Handle None entries in sys.modules.

sys.modules entries can be explicitly set to None to block imports, so
the `name in sys.modules` check should rather be written
`sys.modules.get(name)` (... is not None).

While we're at it, disentangle a bit install_repl_displayhook to exit
early if `not sys.modules.get("IPython") or not get_ipython()`, which
avoids having to throw a local NotIPython exception just to `goto end`.
(Also note that the import of `backend2gui` should not throw an
ImportError, as that mapping has been around since <2011).

Author: anntzer

lib/matplotlib/backends/qt_compat.py

@@ -35,14 +35,16 @@
 _ETS = {"pyqt5": QT_API_PYQT5, "pyside2": QT_API_PYSIDE2,
         "pyqt": QT_API_PYQTv2, "pyside": QT_API_PYSIDE,
         None: None}
-# First, check if anything is already imported.
-if "PyQt5.QtCore" in sys.modules:
+# First, check if anything is already imported.  Use ``sys.modules.get(name)``
+# rather than ``name in sys.modules`` as entries can also have been explicitly
+# set to None.
+if sys.modules.get("PyQt5.QtCore"):
     QT_API = QT_API_PYQT5
-elif "PySide2.QtCore" in sys.modules:
+elif sys.modules.get("PySide2.QtCore"):
     QT_API = QT_API_PYSIDE2
-elif "PyQt4.QtCore" in sys.modules:
+elif sys.modules.get("PyQt4.QtCore"):
     QT_API = QT_API_PYQTv2
-elif "PySide.QtCore" in sys.modules:
+elif sys.modules.get("PySide.QtCore"):
     QT_API = QT_API_PYSIDE
 # Otherwise, check the QT_API environment variable (from Enthought).  This can
 # only override the binding, not the backend (in other words, we check that the

lib/matplotlib/cbook/__init__.py

@@ -54,6 +54,8 @@ def _get_running_interactive_framework():
         One of the following values: "qt5", "qt4", "gtk3", "wx", "tk",
         "macosx", "headless", ``None``.
     """
+    # Use ``sys.modules.get(name)`` rather than ``name in sys.modules`` as
+    # entries can also have been explicitly set to None.
     QtWidgets = (sys.modules.get("PyQt5.QtWidgets")
                  or sys.modules.get("PySide2.QtWidgets"))
     if QtWidgets and QtWidgets.QApplication.instance():
@@ -76,9 +78,9 @@ def _get_running_interactive_framework():
                 if frame.f_code in codes:
                     return "tk"
                 frame = frame.f_back
-    if 'matplotlib.backends._macosx' in sys.modules:
-        if sys.modules["matplotlib.backends._macosx"].event_loop_is_running():
-            return "macosx"
+    macosx = sys.modules.get("matplotlib.backends._macosx")
+    if macosx and macosx.event_loop_is_running():
+        return "macosx"
     if not _c_internal_utils.display_is_valid():
         return "headless"
     return None

lib/matplotlib/pyplot.py

@@ -119,52 +119,41 @@ def install_repl_displayhook():
     global _IP_REGISTERED
     global _INSTALL_FIG_OBSERVER
 
-    class _NotIPython(Exception):
-        pass
-
-    # see if we have IPython hooks around, if use them
-
-    try:
-        if 'IPython' in sys.modules:
-            from IPython import get_ipython
-            ip = get_ipython()
-            if ip is None:
-                raise _NotIPython()
-
-            if _IP_REGISTERED:
-                return
-
-            def post_execute():
-                if matplotlib.is_interactive():
-                    draw_all()
-
-            # IPython >= 2
-            try:
-                ip.events.register('post_execute', post_execute)
-            except AttributeError:
-                # IPython 1.x
-                ip.register_post_execute(post_execute)
-
-            _IP_REGISTERED = post_execute
-            _INSTALL_FIG_OBSERVER = False
+    if _IP_REGISTERED:
+        return
+    # See if we have IPython hooks around, if so use them.
+    # Use ``sys.modules.get(name)`` rather than ``name in sys.modules`` as
+    # entries can also have been explicitly set to None.
+    mod_ipython = sys.modules.get("IPython")
+    if not mod_ipython:
+        _INSTALL_FIG_OBSERVER = True
+        return
+    ip = mod_ipython.get_ipython()
+    if not ip:
+        _INSTALL_FIG_OBSERVER = True
+        return
 
-            # trigger IPython's eventloop integration, if available
-            from IPython.core.pylabtools import backend2gui
+    def post_execute():
+        if matplotlib.is_interactive():
+            draw_all()
 
-            ipython_gui_name = backend2gui.get(get_backend())
-            if ipython_gui_name:
-                ip.enable_gui(ipython_gui_name)
-        else:
-            _INSTALL_FIG_OBSERVER = True
+    try:  # IPython >= 2
+        ip.events.register("post_execute", post_execute)
+    except AttributeError:  # IPython 1.x
+        ip.register_post_execute(post_execute)
+    _IP_REGISTERED = post_execute
+    _INSTALL_FIG_OBSERVER = False
 
-    # import failed or ipython is not running
-    except (ImportError, _NotIPython):
-        _INSTALL_FIG_OBSERVER = True
+    from IPython.core.pylabtools import backend2gui
+    # trigger IPython's eventloop integration, if available
+    ipython_gui_name = backend2gui.get(get_backend())
+    if ipython_gui_name:
+        ip.enable_gui(ipython_gui_name)
 
 
 def uninstall_repl_displayhook():
     """
-    Uninstall the matplotlib display hook.
+    Uninstall the Matplotlib display hook.
 
     .. warning::
 
@@ -174,8 +163,8 @@ def uninstall_repl_displayhook():
     .. warning::
 
        If you are using vanilla python and have installed another
-       display hook this will reset ``sys.displayhook`` to what ever
-       function was there when matplotlib installed it's displayhook,
+       display hook, this will reset ``sys.displayhook`` to what ever
+       function was there when Matplotlib installed its displayhook,
        possibly discarding your changes.
     """
     global _IP_REGISTERED

lib/matplotlib/testing/conftest.py

@@ -48,7 +48,7 @@ def mpl_test_settings(request):
 
             # special case Qt backend importing to avoid conflicts
             if backend.lower().startswith('qt4'):
-                if any(k in sys.modules for k in ('PyQt5', 'PySide2')):
+                if any(sys.modules.get(k) for k in ('PyQt5', 'PySide2')):
                     pytest.skip('Qt5 binding already imported')
                 try:
                     import PyQt4
@@ -59,7 +59,7 @@ def mpl_test_settings(request):
                     except ImportError:
                         pytest.skip("Failed to import a Qt4 binding.")
             elif backend.lower().startswith('qt5'):
-                if any(k in sys.modules for k in ('PyQt4', 'PySide')):
+                if any(sys.modules.get(k) for k in ('PyQt4', 'PySide')):
                     pytest.skip('Qt4 binding already imported')
                 try:
                     import PyQt5