Raise ImportError on failure to import backends.

anntzer · anntzer · commit 5a01cb8d278e · 2018-06-27T15:00:03.000+02:00
This will make backend fallback logic easier (try a backend, if it
raises ImportError, move to the next one).
diff --git a/doc/api/next_api_changes/2018-06-27-AL.rst b/doc/api/next_api_changes/2018-06-27-AL.rst
@@ -0,0 +1,6 @@
+Changes to backend loading
+``````````````````````````
+
+Failure to load backend modules (``macosx`` on non-framework builds and
+``gtk3`` when running headless) now raises `ImportError` (instead of
+`RuntimeError` and `TypeError`, respectively.
diff --git a/lib/matplotlib/backends/backend_gtk3.py b/lib/matplotlib/backends/backend_gtk3.py
@@ -23,13 +23,18 @@
 # see http://groups.google.com/groups?q=screen+dpi+x11&hl=en&lr=&ie=UTF-8&oe=UTF-8&safe=off&selm=7077.26e81ad5%40swift.cs.tcd.ie&rnum=5 for some info about screen dpi
 PIXELS_PER_INCH = 96
 
-cursord = {
-    cursors.MOVE          : Gdk.Cursor.new(Gdk.CursorType.FLEUR),
-    cursors.HAND          : Gdk.Cursor.new(Gdk.CursorType.HAND2),
-    cursors.POINTER       : Gdk.Cursor.new(Gdk.CursorType.LEFT_PTR),
-    cursors.SELECT_REGION : Gdk.Cursor.new(Gdk.CursorType.TCROSS),
-    cursors.WAIT          : Gdk.Cursor.new(Gdk.CursorType.WATCH),
+try:
+    cursord = {
+        cursors.MOVE          : Gdk.Cursor.new(Gdk.CursorType.FLEUR),
+        cursors.HAND          : Gdk.Cursor.new(Gdk.CursorType.HAND2),
+        cursors.POINTER       : Gdk.Cursor.new(Gdk.CursorType.LEFT_PTR),
+        cursors.SELECT_REGION : Gdk.Cursor.new(Gdk.CursorType.TCROSS),
+        cursors.WAIT          : Gdk.Cursor.new(Gdk.CursorType.WATCH),
     }
+except TypeError as exc:
+    # Happens when running headless.  Convert to ImportError to cooperate with
+    # backend switching.
+    raise ImportError(exc)
 
 
 class TimerGTK3(TimerBase):
diff --git a/src/_macosx.m b/src/_macosx.m
@@ -3025,23 +3025,26 @@ static bool verify_framework(void)
      && GetCurrentProcess(&psn)==noErr
      && SetFrontProcess(&psn)==noErr) return true;
 #endif
-    PyErr_SetString(PyExc_RuntimeError,
+    PyErr_SetString(PyExc_ImportError,
         "Python is not installed as a framework. The Mac OS X backend will "
         "not be able to function correctly if Python is not installed as a "
         "framework. See the Python documentation for more information on "
         "installing Python as a framework on Mac OS X. Please either reinstall "
         "Python as a framework, or try one of the other backends. If you are "
-        "using (Ana)Conda please install python.app and replace the use of 'python' "
-        "with 'pythonw'. See 'Working with Matplotlib on OSX' "
-        "in the Matplotlib FAQ for more information.");
+        "using (Ana)Conda please install python.app and replace the use of "
+        "'python' with 'pythonw'. See 'Working with Matplotlib on OSX' in the "
+        "Matplotlib FAQ for more information.");
     return false;
 }
 
 static struct PyMethodDef methods[] = {
    {"show",
     (PyCFunction)show,
     METH_NOARGS,
-    "Show all the figures and enter the main loop.\nThis function does not return until all Matplotlib windows are closed,\nand is normally not needed in interactive sessions."
+    "Show all the figures and enter the main loop.\n"
+    "\n"
+    "This function does not return until all Matplotlib windows are closed,\n"
+    "and is normally not needed in interactive sessions."
    },
    {"choose_save_file",
     (PyCFunction)choose_save_file,
@@ -3053,7 +3056,7 @@ static bool verify_framework(void)
     METH_VARARGS,
     "Sets the active cursor."
    },
-   {NULL,          NULL, 0, NULL}/* sentinel */
+   {NULL, NULL, 0, NULL} /* sentinel */
 };
 
 static struct PyModuleDef moduledef = {
