diff --git a/library/tests/conftest.py b/library/tests/conftest.py
new file mode 100644
index 0000000..7582e08
--- /dev/null
+++ b/library/tests/conftest.py
@@ -0,0 +1,38 @@
+"""Test configuration.
+These allow the mocking of various Python modules
+that might otherwise have runtime side-effects.
+"""
+import sys
+import mock
+import pytest
+
+
+@pytest.fixture(scope='function', autouse=False)
+def GPIO():
+    """Mock RPi.GPIO module."""
+    GPIO = mock.MagicMock()
+    # Fudge for Python < 37 (possibly earlier)
+    sys.modules['RPi'] = mock.Mock()
+    sys.modules['RPi'].GPIO = GPIO
+    sys.modules['RPi.GPIO'] = GPIO
+    yield GPIO
+    del sys.modules['RPi']
+    del sys.modules['RPi.GPIO']
+
+
+@pytest.fixture(scope='function', autouse=False)
+def spidev():
+    """Mock spidev module."""
+    spidev = mock.MagicMock()
+    sys.modules['spidev'] = spidev
+    yield spidev
+    del sys.modules['spidev']
+
+
+@pytest.fixture(scope='function', autouse=False)
+def numpy():
+    """Mock numpy module."""
+    numpy = mock.MagicMock()
+    sys.modules['numpy'] = numpy
+    yield numpy
+    del sys.modules['numpy']
diff --git a/library/tests/test_dimensions.py b/library/tests/test_dimensions.py
index 8de9af4..4eb4b61 100644
--- a/library/tests/test_dimensions.py
+++ b/library/tests/test_dimensions.py
@@ -1,24 +1,16 @@
-import sys
-import mock
+from tools import force_reimport
 
 
-def _mock():
-    sys.modules['numpy'] = mock.Mock()
-    sys.modules['spidev'] = mock.Mock()
-    sys.modules['RPi'] = mock.Mock()
-    sys.modules['RPi.GPIO'] = mock.Mock()
-
-
-def test_128_64_0():
-    _mock()
+def test_128_64_0(GPIO, spidev, numpy):
+    force_reimport('ST7735')
     import ST7735
     display = ST7735.ST7735(port=0, cs=0, dc=24, width=128, height=64, rotation=0)
     assert display.width == 128
     assert display.height == 64
 
 
-def test_128_64_90():
-    _mock()
+def test_128_64_90(GPIO, spidev, numpy):
+    force_reimport('ST7735')
     import ST7735
     display = ST7735.ST7735(port=0, cs=0, dc=24, width=128, height=64, rotation=90)
     assert display.width == 64
diff --git a/library/tests/test_features.py b/library/tests/test_features.py
new file mode 100644
index 0000000..f8fb7d4
--- /dev/null
+++ b/library/tests/test_features.py
@@ -0,0 +1,25 @@
+import mock
+from tools import force_reimport
+
+
+def test_display(GPIO, spidev, numpy):
+    force_reimport('ST7735')
+    import ST7735
+    display = ST7735.ST7735(port=0, cs=0, dc=24)
+    numpy.dstack().flatten().tolist.return_value = [0xff, 0x00, 0xff, 0x00]
+    display.display(mock.MagicMock())
+
+    spidev.SpiDev().xfer3.assert_called_with([0xff, 0x00, 0xff, 0x00])
+
+
+def test_color565(GPIO, spidev, numpy):
+    force_reimport('ST7735')
+    import ST7735
+    assert ST7735.color565(255, 255, 255) == 0xFFFF
+
+
+def test_image_to_data(GPIO, spidev, numpy):
+    force_reimport('ST7735')
+    numpy.dstack().flatten().tolist.return_value = []
+    import ST7735
+    assert ST7735.image_to_data(mock.MagicMock()) == []
diff --git a/library/tests/test_setup.py b/library/tests/test_setup.py
index ac8bb04..8b89f47 100644
--- a/library/tests/test_setup.py
+++ b/library/tests/test_setup.py
@@ -1,13 +1,47 @@
-import sys
 import mock
+from tools import force_reimport
 
 
-def test_setup():
-    sys.modules['numpy'] = mock.Mock()
-    sys.modules['spidev'] = mock.Mock()
-    sys.modules['RPi'] = mock.Mock()
-    sys.modules['RPi.GPIO'] = mock.Mock()
-
+def test_setup(GPIO, spidev, numpy):
+    force_reimport('ST7735')
     import ST7735
     display = ST7735.ST7735(port=0, cs=0, dc=24)
     del display
+
+    GPIO.output.assert_has_calls([
+        mock.call(24, True),
+        mock.call(24, False)
+    ], any_order=True)
+
+
+def test_setup_no_invert(GPIO, spidev, numpy):
+    force_reimport('ST7735')
+    import ST7735
+    display = ST7735.ST7735(port=0, cs=0, dc=24, invert=False)
+    del display
+
+
+def test_setup_with_backlight(GPIO, spidev, numpy):
+    force_reimport('ST7735')
+    import ST7735
+    display = ST7735.ST7735(port=0, cs=0, dc=24, backlight=4)
+    GPIO.setup.assert_called_with(4, GPIO.OUT)
+
+    display.set_backlight(GPIO.HIGH)
+
+    GPIO.output.assert_has_calls([
+        mock.call(4, GPIO.LOW),
+        mock.call(4, GPIO.HIGH),
+        # Dozens of falls with True/False here
+        # due to _init() being called and the display
+        # setup setting the command/data pin
+        mock.call(4, GPIO.HIGH)
+    ], any_order=True)
+
+
+def test_setup_with_reset(GPIO, spidev, numpy):
+    force_reimport('ST7735')
+    import ST7735
+    display = ST7735.ST7735(port=0, cs=0, dc=24, rst=4)
+    GPIO.setup.assert_called_with(4, GPIO.OUT)
+    del display
diff --git a/library/tests/tools.py b/library/tests/tools.py
new file mode 100644
index 0000000..ddf752d
--- /dev/null
+++ b/library/tests/tools.py
@@ -0,0 +1,23 @@
+import sys
+
+
+def force_reimport(module):
+    """Force the module under test to be re-imported.
+
+    Because pytest runs all tests within the same scope (this makes me cry)
+    we have to do some manual housekeeping to avoid tests polluting each other.
+
+    Since conftest.py already does some sys.modules mangling I see no reason not to
+    do the same thing here.
+    """
+    if "." in module:
+        steps = module.split(".")
+    else:
+        steps = [module]
+
+    for i in range(len(steps)):
+        module = ".".join(steps[0:i + 1])
+        try:
+            del sys.modules[module]
+        except KeyError:
+            pass
diff --git a/library/tox.ini b/library/tox.ini
index e53234d..b23507c 100644
--- a/library/tox.ini
+++ b/library/tox.ini
@@ -1,5 +1,5 @@
 [tox]
-envlist = py{27,35},qa
+envlist = py{27,35,37},qa
 skip_missing_interpreters = True
 
 [testenv]