diff --git a/tests/core/test_denylist.py b/tests/core/test_denylist.py
new file mode 100644
index 0000000..7305218
--- /dev/null
+++ b/tests/core/test_denylist.py
@@ -0,0 +1,106 @@
+import warnings
+
+import json
+import tempfile
+from datetime import datetime
+from typing import NamedTuple, Iterator
+
+from my.core.denylist import DenyList
+
+
+class IP(NamedTuple):
+    addr: str
+    dt: datetime
+
+
+def data() -> Iterator[IP]:
+    # random IP addresses
+    yield IP(addr="67.98.113.0", dt=datetime(2020, 1, 1))
+    yield IP(addr="59.40.113.87", dt=datetime(2020, 2, 1))
+    yield IP(addr="161.235.192.228", dt=datetime(2020, 3, 1))
+    yield IP(addr="165.243.139.87", dt=datetime(2020, 4, 1))
+    yield IP(addr="69.69.141.154", dt=datetime(2020, 5, 1))
+    yield IP(addr="50.72.224.80", dt=datetime(2020, 6, 1))
+    yield IP(addr="221.67.89.168", dt=datetime(2020, 7, 1))
+    yield IP(addr="177.113.119.251", dt=datetime(2020, 8, 1))
+    yield IP(addr="93.200.246.215", dt=datetime(2020, 9, 1))
+    yield IP(addr="127.105.171.61", dt=datetime(2020, 10, 1))
+
+
+def test_denylist() -> None:
+    with tempfile.NamedTemporaryFile() as tf, warnings.catch_warnings(record=True):
+
+        # create empty denylist (though file does not have to exist for denylist to work)
+        tf.write(b"[]")
+        tf.flush()
+
+        d = DenyList(tf.name)
+
+        d.load()
+        assert dict(d._deny_map) == {}
+        assert d._deny_raw_list == []
+
+        assert list(d.filter(data())) == list(data())
+        # no data in denylist yet
+        assert len(d._deny_map) == 0
+        assert len(d._deny_raw_list) == 0
+
+        # add some data
+        d.deny(key="addr", value="67.98.113.0")
+        # write and reload to update _deny_map, _deny_raw_list
+        d.write()
+        d.load()
+
+        assert len(d._deny_map) == 1
+        assert len(d._deny_raw_list) == 1
+
+        assert d._deny_raw_list == [{"addr": "67.98.113.0"}]
+
+        filtered = list(d.filter(data()))
+        assert len(filtered) == 9
+        assert "67.98.113.0" not in [i.addr for i in filtered]
+
+        assert dict(d._deny_map) == {"addr": {"67.98.113.0"}}
+
+        denied = list(d.filter(data(), invert=True))
+        assert len(denied) == 1
+
+        assert denied[0] == IP(addr="67.98.113.0", dt=datetime(2020, 1, 1))
+
+        # add some non-JSON primitive data
+
+        d.deny(key="dt", value=datetime(2020, 2, 1))
+
+        # test internal behavior, _deny_raw_list should have been updated,
+        # but _deny_map doesnt get updated by a call to .deny
+        #
+        # if we change this just update the test, is just here to ensure
+        # this is the behaviour
+
+        assert len(d._deny_map) == 1
+
+        # write and load to update _deny_map
+        d.write()
+        d.load()
+
+        assert len(d._deny_map) == 2
+        assert len(d._deny_raw_list) == 2
+
+        assert d._deny_raw_list[-1] == {"dt": "2020-02-01T00:00:00"}
+
+        filtered = list(d.filter(data()))
+        assert len(filtered) == 8
+
+        assert "59.40.113.87" not in [i.addr for i in filtered]
+
+        with open(tf.name, "r") as f:
+            data_json = json.loads(f.read())
+
+        assert data_json == [
+            {
+                "addr": "67.98.113.0",
+            },
+            {
+                "dt": "2020-02-01T00:00:00",
+            },
+        ]