Use pytest fixture+marker instead of switch_backend.

Non default backends can be specified by marking test functions with
`@pytest.mark.backend('backend')`.

Note that because switch_backend calls `matplotlib.testing.setup`, and
it appears _below_ the previous `@cleanup` decorator in the PGF tests,
the specified 'classic' style is not actually used for those tests.

lib/matplotlib/tests/conftest.py

@@ -13,16 +13,33 @@ def mpl_test_settings(request):
     original_units_registry = matplotlib.units.registry.copy()
     original_settings = matplotlib.rcParams.copy()
 
+    backend = None
+    backend_marker = request.keywords.get('backend')
+    if backend_marker is not None:
+        assert len(backend_marker.args) == 1, \
+            "Marker 'backend' must specify 1 backend."
+        backend = backend_marker.args[0]
+        prev_backend = matplotlib.get_backend()
+
     style = 'classic'
     style_marker = request.keywords.get('style')
     if style_marker is not None:
         assert len(style_marker.args) == 1, \
             "Marker 'style' must specify 1 style."
         style = style_marker.args[0]
 
+    matplotlib.testing.setup()
+    if backend is not None:
+        # This import must come after setup() so it doesn't load the default
+        # backend prematurely.
+        import matplotlib.pyplot as plt
+        plt.switch_backend(backend)
     matplotlib.style.use(style)
     try:
         yield
     finally:
+        if backend is not None:
+            import matplotlib.pyplot as plt
+            plt.switch_backend(prev_backend)
         _do_cleanup(original_units_registry,
                     original_settings)

lib/matplotlib/tests/test_backend_pgf.py

@@ -12,7 +12,7 @@
 import matplotlib.pyplot as plt
 from matplotlib.compat import subprocess
 from matplotlib.testing.compare import compare_images, ImageComparisonFailure
-from matplotlib.testing.decorators import _image_directories, switch_backend
+from matplotlib.testing.decorators import _image_directories
 
 baseline_dir, result_dir = _image_directories(lambda: 'dummy func')
 
@@ -82,8 +82,8 @@ def create_figure():
 
 # test compiling a figure to pdf with xelatex
 @needs_xelatex
-@pytest.mark.style('classic')
-@switch_backend('pgf')
+@pytest.mark.style('default')
+@pytest.mark.backend('pgf')
 def test_xelatex():
     rc_xelatex = {'font.family': 'serif',
                   'pgf.rcfonts': False}
@@ -94,8 +94,8 @@ def test_xelatex():
 
 # test compiling a figure to pdf with pdflatex
 @needs_pdflatex
-@pytest.mark.style('classic')
-@switch_backend('pgf')
+@pytest.mark.style('default')
+@pytest.mark.backend('pgf')
 def test_pdflatex():
     import os
     if os.environ.get('APPVEYOR', False):
@@ -116,8 +116,8 @@ def test_pdflatex():
 # test updating the rc parameters for each figure
 @needs_xelatex
 @needs_pdflatex
-@pytest.mark.style('classic')
-@switch_backend('pgf')
+@pytest.mark.style('default')
+@pytest.mark.backend('pgf')
 def test_rcupdate():
     rc_sets = []
     rc_sets.append({'font.family': 'sans-serif',
@@ -147,8 +147,8 @@ def test_rcupdate():
 
 # test backend-side clipping, since large numbers are not supported by TeX
 @needs_xelatex
-@pytest.mark.style('classic')
-@switch_backend('pgf')
+@pytest.mark.style('default')
+@pytest.mark.backend('pgf')
 def test_pathclip():
     rc_xelatex = {'font.family': 'serif',
                   'pgf.rcfonts': False}
@@ -164,8 +164,8 @@ def test_pathclip():
 
 # test mixed mode rendering
 @needs_xelatex
-@pytest.mark.style('classic')
-@switch_backend('pgf')
+@pytest.mark.style('default')
+@pytest.mark.backend('pgf')
 def test_mixedmode():
     rc_xelatex = {'font.family': 'serif',
                   'pgf.rcfonts': False}
@@ -179,8 +179,8 @@ def test_mixedmode():
 
 # test bbox_inches clipping
 @needs_xelatex
-@pytest.mark.style('classic')
-@switch_backend('pgf')
+@pytest.mark.style('default')
+@pytest.mark.backend('pgf')
 def test_bbox_inches():
     rc_xelatex = {'font.family': 'serif',
                   'pgf.rcfonts': False}

lib/matplotlib/tests/test_backend_qt4.py

@@ -2,7 +2,6 @@
                         unicode_literals)
 
 from matplotlib import pyplot as plt
-from matplotlib.testing.decorators import switch_backend
 from matplotlib._pylab_helpers import Gcf
 import matplotlib
 import copy
@@ -34,7 +33,7 @@
     pytestmark = pytest.mark.xfail(reason='Qt4 is not available')
 
 
-@switch_backend('Qt4Agg')
+@pytest.mark.backend('Qt4Agg')
 def test_fig_close():
     # save the state of Gcf.figs
     init_figs = copy.copy(Gcf.figs)
@@ -83,7 +82,7 @@ def test_fig_close():
         'non_unicode_key',
     ]
 )
-@switch_backend('Qt4Agg')
+@pytest.mark.backend('Qt4Agg')
 def test_correct_key(qt_key, qt_mods, answer):
     """
     Make a figure

lib/matplotlib/tests/test_backend_qt5.py

@@ -2,7 +2,6 @@
                         unicode_literals)
 
 from matplotlib import pyplot as plt
-from matplotlib.testing.decorators import switch_backend
 from matplotlib._pylab_helpers import Gcf
 import matplotlib
 import copy
@@ -27,7 +26,7 @@
 _, ShiftModifier, ShiftKey = MODIFIER_KEYS[SHIFT]
 
 
-@switch_backend('Qt5Agg')
+@pytest.mark.backend('Qt5Agg')
 def test_fig_close():
     # save the state of Gcf.figs
     init_figs = copy.copy(Gcf.figs)
@@ -76,7 +75,7 @@ def test_fig_close():
         'non_unicode_key',
     ]
 )
-@switch_backend('Qt5Agg')
+@pytest.mark.backend('Qt5Agg')
 def test_correct_key(qt_key, qt_mods, answer):
     """
     Make a figure