diff --git a/.github/workflows/main.yml b/.github/workflows/main.yml
index c447f34..bdfda45 100644
--- a/.github/workflows/main.yml
+++ b/.github/workflows/main.yml
@@ -11,8 +11,8 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        python: ["2.7", "3.6", "3.7", "3.8", "3.9", "3.10"]
-        toxenv: [py]
+        python: ["3.6", "3.7", "3.8", "3.9", "3.10"]
+        toxenv: [py, py-type]
         include:
           - python: "3.10"
             toxenv: flake8
diff --git a/poetry.lock b/poetry.lock
index b7d1e5b..c3f190a 100644
--- a/poetry.lock
+++ b/poetry.lock
@@ -1,6 +1,6 @@
 [[package]]
 name = "appnope"
-version = "0.1.2"
+version = "0.1.3"
 description = "Disable App Nap on macOS >= 10.9"
 category = "dev"
 optional = false
@@ -16,17 +16,17 @@ python-versions = ">=2.7, !=3.0.*, !=3.1.*, !=3.2.*, !=3.3.*"
 
 [[package]]
 name = "attrs"
-version = "21.2.0"
+version = "21.4.0"
 description = "Classes Without Boilerplate"
 category = "dev"
 optional = false
 python-versions = ">=2.7, !=3.0.*, !=3.1.*, !=3.2.*, !=3.3.*, !=3.4.*"
 
 [package.extras]
-dev = ["coverage[toml] (>=5.0.2)", "hypothesis", "pympler", "pytest (>=4.3.0)", "six", "mypy", "pytest-mypy-plugins", "zope.interface", "furo", "sphinx", "sphinx-notfound-page", "pre-commit"]
+dev = ["coverage[toml] (>=5.0.2)", "hypothesis", "pympler", "pytest (>=4.3.0)", "six", "mypy", "pytest-mypy-plugins", "zope.interface", "furo", "sphinx", "sphinx-notfound-page", "pre-commit", "cloudpickle"]
 docs = ["furo", "sphinx", "zope.interface", "sphinx-notfound-page"]
-tests = ["coverage[toml] (>=5.0.2)", "hypothesis", "pympler", "pytest (>=4.3.0)", "six", "mypy", "pytest-mypy-plugins", "zope.interface"]
-tests_no_zope = ["coverage[toml] (>=5.0.2)", "hypothesis", "pympler", "pytest (>=4.3.0)", "six", "mypy", "pytest-mypy-plugins"]
+tests = ["coverage[toml] (>=5.0.2)", "hypothesis", "pympler", "pytest (>=4.3.0)", "six", "mypy", "pytest-mypy-plugins", "zope.interface", "cloudpickle"]
+tests_no_zope = ["coverage[toml] (>=5.0.2)", "hypothesis", "pympler", "pytest (>=4.3.0)", "six", "mypy", "pytest-mypy-plugins", "cloudpickle"]
 
 [[package]]
 name = "backcall"
@@ -36,52 +36,35 @@ category = "dev"
 optional = false
 python-versions = "*"
 
-[[package]]
-name = "backports.entry-points-selectable"
-version = "1.1.0"
-description = "Compatibility shim providing selectable entry points for older implementations"
-category = "dev"
-optional = false
-python-versions = ">=2.7"
-
-[package.extras]
-docs = ["sphinx", "jaraco.packaging (>=8.2)", "rst.linker (>=1.9)"]
-testing = ["pytest (>=4.6)", "pytest-flake8", "pytest-cov", "pytest-black (>=0.3.7)", "pytest-mypy", "pytest-checkdocs (>=2.4)", "pytest-enabler (>=1.0.1)"]
-
 [[package]]
 name = "black"
-version = "21.9b0"
+version = "22.3.0"
 description = "The uncompromising code formatter."
 category = "dev"
 optional = false
 python-versions = ">=3.6.2"
 
 [package.dependencies]
-click = ">=7.1.2"
+click = ">=8.0.0"
 mypy-extensions = ">=0.4.3"
-pathspec = ">=0.9.0,<1"
+pathspec = ">=0.9.0"
 platformdirs = ">=2"
-regex = ">=2020.1.8"
-tomli = ">=0.2.6,<2.0.0"
-typing-extensions = [
-    {version = ">=3.10.0.0", markers = "python_version < \"3.10\""},
-    {version = "!=3.10.0.1", markers = "python_version >= \"3.10\""},
-]
+tomli = {version = ">=1.1.0", markers = "python_version < \"3.11\""}
+typing-extensions = {version = ">=3.10.0.0", markers = "python_version < \"3.10\""}
 
 [package.extras]
 colorama = ["colorama (>=0.4.3)"]
-d = ["aiohttp (>=3.6.0)", "aiohttp-cors (>=0.4.0)"]
+d = ["aiohttp (>=3.7.4)"]
 jupyter = ["ipython (>=7.8.0)", "tokenize-rt (>=3.2.0)"]
-python2 = ["typed-ast (>=1.4.2)"]
 uvloop = ["uvloop (>=0.15.2)"]
 
 [[package]]
 name = "click"
-version = "8.0.3"
+version = "8.1.2"
 description = "Composable command line interface toolkit"
 category = "dev"
 optional = false
-python-versions = ">=3.6"
+python-versions = ">=3.7"
 
 [package.dependencies]
 colorama = {version = "*", markers = "platform_system == \"Windows\""}
@@ -94,40 +77,20 @@ category = "dev"
 optional = false
 python-versions = ">=2.7, !=3.0.*, !=3.1.*, !=3.2.*, !=3.3.*, !=3.4.*"
 
-[[package]]
-name = "configparser"
-version = "4.0.2"
-description = "Updated configparser from Python 3.7 for Python 2.6+."
-category = "main"
-optional = false
-python-versions = ">=2.6"
-
-[package.extras]
-docs = ["sphinx", "jaraco.packaging (>=3.2)", "rst.linker (>=1.9)"]
-testing = ["pytest (>=3.5,!=3.7.3)", "pytest-checkdocs (>=1.2)", "pytest-flake8", "pytest-black-multipy"]
-
-[[package]]
-name = "contextlib2"
-version = "0.6.0.post1"
-description = "Backports and enhancements for the contextlib module"
-category = "main"
-optional = false
-python-versions = ">=2.7, !=3.0.*, !=3.1.*, !=3.2.*, !=3.3.*"
-
 [[package]]
 name = "coverage"
-version = "6.0.2"
+version = "6.3.2"
 description = "Code coverage measurement for Python"
 category = "dev"
 optional = false
-python-versions = ">=3.6"
+python-versions = ">=3.7"
 
 [package.extras]
 toml = ["tomli"]
 
 [[package]]
 name = "decorator"
-version = "5.1.0"
+version = "5.1.1"
 description = "Decorators for Humans"
 category = "dev"
 optional = false
@@ -135,7 +98,7 @@ python-versions = ">=3.5"
 
 [[package]]
 name = "distlib"
-version = "0.3.3"
+version = "0.3.4"
 description = "Distribution utilities"
 category = "dev"
 optional = false
@@ -151,11 +114,11 @@ python-versions = ">=2.7, !=3.0.*, !=3.1.*, !=3.2.*, !=3.3.*, !=3.4.*"
 
 [[package]]
 name = "filelock"
-version = "3.3.1"
+version = "3.6.0"
 description = "A platform independent file lock."
 category = "dev"
 optional = false
-python-versions = ">=3.6"
+python-versions = ">=3.7"
 
 [package.extras]
 docs = ["furo (>=2021.8.17b43)", "sphinx (>=4.1)", "sphinx-autodoc-typehints (>=1.12)"]
@@ -176,7 +139,7 @@ pyflakes = ">=2.4.0,<2.5.0"
 
 [[package]]
 name = "flake8-black"
-version = "0.2.3"
+version = "0.2.5"
 description = "flake8 plugin to call black as a code style validator"
 category = "dev"
 optional = false
@@ -189,7 +152,7 @@ toml = "*"
 
 [[package]]
 name = "flake8-bugbear"
-version = "21.9.2"
+version = "21.11.29"
 description = "A plugin for flake8 finding likely bugs and design problems in your program. Contains warnings that don't belong in pyflakes and pycodestyle."
 category = "dev"
 optional = false
@@ -200,18 +163,18 @@ attrs = ">=19.2.0"
 flake8 = ">=3.0.0"
 
 [package.extras]
-dev = ["coverage", "black", "hypothesis", "hypothesmith"]
+dev = ["coverage", "hypothesis", "hypothesmith (>=0.2)", "pre-commit"]
 
 [[package]]
 name = "flake8-comprehensions"
-version = "3.7.0"
+version = "3.8.0"
 description = "A flake8 plugin to help you write better list/set/dict comprehensions."
 category = "dev"
 optional = false
-python-versions = ">=3.6"
+python-versions = ">=3.7"
 
 [package.dependencies]
-flake8 = ">=3.0,<3.2.0 || >3.2.0,<5"
+flake8 = ">=3.0,<3.2.0 || >3.2.0"
 
 [[package]]
 name = "flake8-docstrings"
@@ -276,16 +239,13 @@ flake8-plugin-utils = ">=1.0,<2.0"
 
 [[package]]
 name = "importlib-metadata"
-version = "2.1.1"
+version = "2.1.3"
 description = "Read metadata from Python packages"
 category = "main"
 optional = false
 python-versions = "!=3.0.*,!=3.1.*,!=3.2.*,!=3.3.*,!=3.4.*,>=2.7"
 
 [package.dependencies]
-configparser = {version = ">=3.5", markers = "python_version < \"3\""}
-contextlib2 = {version = "*", markers = "python_version < \"3\""}
-pathlib2 = {version = "*", markers = "python_version < \"3\""}
 zipp = ">=0.5"
 
 [package.extras]
@@ -302,7 +262,7 @@ python-versions = "*"
 
 [[package]]
 name = "ipython"
-version = "7.28.0"
+version = "7.32.0"
 description = "IPython: Productive Interactive Computing"
 category = "dev"
 optional = false
@@ -334,7 +294,7 @@ test = ["nose (>=0.10.1)", "requests", "testpath", "pygments", "nbformat", "ipyk
 
 [[package]]
 name = "jedi"
-version = "0.18.0"
+version = "0.18.1"
 description = "An autocompletion tool for Python that can be used for text editors."
 category = "dev"
 optional = false
@@ -345,7 +305,7 @@ parso = ">=0.8.0,<0.9.0"
 
 [package.extras]
 qa = ["flake8 (==3.8.3)", "mypy (==0.782)"]
-testing = ["Django (<3.1)", "colorama", "docopt", "pytest (<6.0.0)"]
+testing = ["Django (<3.1)", "colorama", "docopt", "pytest (<7.0.0)"]
 
 [[package]]
 name = "matplotlib-inline"
@@ -366,6 +326,24 @@ category = "dev"
 optional = false
 python-versions = "*"
 
+[[package]]
+name = "mypy"
+version = "0.942"
+description = "Optional static typing for Python"
+category = "dev"
+optional = false
+python-versions = ">=3.6"
+
+[package.dependencies]
+mypy-extensions = ">=0.4.3"
+tomli = ">=1.1.0"
+typing-extensions = ">=3.10"
+
+[package.extras]
+dmypy = ["psutil (>=4.0)"]
+python2 = ["typed-ast (>=1.4.0,<2)"]
+reports = ["lxml"]
+
 [[package]]
 name = "mypy-extensions"
 version = "0.4.3"
@@ -376,18 +354,18 @@ python-versions = "*"
 
 [[package]]
 name = "packaging"
-version = "21.0"
+version = "21.3"
 description = "Core utilities for Python packages"
 category = "dev"
 optional = false
 python-versions = ">=3.6"
 
 [package.dependencies]
-pyparsing = ">=2.0.2"
+pyparsing = ">=2.0.2,<3.0.5 || >3.0.5"
 
 [[package]]
 name = "parso"
-version = "0.8.2"
+version = "0.8.3"
 description = "A Python Parser"
 category = "dev"
 optional = false
@@ -397,18 +375,6 @@ python-versions = ">=3.6"
 qa = ["flake8 (==3.8.3)", "mypy (==0.782)"]
 testing = ["docopt", "pytest (<6.0.0)"]
 
-[[package]]
-name = "pathlib2"
-version = "2.3.6"
-description = "Object-oriented filesystem paths"
-category = "main"
-optional = false
-python-versions = "*"
-
-[package.dependencies]
-scandir = {version = "*", markers = "python_version < \"3.5\""}
-six = "*"
-
 [[package]]
 name = "pathspec"
 version = "0.9.0"
@@ -450,11 +416,11 @@ python-versions = "*"
 
 [[package]]
 name = "platformdirs"
-version = "2.4.0"
+version = "2.5.1"
 description = "A small Python module for determining appropriate platform-specific dirs, e.g. a \"user data dir\"."
 category = "dev"
 optional = false
-python-versions = ">=3.6"
+python-versions = ">=3.7"
 
 [package.extras]
 docs = ["Sphinx (>=4)", "furo (>=2021.7.5b38)", "proselint (>=0.10.2)", "sphinx-autodoc-typehints (>=1.12)"]
@@ -474,7 +440,7 @@ testing = ["pytest", "pytest-benchmark"]
 
 [[package]]
 name = "prompt-toolkit"
-version = "3.0.21"
+version = "3.0.29"
 description = "Library for building powerful interactive command lines in Python"
 category = "dev"
 optional = false
@@ -493,11 +459,11 @@ python-versions = "*"
 
 [[package]]
 name = "py"
-version = "1.10.0"
+version = "1.11.0"
 description = "library with cross-python path, ini-parsing, io, code, log facilities"
 category = "dev"
 optional = false
-python-versions = ">=2.7, !=3.0.*, !=3.1.*, !=3.2.*, !=3.3.*"
+python-versions = ">=2.7, !=3.0.*, !=3.1.*, !=3.2.*, !=3.3.*, !=3.4.*"
 
 [[package]]
 name = "pycodestyle"
@@ -531,7 +497,7 @@ python-versions = ">=2.7, !=3.0.*, !=3.1.*, !=3.2.*, !=3.3.*"
 
 [[package]]
 name = "pygments"
-version = "2.10.0"
+version = "2.11.2"
 description = "Pygments is a syntax highlighting package written in Python."
 category = "dev"
 optional = false
@@ -539,14 +505,14 @@ python-versions = ">=3.5"
 
 [[package]]
 name = "pyparsing"
-version = "3.0.0"
-description = "Python parsing module"
+version = "3.0.8"
+description = "pyparsing module - Classes and methods to define and execute parsing grammars"
 category = "dev"
 optional = false
-python-versions = ">=3.6"
+python-versions = ">=3.6.8"
 
 [package.extras]
-diagrams = ["jinja2", "railroad-diagrams"]
+diagrams = ["railroad-diagrams", "jinja2"]
 
 [[package]]
 name = "pytest"
@@ -569,17 +535,9 @@ toml = "*"
 [package.extras]
 testing = ["argcomplete", "hypothesis (>=3.56)", "mock", "nose", "requests", "xmlschema"]
 
-[[package]]
-name = "regex"
-version = "2021.10.23"
-description = "Alternative regular expression module, to replace re."
-category = "dev"
-optional = false
-python-versions = "*"
-
 [[package]]
 name = "rstcheck"
-version = "3.3.1"
+version = "3.5.0"
 description = "Checks syntax of reStructuredText and code blocks nested within it"
 category = "dev"
 optional = false
@@ -588,14 +546,6 @@ python-versions = "*"
 [package.dependencies]
 docutils = ">=0.7"
 
-[[package]]
-name = "scandir"
-version = "1.10.0"
-description = "scandir, a better directory iterator and faster os.walk()"
-category = "main"
-optional = false
-python-versions = "*"
-
 [[package]]
 name = "six"
 version = "1.16.0"
@@ -606,7 +556,7 @@ python-versions = ">=2.7, !=3.0.*, !=3.1.*, !=3.2.*"
 
 [[package]]
 name = "snowballstemmer"
-version = "2.1.0"
+version = "2.2.0"
 description = "This package provides 29 stemmers for 28 languages generated from Snowball algorithms."
 category = "dev"
 optional = false
@@ -622,15 +572,15 @@ python-versions = ">=2.6, !=3.0.*, !=3.1.*, !=3.2.*"
 
 [[package]]
 name = "tomli"
-version = "1.2.1"
+version = "2.0.1"
 description = "A lil' TOML parser"
 category = "dev"
 optional = false
-python-versions = ">=3.6"
+python-versions = ">=3.7"
 
 [[package]]
 name = "tox"
-version = "3.24.4"
+version = "3.25.0"
 description = "tox is a generic virtualenv management and test command line tool"
 category = "dev"
 optional = false
@@ -648,11 +598,11 @@ virtualenv = ">=16.0.0,<20.0.0 || >20.0.0,<20.0.1 || >20.0.1,<20.0.2 || >20.0.2,
 
 [package.extras]
 docs = ["pygments-github-lexers (>=0.0.5)", "sphinx (>=2.0.0)", "sphinxcontrib-autoprogram (>=0.1.5)", "towncrier (>=18.5.0)"]
-testing = ["flaky (>=3.4.0)", "freezegun (>=0.3.11)", "psutil (>=5.6.1)", "pytest (>=4.0.0)", "pytest-cov (>=2.5.1)", "pytest-mock (>=1.10.0)", "pytest-randomly (>=1.0.0)", "pytest-xdist (>=1.22.2)", "pathlib2 (>=2.3.3)"]
+testing = ["flaky (>=3.4.0)", "freezegun (>=0.3.11)", "pytest (>=4.0.0)", "pytest-cov (>=2.5.1)", "pytest-mock (>=1.10.0)", "pytest-randomly (>=1.0.0)", "psutil (>=5.6.1)", "pathlib2 (>=2.3.3)"]
 
 [[package]]
 name = "traitlets"
-version = "5.1.0"
+version = "5.1.1"
 description = "Traitlets Python configuration system"
 category = "dev"
 optional = false
@@ -663,29 +613,28 @@ test = ["pytest"]
 
 [[package]]
 name = "typing-extensions"
-version = "3.10.0.2"
-description = "Backported and Experimental Type Hints for Python 3.5+"
+version = "4.1.1"
+description = "Backported and Experimental Type Hints for Python 3.6+"
 category = "dev"
 optional = false
-python-versions = "*"
+python-versions = ">=3.6"
 
 [[package]]
 name = "virtualenv"
-version = "20.9.0"
+version = "20.14.1"
 description = "Virtual Python Environment builder"
 category = "dev"
 optional = false
 python-versions = "!=3.0.*,!=3.1.*,!=3.2.*,!=3.3.*,!=3.4.*,>=2.7"
 
 [package.dependencies]
-"backports.entry-points-selectable" = ">=1.0.4"
 distlib = ">=0.3.1,<1"
 filelock = ">=3.2,<4"
 platformdirs = ">=2,<3"
 six = ">=1.9.0,<2"
 
 [package.extras]
-docs = ["proselint (>=0.10.2)", "sphinx (>=3)", "sphinx-argparse (>=0.2.5)", "sphinx-rtd-theme (>=0.4.3)", "towncrier (>=19.9.0rc1)"]
+docs = ["proselint (>=0.10.2)", "sphinx (>=3)", "sphinx-argparse (>=0.2.5)", "sphinx-rtd-theme (>=0.4.3)", "towncrier (>=21.3)"]
 testing = ["coverage (>=4)", "coverage-enable-subprocess (>=1)", "flaky (>=3)", "pytest (>=4)", "pytest-env (>=0.6.2)", "pytest-freezegun (>=0.4.1)", "pytest-mock (>=2)", "pytest-randomly (>=1)", "pytest-timeout (>=1)", "packaging (>=20.0)"]
 
 [[package]]
@@ -704,125 +653,139 @@ category = "main"
 optional = false
 python-versions = ">=2.7"
 
-[package.dependencies]
-contextlib2 = {version = "*", markers = "python_version < \"3.4\""}
-
 [package.extras]
 docs = ["sphinx", "jaraco.packaging (>=3.2)", "rst.linker (>=1.9)"]
 testing = ["pathlib2", "unittest2", "jaraco.itertools", "func-timeout"]
 
 [metadata]
 lock-version = "1.1"
-python-versions = "~2.7 || ^3.5"
-content-hash = "5bda3522682ddac475f32fd4de0a08658f91040ba8e32a37192271b2af6abe0c"
+python-versions = "^3.5"
+content-hash = "980f0772bf3b2936ed86f78bd4c7d993349b14754245c6d557bcc7abc50c8728"
 
 [metadata.files]
 appnope = [
-    {file = "appnope-0.1.2-py2.py3-none-any.whl", hash = "sha256:93aa393e9d6c54c5cd570ccadd8edad61ea0c4b9ea7a01409020c9aa019eb442"},
-    {file = "appnope-0.1.2.tar.gz", hash = "sha256:dd83cd4b5b460958838f6eb3000c660b1f9caf2a5b1de4264e941512f603258a"},
+    {file = "appnope-0.1.3-py2.py3-none-any.whl", hash = "sha256:265a455292d0bd8a72453494fa24df5a11eb18373a60c7c0430889f22548605e"},
+    {file = "appnope-0.1.3.tar.gz", hash = "sha256:02bd91c4de869fbb1e1c50aafc4098827a7a54ab2f39d9dcba6c9547ed920e24"},
 ]
 atomicwrites = [
     {file = "atomicwrites-1.4.0-py2.py3-none-any.whl", hash = "sha256:6d1784dea7c0c8d4a5172b6c620f40b6e4cbfdf96d783691f2e1302a7b88e197"},
     {file = "atomicwrites-1.4.0.tar.gz", hash = "sha256:ae70396ad1a434f9c7046fd2dd196fc04b12f9e91ffb859164193be8b6168a7a"},
 ]
 attrs = [
-    {file = "attrs-21.2.0-py2.py3-none-any.whl", hash = "sha256:149e90d6d8ac20db7a955ad60cf0e6881a3f20d37096140088356da6c716b0b1"},
-    {file = "attrs-21.2.0.tar.gz", hash = "sha256:ef6aaac3ca6cd92904cdd0d83f629a15f18053ec84e6432106f7a4d04ae4f5fb"},
+    {file = "attrs-21.4.0-py2.py3-none-any.whl", hash = "sha256:2d27e3784d7a565d36ab851fe94887c5eccd6a463168875832a1be79c82828b4"},
+    {file = "attrs-21.4.0.tar.gz", hash = "sha256:626ba8234211db98e869df76230a137c4c40a12d72445c45d5f5b716f076e2fd"},
 ]
 backcall = [
     {file = "backcall-0.2.0-py2.py3-none-any.whl", hash = "sha256:fbbce6a29f263178a1f7915c1940bde0ec2b2a967566fe1c65c1dfb7422bd255"},
     {file = "backcall-0.2.0.tar.gz", hash = "sha256:5cbdbf27be5e7cfadb448baf0aa95508f91f2bbc6c6437cd9cd06e2a4c215e1e"},
 ]
-"backports.entry-points-selectable" = [
-    {file = "backports.entry_points_selectable-1.1.0-py2.py3-none-any.whl", hash = "sha256:a6d9a871cde5e15b4c4a53e3d43ba890cc6861ec1332c9c2428c92f977192acc"},
-    {file = "backports.entry_points_selectable-1.1.0.tar.gz", hash = "sha256:988468260ec1c196dab6ae1149260e2f5472c9110334e5d51adcb77867361f6a"},
-]
 black = [
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-    {file = "typing_extensions-3.10.0.2.tar.gz", hash = "sha256:49f75d16ff11f1cd258e1b988ccff82a3ca5570217d7ad8c5f48205dd99a677e"},
+    {file = "typing_extensions-4.1.1-py3-none-any.whl", hash = "sha256:21c85e0fe4b9a155d0799430b0ad741cdce7e359660ccbd8b530613e8df88ce2"},
+    {file = "typing_extensions-4.1.1.tar.gz", hash = "sha256:1a9462dcc3347a79b1f1c0271fbe79e844580bb598bafa1ed208b94da3cdcd42"},
 ]
 virtualenv = [
-    {file = "virtualenv-20.9.0-py2.py3-none-any.whl", hash = "sha256:1d145deec2da86b29026be49c775cc5a9aab434f85f7efef98307fb3965165de"},
-    {file = "virtualenv-20.9.0.tar.gz", hash = "sha256:bb55ace18de14593947354e5e6cd1be75fb32c3329651da62e92bf5d0aab7213"},
+    {file = "virtualenv-20.14.1-py2.py3-none-any.whl", hash = "sha256:e617f16e25b42eb4f6e74096b9c9e37713cf10bf30168fb4a739f3fa8f898a3a"},
+    {file = "virtualenv-20.14.1.tar.gz", hash = "sha256:ef589a79795589aada0c1c5b319486797c03b67ac3984c48c669c0e4f50df3a5"},
 ]
 wcwidth = [
     {file = "wcwidth-0.2.5-py2.py3-none-any.whl", hash = "sha256:beb4802a9cebb9144e99086eff703a642a13d6a0052920003a230f3294bbe784"},
diff --git a/pyproject.toml b/pyproject.toml
index 6d0b461..42ff47b 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -1,6 +1,6 @@
 [tool.poetry]
 name = "flatten-dict"
-version = "0.4.3"
+version = "0.5.3"
 description = "A flexible utility for flattening and unflattening dict-like objects in Python."
 authors = ["Ian Lin <you@example.com>"]
 license = "MIT"
@@ -12,14 +12,14 @@ classifiers = [
 ]
 
 [tool.poetry.dependencies]
-python = "~2.7 || ^3.5"
+python = "^3.5"
 six = "^1.12"
-pathlib2 = { version = "^2.3", python = "<3.4" }
 importlib-metadata = {version = "*", python = "<3.8"}
 
 [tool.poetry.dev-dependencies]
 pytest = { version = "^6.0", python = "^3.8" }
 flake8 = { version = "^4.0", python = "^3.8" }
+mypy = { version = "^0.942", python = "^3.8" }
 pep8-naming = { version = "^0.12.0", python = "^3.8"}
 flake8-import-order = { version = "^0.18.1", python = "^3.8"}
 flake8-bugbear = { version = "^21.4.0", python = "^3.8"}
@@ -30,7 +30,7 @@ flake8-return = { version = "^1.1.2", python = "^3.8"}
 flake8-black = { version = "^0.2.1", python = "^3.8"}
 tox = { version = "^3.19", python = "^3.8" }
 coverage = { version = "^6.0", python = "^3.8" }
-black = { version = "= 21.9b0", python = "^3.8" }
+black = { version = "= 22.3.0", python = "^3.8" }
 rstcheck = { version = "^3.3.1", python = "^3.8" }
 docutils = { version = "^0.17", python = "^3.8" }
 ipython = { version = "^7.26.0", python = "^3.8" }
diff --git a/setup.cfg b/setup.cfg
index 91552b9..abcd3d8 100644
--- a/setup.cfg
+++ b/setup.cfg
@@ -85,3 +85,12 @@ max-line-length = 99
 import-order-style = pep8
 application-import-names = flatten_dict
 docstring-convention = numpy
+
+
+[mypy]
+warn_return_any = True
+warn_unused_configs = True
+disallow_untyped_defs = True
+
+[mypy-pytest,six,importlib_metadata]
+ignore_missing_imports = True
diff --git a/src/flatten_dict/__init__.py b/src/flatten_dict/__init__.py
index bfcfb93..0f2564e 100644
--- a/src/flatten_dict/__init__.py
+++ b/src/flatten_dict/__init__.py
@@ -1,12 +1,25 @@
-from .flatten_dict import flatten, unflatten  # noqa: F401
+import sys
 
+from .flatten_dict import (
+    flatten,
+    unflatten,
+    TReducerCallable,
+    TSplitterCallable,
+)  # noqa: F401
 
-__all__ = ["flatten", "unflatten", "splitter"]
 
-try:
+__all__ = [
+    "flatten",
+    "unflatten",
+    "splitter",
+    "TReducerCallable",
+    "TSplitterCallable",
+]
+
+if sys.version_info >= (3, 8):
     # for Python >= 3.8
     from importlib.metadata import version
-except ImportError:
+else:
     # for Python < 3.8, the package importlib-metadata will be installed
     from importlib_metadata import version
 
diff --git a/src/flatten_dict/flatten_dict.py b/src/flatten_dict/flatten_dict.py
index e392fc5..2f125f0 100644
--- a/src/flatten_dict/flatten_dict.py
+++ b/src/flatten_dict/flatten_dict.py
@@ -1,24 +1,64 @@
 import inspect
+import sys
+from collections.abc import Mapping
+from typing import (
+    cast,
+    Any,
+    Callable,
+    Dict,
+    Hashable,
+    Iterable,
+    Optional,
+    Sequence,
+    Union,
+    Tuple,
+    TypeVar,
+    overload,
+)
+import typing
+
+if sys.version_info < (3, 8):
+    from typing_extensions import Literal
+else:
+    from typing import Literal
 
-try:
-    from collections.abc import Mapping
-except ImportError:
-    from collections import Mapping
 
 import six
 
 from .reducers import tuple_reducer, path_reducer, dot_reducer, underscore_reducer
 from .splitters import tuple_splitter, path_splitter, dot_splitter, underscore_splitter
 
+TKeyIn = TypeVar("TKeyIn", bound=Hashable)
+TKeyIn2 = TypeVar("TKeyIn2", bound=Hashable)
+TKeyOut = TypeVar("TKeyOut", bound=Hashable)
+
+TReducerCallable2Args = Callable[
+    [Optional[Union[TKeyIn, TKeyOut]], Union[TKeyIn, int]], TKeyOut
+]
+
+TReducerCallable3Args = Callable[
+    [
+        Optional[Union[TKeyIn, TKeyOut]],
+        Union[TKeyIn, int],
+        Union[typing.Mapping[TKeyIn, Any], Iterable[Any]],
+    ],
+    TKeyOut,
+]
+
+
+TReducerCallable = Union[TReducerCallable2Args, TReducerCallable3Args]
+
+TSplitterCallable = Callable[[TKeyIn], Tuple[TKeyOut, ...]]
 
-REDUCER_DICT = {
+
+REDUCER_DICT: Dict[str, TReducerCallable2Args] = {
     "tuple": tuple_reducer,
     "path": path_reducer,
     "dot": dot_reducer,
     "underscore": underscore_reducer,
 }
 
-SPLITTER_DICT = {
+SPLITTER_DICT: Dict[str, TSplitterCallable] = {
     "tuple": tuple_splitter,
     "path": path_splitter,
     "dot": dot_splitter,
@@ -26,14 +66,118 @@
 }
 
 
+@overload
 def flatten(
-    d,
-    reducer="tuple",
-    inverse=False,
-    max_flatten_depth=None,
-    enumerate_types=(),
-    keep_empty_types=(),
-):
+    d: typing.Mapping[TKeyIn, Any],
+    reducer: Union[
+        TReducerCallable2Args[TKeyIn, TKeyOut], TReducerCallable3Args[TKeyIn, TKeyOut]
+    ],
+    inverse: bool = False,
+    max_flatten_depth: Optional[int] = None,
+    enumerate_types: Tuple[type, ...] = (),
+    keep_empty_types: Tuple[type, ...] = (),
+) -> Dict[TKeyOut, Any]:
+    pass
+
+
+@overload
+def flatten(
+    d: typing.Mapping[TKeyIn, Any],
+    reducer: Literal["tuple"] = "tuple",
+    inverse: bool = False,
+    max_flatten_depth: Optional[int] = None,
+    enumerate_types: Tuple[type, ...] = (),
+    keep_empty_types: Tuple[type, ...] = (),
+) -> Dict[Tuple[Any, ...], Any]:
+    pass
+
+
+@overload
+def flatten(
+    d: typing.Mapping[TKeyIn, Any],
+    reducer: Literal["dot", "path", "underscore"],
+    inverse: bool = False,
+    max_flatten_depth: Optional[int] = None,
+    enumerate_types: Tuple[type, ...] = (),
+    keep_empty_types: Tuple[type, ...] = (),
+) -> Dict[Union[TKeyIn, str], Any]:
+    pass
+
+
+@overload
+def flatten(
+    d: typing.Mapping[TKeyIn, Any],
+    reducer: str = "tuple",
+    inverse: bool = False,
+    max_flatten_depth: Optional[int] = None,
+    enumerate_types: Tuple[type, ...] = (),
+    keep_empty_types: Tuple[type, ...] = (),
+) -> Union[Dict[Tuple[Any, ...], Any], Dict[Union[TKeyIn, str], Any]]:
+    pass
+
+
+@overload
+def flatten(
+    d: Sequence[Any],
+    reducer: Union[
+        TReducerCallable2Args[TKeyIn, TKeyOut], TReducerCallable3Args[TKeyIn, TKeyOut]
+    ],
+    inverse: bool = False,
+    max_flatten_depth: Optional[int] = None,
+    enumerate_types: Tuple[type, ...] = (),
+    keep_empty_types: Tuple[type, ...] = (),
+) -> Dict[TKeyOut, Any]:
+    pass
+
+
+@overload
+def flatten(
+    d: Sequence[Any],
+    reducer: Literal["tuple"] = "tuple",
+    inverse: bool = False,
+    max_flatten_depth: Optional[int] = None,
+    enumerate_types: Tuple[type, ...] = (),
+    keep_empty_types: Tuple[type, ...] = (),
+) -> Dict[Tuple[Any, ...], Any]:
+    pass
+
+
+@overload
+def flatten(
+    d: Sequence[Any],
+    reducer: Literal["dot", "path", "underscore"],
+    inverse: bool = False,
+    max_flatten_depth: Optional[int] = None,
+    enumerate_types: Tuple[type, ...] = (),
+    keep_empty_types: Tuple[type, ...] = (),
+) -> Dict[Union[int, str], Any]:
+    pass
+
+
+@overload
+def flatten(
+    d: Sequence[Any],
+    reducer: str = "tuple",
+    inverse: bool = False,
+    max_flatten_depth: Optional[int] = None,
+    enumerate_types: Tuple[type, ...] = (),
+    keep_empty_types: Tuple[type, ...] = (),
+) -> Union[Dict[Tuple[Any, ...], Any], Dict[Union[int, str], Any]]:
+    pass
+
+
+def flatten(
+    d: Any,
+    reducer: Union[
+        str,
+        TReducerCallable2Args[TKeyIn, TKeyOut],
+        TReducerCallable3Args[TKeyIn, TKeyOut],
+    ] = "tuple",
+    inverse: bool = False,
+    max_flatten_depth: Optional[int] = None,
+    enumerate_types: Tuple[type, ...] = (),
+    keep_empty_types: Tuple[type, ...] = (),
+) -> Any:
     """Flatten `Mapping` object.
 
     Parameters
@@ -81,27 +225,34 @@ def flatten(
     if max_flatten_depth is not None and max_flatten_depth < 1:
         raise ValueError("max_flatten_depth should not be less than 1.")
 
-    if isinstance(reducer, str):
-        reducer = REDUCER_DICT[reducer]
-    try:
-        # Python 3
-        reducer_accepts_parent_obj = len(inspect.signature(reducer).parameters) == 3
-    except AttributeError:
-        # Python 2
-        reducer_accepts_parent_obj = len(inspect.getargspec(reducer)[0]) == 3
+    reducer = REDUCER_DICT[reducer] if isinstance(reducer, str) else reducer
+    _reducer2: Optional[TReducerCallable2Args[TKeyIn, TKeyOut]] = None
+    _reducer3: Optional[TReducerCallable3Args[TKeyIn, TKeyOut]] = None
+    if len(inspect.signature(reducer).parameters) == 3:
+        _reducer3 = cast(TReducerCallable3Args[TKeyIn, TKeyOut], reducer)
+    else:
+        _reducer2 = cast(TReducerCallable2Args[TKeyIn, TKeyOut], reducer)
+
     flat_dict = {}
 
-    def _flatten(_d, depth, parent=None):
-        key_value_iterable = (
-            enumerate(_d) if isinstance(_d, enumerate_types) else six.viewitems(_d)
+    def _flatten(
+        _d: Union[
+            typing.Mapping[TKeyIn, Any], Any
+        ],  # either mapping or one of enumerate_types
+        depth: int,
+        parent: TKeyOut = None,
+    ) -> bool:
+        key_value_iterable: Iterable[Tuple[Union[int, TKeyIn], Any]] = (
+            enumerate(_d) if isinstance(_d, enumerate_types) else _d.items()
         )
         has_item = False
         for key, value in key_value_iterable:
             has_item = True
-            if reducer_accepts_parent_obj:
-                flat_key = reducer(parent, key, _d)
-            else:
-                flat_key = reducer(parent, key)
+            if _reducer3 is not None:
+                flat_key = _reducer3(parent, key, _d)
+            elif _reducer2 is not None:
+                flat_key = _reducer2(parent, key)
+
             if isinstance(value, flattenable_types) and (
                 max_flatten_depth is None or depth < max_flatten_depth
             ):
@@ -125,7 +276,7 @@ def _flatten(_d, depth, parent=None):
     return flat_dict
 
 
-def nested_set_dict(d, keys, value):
+def nested_set_dict(d: Dict[TKeyIn, Any], keys: Sequence[TKeyIn], value: Any) -> None:
     """Set a value to a sequence of nested keys.
 
     Parameters
@@ -145,7 +296,43 @@ def nested_set_dict(d, keys, value):
     nested_set_dict(d, keys[1:], value)
 
 
-def unflatten(d, splitter="tuple", inverse=False):
+@overload
+def unflatten(
+    d: typing.Mapping[str, Any],
+    splitter: Literal["dot", "underscore", "path"],
+    inverse: bool = False,
+) -> Dict[str, Any]:
+    pass
+
+
+@overload
+def unflatten(
+    d: typing.Mapping[Tuple[TKeyIn, ...], Any],
+    splitter: Literal["tuple"] = "tuple",
+    inverse: bool = False,
+) -> Dict[TKeyIn, Any]:
+    pass
+
+
+@overload
+def unflatten(
+    d: Union[typing.Mapping[str, Any], typing.Mapping[Tuple[TKeyIn, ...], Any]],
+    splitter: str = "tuple",
+    inverse: bool = False,
+) -> Union[Dict[str, Any], Dict[TKeyIn, Any]]:
+    pass
+
+
+@overload
+def unflatten(
+    d: typing.Mapping[TKeyIn, Any],
+    splitter: Callable[[TKeyIn], Tuple[TKeyOut, ...]],
+    inverse: bool = False,
+) -> Dict[TKeyOut, Any]:
+    pass
+
+
+def unflatten(d: Any, splitter: Any = "tuple", inverse: bool = False) -> Dict[Any, Any]:
     """Unflatten dict-like object.
 
     Parameters
@@ -169,7 +356,7 @@ def unflatten(d, splitter="tuple", inverse=False):
     if isinstance(splitter, str):
         splitter = SPLITTER_DICT[splitter]
 
-    unflattened_dict = {}
+    unflattened_dict: Dict[Any, Any] = {}
     for flat_key, value in six.viewitems(d):
         if inverse:
             flat_key, value = value, flat_key
diff --git a/src/flatten_dict/py.typed b/src/flatten_dict/py.typed
new file mode 100644
index 0000000..e69de29
diff --git a/src/flatten_dict/reducers.py b/src/flatten_dict/reducers.py
index 1bcd068..fe0644e 100644
--- a/src/flatten_dict/reducers.py
+++ b/src/flatten_dict/reducers.py
@@ -1,34 +1,45 @@
-def tuple_reducer(k1, k2):
+from typing import Callable, Tuple, TypeVar, Optional, Union
+
+T = TypeVar("T")
+
+
+def tuple_reducer(k1: Optional[Tuple[T, ...]], k2: T) -> Tuple[T, ...]:
     if k1 is None:
         return (k2,)
     else:
         return k1 + (k2,)
 
 
-def path_reducer(k1, k2):
+def path_reducer(k1: Optional[str], k2: Union[str, int]) -> str:
     import os.path
 
     if k1 is None:
-        return k2
+        return str(k2)
     else:
-        return os.path.join(k1, k2)
+        return os.path.join(k1, str(k2))
 
 
-def dot_reducer(k1, k2):
+def dot_reducer(
+    k1: Optional[Union[T, str, int]], k2: Union[T, int]
+) -> Union[T, str, int]:
     if k1 is None:
         return k2
     else:
         return "{}.{}".format(k1, k2)
 
 
-def underscore_reducer(k1, k2):
+def underscore_reducer(
+    k1: Optional[Union[T, str, int]], k2: Union[T, int]
+) -> Union[T, str, int]:
     if k1 is None:
         return k2
     else:
         return "{}_{}".format(k1, k2)
 
 
-def make_reducer(delimiter):
+def make_reducer(
+    delimiter: str,
+) -> Callable[[Union[T, str, int, None], Union[T, str, int]], Union[T, str, int]]:
     """Create a reducer with a custom delimiter.
 
     Parameters
@@ -42,7 +53,9 @@ def make_reducer(delimiter):
         Callable that can be passed to `flatten()`'s `reducer` argument.
     """
 
-    def f(k1, k2):
+    def f(
+        k1: Optional[Union[T, str, int]], k2: Union[T, str, int]
+    ) -> Union[T, str, int]:
         if k1 is None:
             return k2
         else:
diff --git a/src/flatten_dict/splitters.py b/src/flatten_dict/splitters.py
index 94a81b4..c2e4fe4 100644
--- a/src/flatten_dict/splitters.py
+++ b/src/flatten_dict/splitters.py
@@ -1,27 +1,30 @@
-def tuple_splitter(flat_key):
+from typing import Tuple, Callable, TypeVar, Hashable
+
+THashable = TypeVar("THashable", bound=Hashable)
+
+
+def tuple_splitter(flat_key: Tuple[THashable, ...]) -> Tuple[THashable, ...]:
     return flat_key
 
 
-def path_splitter(flat_key):
-    try:
-        from pathlib import PurePath
-    except ImportError:
-        from pathlib2 import PurePath
+def path_splitter(flat_key: str) -> Tuple[str, ...]:
+    from pathlib import PurePath
+
     keys = PurePath(flat_key).parts
     return keys
 
 
-def dot_splitter(flat_key):
+def dot_splitter(flat_key: str) -> Tuple[str, ...]:
     keys = tuple(flat_key.split("."))
     return keys
 
 
-def underscore_splitter(flat_key):
+def underscore_splitter(flat_key: str) -> Tuple[str, ...]:
     keys = tuple(flat_key.split("_"))
     return keys
 
 
-def make_splitter(delimiter):
+def make_splitter(delimiter: str) -> Callable[[str], Tuple[str, ...]]:
     """Create a reducer with a custom delimiter.
 
     Parameters
@@ -35,7 +38,7 @@ def make_splitter(delimiter):
         Callable that can be passed to ``unflatten``'s ``splitter`` argument.
     """
 
-    def f(flat_key):
+    def f(flat_key: str) -> Tuple[str, ...]:
         keys = tuple(flat_key.split(delimiter))
         return keys
 
diff --git a/src/flatten_dict/tests/flatten_dict_test.py b/src/flatten_dict/tests/flatten_dict_test.py
index 581bc11..514440a 100644
--- a/src/flatten_dict/tests/flatten_dict_test.py
+++ b/src/flatten_dict/tests/flatten_dict_test.py
@@ -3,11 +3,31 @@
 import os.path
 import json
 from types import GeneratorType
+import sys
+from typing import (
+    Any,
+    Callable,
+    Dict,
+    Iterable,
+    Iterator,
+    List,
+    Mapping,
+    Optional,
+    Sequence,
+    Tuple,
+    TypeVar,
+    Union,
+)
+
+if sys.version_info >= (3, 8):
+    from typing import Literal
+else:
+    from typing_extensions import Literal
 
-import six
 import pytest
 
-from flatten_dict import flatten, unflatten
+from flatten_dict import flatten, unflatten, TReducerCallable
+from flatten_dict.flatten_dict import TKeyIn
 from flatten_dict.reducers import (
     tuple_reducer,
     path_reducer,
@@ -21,9 +41,11 @@
     make_splitter,
 )
 
+TNormalDict = Dict[str, Union[str, Dict[str, Union[str, Dict[str, str]]]]]
+
 
 @pytest.fixture
-def normal_dict():
+def normal_dict() -> TNormalDict:
     return {
         "a": "0",
         "b": {
@@ -40,8 +62,11 @@ def normal_dict():
     }
 
 
+TFlatTupleDict = Dict[Tuple[str, ...], str]
+
+
 @pytest.fixture
-def flat_tuple_dict():
+def flat_tuple_dict() -> TFlatTupleDict:
     return {
         ("a",): "0",
         ("b", "a"): "1.0",
@@ -52,8 +77,11 @@ def flat_tuple_dict():
     }
 
 
+TFlatTupleDictDepth2 = Dict[Tuple[str, ...], Union[str, Dict[str, str]]]
+
+
 @pytest.fixture
-def flat_tuple_dict_depth2():
+def flat_tuple_dict_depth2() -> TFlatTupleDictDepth2:
     return {
         ("a",): "0",
         ("b", "a"): "1.0",
@@ -63,8 +91,17 @@ def flat_tuple_dict_depth2():
     }
 
 
+TNormalDictWithNestedLists = Dict[
+    str,
+    Union[
+        str,
+        List[Union[str, Dict[str, str], List[str], Dict[str, List[str]]]],
+    ],
+]
+
+
 @pytest.fixture
-def normal_dict_with_nested_lists():
+def normal_dict_with_nested_lists() -> TNormalDictWithNestedLists:
     return {
         "aaa": "0",
         "bbb": ["1.1", "1.2", "1.3"],
@@ -78,7 +115,7 @@ def normal_dict_with_nested_lists():
 
 
 @pytest.fixture
-def flat_dict_with_nested_lists_with_list_syntax():
+def flat_dict_with_nested_lists_with_list_syntax() -> Dict[str, str]:
     return {
         "aaa": "0",
         "bbb[0]": "1.1",
@@ -101,33 +138,41 @@ def flat_dict_with_nested_lists_with_list_syntax():
     }
 
 
-def get_flat_tuple_dict(flat_tuple_dict):
+T = TypeVar("T")
+
+
+def get_flat_tuple_dict(flat_tuple_dict: T) -> T:
     return flat_tuple_dict
 
 
-def get_flat_path_dict(flat_tuple_dict):
-    return {os.path.join(*k): v for k, v in six.viewitems(flat_tuple_dict)}
+def get_flat_path_dict(flat_tuple_dict: TFlatTupleDict) -> Dict[str, str]:
+    return {os.path.join(*k): v for k, v in flat_tuple_dict.items()}
 
 
-def get_flat_underscore_dict(flat_tuple_dict):
-    return {"_".join(k): v for k, v in six.viewitems(flat_tuple_dict)}
+def get_flat_underscore_dict(flat_tuple_dict: TFlatTupleDict) -> Dict[str, str]:
+    return {"_".join(k): v for k, v in flat_tuple_dict.items()}
+
+
+TInvFlatTupleDict = Dict[str, Tuple[str, ...]]
 
 
 @pytest.fixture
-def inv_flat_tuple_dict(flat_tuple_dict):
-    return {v: k for k, v in six.viewitems(flat_tuple_dict)}
+def inv_flat_tuple_dict(flat_tuple_dict: TFlatTupleDict) -> TInvFlatTupleDict:
+    return {v: k for k, v in flat_tuple_dict.items()}
 
 
-def test_flatten_dict(normal_dict, flat_tuple_dict):
+def test_flatten_dict(
+    normal_dict: TNormalDict, flat_tuple_dict: TFlatTupleDict
+) -> None:
     assert flatten(normal_dict) == flat_tuple_dict
 
 
-def test_flatten_dict_invalid_depth_limit(normal_dict):
+def test_flatten_dict_invalid_depth_limit(normal_dict: TNormalDict) -> None:
     with pytest.raises(ValueError):
         flatten(normal_dict, max_flatten_depth=0)
 
 
-def test_flatten_dict_depth_limit_1(normal_dict):
+def test_flatten_dict_depth_limit_1(normal_dict: TNormalDict) -> None:
     flattened = flatten(normal_dict, max_flatten_depth=1)
     values_before = sorted(
         flattened.values(), key=lambda x: json.dumps(x, sort_keys=True)
@@ -138,15 +183,19 @@ def test_flatten_dict_depth_limit_1(normal_dict):
     assert values_before == values_after
 
 
-def test_flatten_dict_depth_limit_2(normal_dict, flat_tuple_dict_depth2):
+def test_flatten_dict_depth_limit_2(
+    normal_dict: TNormalDict, flat_tuple_dict_depth2: TFlatTupleDictDepth2
+) -> None:
     assert flatten(normal_dict, max_flatten_depth=2) == flat_tuple_dict_depth2
 
 
-def test_flatten_dict_irrelevant_depth_limit(normal_dict, flat_tuple_dict):
+def test_flatten_dict_irrelevant_depth_limit(
+    normal_dict: TNormalDict, flat_tuple_dict: TFlatTupleDict
+) -> None:
     assert flatten(normal_dict, max_flatten_depth=3) == flat_tuple_dict
 
 
-def test_flatten_nonflattenable_type():
+def test_flatten_nonflattenable_type() -> None:
     with pytest.raises(ValueError):
         flatten([])
 
@@ -163,17 +212,24 @@ def test_flatten_nonflattenable_type():
     ],
 )
 def test_flatten_dict_with_reducer(
-    normal_dict, flat_tuple_dict, reducer, expected_flat_dict_func
-):
+    normal_dict: TNormalDict,
+    flat_tuple_dict: TFlatTupleDict,
+    reducer: TReducerCallable,
+    expected_flat_dict_func: Callable[
+        [TFlatTupleDict], Union[TFlatTupleDict, Dict[str, str]]
+    ],
+) -> None:
     expected_flat_dict = expected_flat_dict_func(flat_tuple_dict)
     assert flatten(normal_dict, reducer=reducer) == expected_flat_dict
 
 
-def test_flatten_dict_inverse(normal_dict, inv_flat_tuple_dict):
+def test_flatten_dict_inverse(
+    normal_dict: TNormalDict, inv_flat_tuple_dict: TInvFlatTupleDict
+) -> None:
     assert flatten(normal_dict, inverse=True) == inv_flat_tuple_dict
 
 
-def test_flatten_dict_inverse_with_duplicated_value(normal_dict):
+def test_flatten_dict_inverse_with_duplicated_value(normal_dict: TNormalDict) -> None:
     dup_val_dict = normal_dict.copy()
     dup_val_dict["a"] = "2.1.1"
     with pytest.raises(ValueError):
@@ -181,9 +237,14 @@ def test_flatten_dict_inverse_with_duplicated_value(normal_dict):
 
 
 def test_flatten_dict_with_list_syntax(
-    normal_dict_with_nested_lists, flat_dict_with_nested_lists_with_list_syntax
-):
-    def _reducer(parent_path, key, parent_obj):
+    normal_dict_with_nested_lists: TNormalDictWithNestedLists,
+    flat_dict_with_nested_lists_with_list_syntax: Dict[str, str],
+) -> None:
+    def _reducer(
+        parent_path: Optional[Union[str, int]],
+        key: Union[str, int],
+        parent_obj: Union[Iterable[Any], Mapping[str, Any]],
+    ) -> Union[str, int]:
         if parent_path is None:
             return key
         elif isinstance(parent_obj, list):
@@ -199,11 +260,15 @@ def _reducer(parent_path, key, parent_obj):
     )
 
 
-def test_unflatten_dict(normal_dict, flat_tuple_dict):
+def test_unflatten_dict(
+    normal_dict: TNormalDict, flat_tuple_dict: TFlatTupleDict
+) -> None:
     assert unflatten(flat_tuple_dict) == normal_dict
 
 
-def test_unflatten_dict_inverse(normal_dict, inv_flat_tuple_dict):
+def test_unflatten_dict_inverse(
+    normal_dict: TNormalDict, inv_flat_tuple_dict: TInvFlatTupleDict
+) -> None:
     assert unflatten(inv_flat_tuple_dict, inverse=True) == normal_dict
 
 
@@ -219,22 +284,30 @@ def test_unflatten_dict_inverse(normal_dict, inv_flat_tuple_dict):
     ],
 )
 def test_unflatten_dict_with_splitter(
-    normal_dict, flat_tuple_dict, splitter, flat_dict_func
-):
+    normal_dict: TNormalDict,
+    flat_tuple_dict: TFlatTupleDict,
+    splitter: Callable[[str], Tuple[Any, ...]],
+    flat_dict_func: Callable[[TFlatTupleDict], Dict[str, str]],
+) -> None:
     flat_dict = flat_dict_func(flat_tuple_dict)
     assert unflatten(flat_dict, splitter=splitter) == normal_dict
 
 
 def test_unflatten_dict_inverse_with_duplicated_value(
-    flat_tuple_dict, inv_flat_tuple_dict
-):
+    inv_flat_tuple_dict: TInvFlatTupleDict,
+) -> None:
     inv_flat_tuple_dict["2.1.1"] = ("c", "b", "a")
     with pytest.raises(ValueError):
         unflatten(inv_flat_tuple_dict, inverse=True)
 
 
+TDictWithLists = Dict[
+    str, Union[str, Dict[str, Union[str, Dict[str, Union[str, Sequence[str]]]]]]
+]
+
+
 @pytest.fixture
-def dict_with_list():
+def dict_with_list() -> TDictWithLists:
     return {
         "a": "0",
         "b": {
@@ -252,8 +325,11 @@ def dict_with_list():
     }
 
 
+TFlatTupleDictWithList = Dict[Tuple[str, ...], Union[str, Sequence[str]]]
+
+
 @pytest.fixture
-def flat_tuple_dict_with_list():
+def flat_tuple_dict_with_list() -> TFlatTupleDictWithList:
     return {
         ("a",): "0",
         ("b", "a"): "1.0",
@@ -265,8 +341,13 @@ def flat_tuple_dict_with_list():
     }
 
 
+TFlatTupleDictWithListDepth2 = Dict[
+    Tuple[str, ...], Union[str, Dict[str, Union[str, Sequence[str]]]]
+]
+
+
 @pytest.fixture
-def flat_tuple_dict_with_list_depth2():
+def flat_tuple_dict_with_list_depth2() -> TFlatTupleDictWithListDepth2:
     return {
         ("a",): "0",
         ("b", "a"): "1.0",
@@ -280,33 +361,42 @@ def flat_tuple_dict_with_list_depth2():
     }
 
 
-def test_flatten_dict_with_list(dict_with_list, flat_tuple_dict_with_list):
+def test_flatten_dict_with_list(
+    dict_with_list: TDictWithLists, flat_tuple_dict_with_list: TFlatTupleDictWithList
+) -> None:
     assert flatten(dict_with_list) == flat_tuple_dict_with_list
 
 
-def test_flatten_dict_with_list_depth_limit_1(dict_with_list):
-    flattened = flatten(dict_with_list, max_flatten_depth=1)
+def test_flatten_dict_with_list_depth_limit_1(dict_with_list: TDictWithLists) -> None:
+    # TODO: type of key should be possible to infer
+    flattened: Dict[Tuple[str, ...], str] = flatten(dict_with_list, max_flatten_depth=1)
     before = sorted(dict_with_list.items(), key=lambda x: x[0])
-    after = sorted(flattened.items(), key=lambda x: x[0])
+    _after = sorted(flattened.items(), key=lambda x: x[0])
     # flatten with the default reducer transforms keys to tuples
-    after = [(x[0][0], x[1]) for x in after]
+    after = [(key, val) for (key,), val in _after]
     assert before == after
 
 
 def test_flatten_dict_with_list_depth_limit_2(
-    dict_with_list, flat_tuple_dict_with_list_depth2
-):
+    dict_with_list: TDictWithLists,
+    flat_tuple_dict_with_list_depth2: TFlatTupleDictWithListDepth2,
+) -> None:
     assert (
         flatten(dict_with_list, max_flatten_depth=2) == flat_tuple_dict_with_list_depth2
     )
 
 
 def test_flatten_dict_with_list_irrelevant_depth_limit(
-    dict_with_list, flat_tuple_dict_with_list
-):
+    dict_with_list: TDictWithLists, flat_tuple_dict_with_list: TFlatTupleDictWithList
+) -> None:
     assert flatten(dict_with_list, max_flatten_depth=3) == flat_tuple_dict_with_list
 
 
+TFlatDictCallable = Callable[
+    [TFlatTupleDictWithList], Union[TDictWithLists, TFlatTupleDictWithList]
+]
+
+
 @pytest.mark.parametrize(
     "reducer, expected_flat_dict_func",
     [
@@ -319,13 +409,18 @@ def test_flatten_dict_with_list_irrelevant_depth_limit(
     ],
 )
 def test_flatten_dict_with_list_with_reducer(
-    dict_with_list, flat_tuple_dict_with_list, reducer, expected_flat_dict_func
-):
+    dict_with_list: TDictWithLists,
+    flat_tuple_dict_with_list: TFlatTupleDictWithList,
+    reducer: TReducerCallable,
+    expected_flat_dict_func: TFlatDictCallable,
+) -> None:
     expected_flat_dict = expected_flat_dict_func(flat_tuple_dict_with_list)
     assert flatten(dict_with_list, reducer=reducer) == expected_flat_dict
 
 
-def test_unflatten_dict_with_list(dict_with_list, flat_tuple_dict_with_list):
+def test_unflatten_dict_with_list(
+    dict_with_list: TDictWithLists, flat_tuple_dict_with_list: TFlatTupleDictWithList
+) -> None:
     assert unflatten(flat_tuple_dict_with_list) == dict_with_list
 
 
@@ -341,14 +436,23 @@ def test_unflatten_dict_with_list(dict_with_list, flat_tuple_dict_with_list):
     ],
 )
 def test_unflatten_dict_with_list_with_splitter(
-    dict_with_list, flat_tuple_dict_with_list, splitter, flat_dict_func
-):
+    dict_with_list: TDictWithLists,
+    flat_tuple_dict_with_list: TFlatTupleDictWithList,
+    splitter: Callable[[TKeyIn], Tuple[str, ...]],
+    flat_dict_func: Callable[
+        [TFlatTupleDictWithList],
+        Dict[TKeyIn, Union[str, List[str]]],
+    ],
+) -> None:
     flat_dict = flat_dict_func(flat_tuple_dict_with_list)
     assert unflatten(flat_dict, splitter=splitter) == dict_with_list
 
 
+TFlatTupleDictWithEnumeratedList = Dict[Tuple[Union[str, int], ...], str]
+
+
 @pytest.fixture
-def flat_tuple_dict_with_enumerated_list():
+def flat_tuple_dict_with_enumerated_list() -> TFlatTupleDictWithEnumeratedList:
     return {
         ("a",): "0",
         ("b", "a"): "1.0",
@@ -361,20 +465,26 @@ def flat_tuple_dict_with_enumerated_list():
 
 
 def test_flatten_dict_with_list_with_enumerate_types(
-    dict_with_list, flat_tuple_dict_with_enumerated_list
-):
+    dict_with_list: TDictWithLists,
+    flat_tuple_dict_with_enumerated_list: TFlatTupleDictWithEnumeratedList,
+) -> None:
     assert (
         flatten(dict_with_list, enumerate_types=(list,))
         == flat_tuple_dict_with_enumerated_list
     )
 
 
-def test_flatten_list():
+def test_flatten_list() -> None:
     assert flatten([1, 2], enumerate_types=(list,)) == {(0,): 1, (1,): 2}
 
 
+TDictWithGenerator = Dict[
+    str, Union[str, Dict[str, Union[str, Dict[str, Union[str, Iterator[str]]]]]]
+]
+
+
 @pytest.fixture
-def dict_with_generator():
+def dict_with_generator() -> TDictWithGenerator:
     return {
         "a": "0",
         "b": {
@@ -386,23 +496,29 @@ def dict_with_generator():
             "b": {
                 "a": "2.1.0",
                 "b": ("2.1.1.%d" % i for i in range(2)),
-                "c": (i for i in ()),  # empty generator
+                "c": (i for i in ""),  # empty generator
             },
         },
     }
 
 
 def test_flatten_dict_with_generator_with_enumerate_types(
-    dict_with_generator, flat_tuple_dict_with_enumerated_list
-):
+    dict_with_generator: TDictWithGenerator,
+    flat_tuple_dict_with_enumerated_list: TFlatTupleDictWithEnumeratedList,
+) -> None:
     assert (
         flatten(dict_with_generator, enumerate_types=(GeneratorType,))
         == flat_tuple_dict_with_enumerated_list
     )
 
 
+TDictWithEmptyDict = Dict[
+    str, Union[str, Dict[str, Union[str, Dict[str, Union[str, Dict]]]]]
+]
+
+
 @pytest.fixture
-def dict_with_empty_dict():
+def dict_with_empty_dict() -> TDictWithEmptyDict:
     return {
         "a": "0",
         "b": {
@@ -420,8 +536,11 @@ def dict_with_empty_dict():
     }
 
 
+TFlatTupleDictWithEmptyDict = Dict[Tuple[str, ...], Union[str, Dict]]
+
+
 @pytest.fixture
-def flat_tuple_dict_with_empty_dict():
+def flat_tuple_dict_with_empty_dict() -> TFlatTupleDictWithEmptyDict:
     return {
         ("a",): "0",
         ("b", "a"): "1.0",
@@ -433,28 +552,41 @@ def flat_tuple_dict_with_empty_dict():
     }
 
 
-def test_flatten_dict_with_empty_dict(dict_with_empty_dict, flat_tuple_dict):
+def test_flatten_dict_with_empty_dict(
+    dict_with_empty_dict: TDictWithEmptyDict,
+    flat_tuple_dict: TFlatTupleDictWithEmptyDict,
+) -> None:
     assert flatten(dict_with_empty_dict) == flat_tuple_dict
 
 
 def test_flatten_dict_with_empty_dict_kept(
-    dict_with_empty_dict, flat_tuple_dict_with_empty_dict
-):
+    dict_with_empty_dict: TDictWithEmptyDict,
+    flat_tuple_dict_with_empty_dict: TFlatTupleDictWithEmptyDict,
+) -> None:
     assert (
         flatten(dict_with_empty_dict, keep_empty_types=(dict,))
         == flat_tuple_dict_with_empty_dict
     )
 
 
-def test_flatten_dict_with_keep_empty_types(normal_dict, flat_tuple_dict):
+def test_flatten_dict_with_keep_empty_types(
+    normal_dict: TNormalDict, flat_tuple_dict: TFlatTupleDict
+) -> None:
     assert flatten(normal_dict, keep_empty_types=(dict, str)) == flat_tuple_dict
 
 
 @pytest.mark.parametrize(
     "delimiter, delimiter_equivalent", [(".", "dot"), ("_", "underscore")]
 )
-def test_make_reducer(normal_dict, delimiter, delimiter_equivalent):
-    reducer = make_reducer(delimiter)
+def test_make_reducer(
+    normal_dict: TNormalDict,
+    delimiter: str,
+    delimiter_equivalent: Literal["dot", "underscore"],
+) -> None:
+    # use explicit type to narrow default reducer
+    reducer: Callable[
+        [Union[str, int, None], Union[str, int]], Union[str, int]
+    ] = make_reducer(delimiter)
     flattened_dict_using_make_reducer = flatten(normal_dict, reducer=reducer)
     flattened_dict_using_equivalent_reducer = flatten(
         normal_dict, reducer=delimiter_equivalent
@@ -465,7 +597,11 @@ def test_make_reducer(normal_dict, delimiter, delimiter_equivalent):
 @pytest.mark.parametrize(
     "delimiter, delimiter_equivalent", [(".", "dot"), ("_", "underscore")]
 )
-def test_make_splitter(normal_dict, delimiter, delimiter_equivalent):
+def test_make_splitter(
+    normal_dict: TNormalDict,
+    delimiter: str,
+    delimiter_equivalent: Literal["dot", "underscore"],
+) -> None:
     splitter = make_splitter(delimiter)
     flat_dict = flatten(normal_dict, delimiter_equivalent)
     unflattened_dict_using_make_splitter = unflatten(flat_dict, splitter=splitter)
diff --git a/tox.ini b/tox.ini
index fa2476b..bd7eb79 100644
--- a/tox.ini
+++ b/tox.ini
@@ -1,6 +1,6 @@
 [tox]
 isolated_build = true
-envlist = py{27, 36, 37, 38, 39, 310}, flake8
+envlist = py{36, 37, 38, 39, 310}, py{,36, 37, 38, 39, 310}-type, flake8
 
 [testenv]
 deps =
@@ -20,3 +20,9 @@ commands =
     # use poetry to install all dev dependencies
     poetry install
     poetry run flake8
+
+[testenv:py{, 36, 37, 38, 39, 310}-type]
+deps =
+    mypy
+commands =
+    mypy src