Use the standard Python unittest library.

Let's keep it simple and not use pytest.

README.md

@@ -137,7 +137,7 @@ The Flickr API does not support downloading original video files. If this script
 
 Running unit tests
 ==================
-Run [`py.test`](http://pytest.org/).
+Run `python -m unittest`
 
 
 TODO

test_flickrmirrorer.py

@@ -0,0 +1,29 @@
+import unittest
+
+from flickrmirrorer import get_photo_datetime
+
+class Tests(unittest.TestCase):
+    def test_unparseable_title_timestamp(self):
+        timestamp = get_photo_datetime({
+            'datetakenunknown': '1',
+            'datetaken': '2014-10-01 13:45:37',
+            'title': 'flaskpost'
+        })
+
+        # Fall back on datetaken if we can't parse the date from the title
+        self.assertEqual(timestamp.isoformat(), "2014-10-01T13:45:37")
+
+    def test_plain_title_timestamp(self):
+        timestamp = get_photo_datetime({
+            'datetakenunknown': '1',
+            'datetaken': '2014-10-01 13:45:37',
+            'title': '20151130_135610'
+        })
+        self.assertEqual(timestamp.isoformat(), "2015-11-30T13:56:10")
+
+    def test_known_timestamp(self):
+        timestamp = get_photo_datetime({
+            'datetakenunknown': '0',
+            'datetaken': '2015-11-02 12:35:07'
+        })
+        self.assertEqual(timestamp.isoformat(), "2015-11-02T12:35:07")

tox.ini

@@ -1,4 +1,2 @@
 [flake8]
 max-line-length = 120
-[pytest]
-python_files = tests.py