tuple(utils.path_splitter(given)) == tuple(expected)
 
 
 @given(lists(sampled_from(string.ascii_letters), min_size=3).filter(all))
-def test_path_splitter_splits_path_string_by_separator_and_removes_extension(x):
+def test_path_splitter_splits_path_string_by_separator_and_removes_extension(
+    x: List[str],
+) -> None:
     z = str(pathlib.Path(*x[:-2])) + "." + x[-1]
     y = tuple(pathlib.Path(z).parts)
     assert tuple(utils.path_splitter(z)) == y[:-1] + (
diff --git a/tox.ini b/tox.ini
index fb2c58f..7bc66bc 100644
--- a/tox.ini
+++ b/tox.ini
@@ -56,8 +56,11 @@ skip_install = true
 [testenv:mypy]
 deps =
     mypy
+    hypothesis
+    pytest
+    pytest-mock
 commands =
-    mypy --strict natsort
+    mypy --strict natsort tests
 skip_install = true
 
 # Build documentation.