matplotlib · jklymak · Jul 4, 2018 · Jun 27, 2018
diff --git a/doc/api/next_api_changes/2018-06-27-AL.rst b/doc/api/next_api_changes/2018-06-27-AL.rst
@@ -4,3 +4,10 @@ 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.
+
+Third-party backends that integrate with an interactive framework are now
+encouraged to define the ``required_interactive_framework`` global value to one
+of the following values: "qt5", "qt4", "gtk3", "wx", "tk", or "macosx". This
+information will be used to determine whether it is possible to switch from a
+backend to another (specifically, whether they use the same interactive
+framework).
diff --git a/lib/matplotlib/backends/__init__.py b/lib/matplotlib/backends/__init__.py
@@ -1,5 +1,7 @@
 import importlib
 import logging
+import os
+import sys
 import traceback
 
 import matplotlib
@@ -14,6 +16,54 @@
     if not line.startswith('  File "<frozen importlib._bootstrap'))
 
 
+def _get_running_interactive_framework():
+    """
+    Return the interactive framework whose event loop is currently running, if
+    any, or "headless" if no event loop can be started, or None.
+
+    Returns
+    -------
+    Optional[str]
+        One of the following values: "qt5", "qt4", "gtk3", "wx", "tk",
+        "macosx", "headless", ``None``.
+    """
+    QtWidgets = (sys.modules.get("PyQt5.QtWidgets")
+                 or sys.modules.get("PySide2.QtWidgets"))
+    if QtWidgets and QtWidgets.QApplication.instance():
+        return "qt5"
+    QtGui = (sys.modules.get("PyQt4.QtGui")
+             or sys.modules.get("PySide.QtGui"))
+    if QtGui and QtGui.QApplication.instance():
+        return "qt4"
+    Gtk = (sys.modules.get("gi.repository.Gtk")
+           or sys.modules.get("pgi.repository.Gtk"))
+    if Gtk and Gtk.main_level():
+        return "gtk3"
+    wx = sys.modules.get("wx")
+    if wx and wx.GetApp():
+        return "wx"
+    tkinter = sys.modules.get("tkinter")
+    if tkinter:
+        for frame in sys._current_frames().values():
+            while frame:
+                if frame.f_code == tkinter.mainloop.__code__:
+                    return "tk"
+                frame = frame.f_back
+    try:
+        from matplotlib.backends import _macosx
+    except ImportError:
+        pass
+    else:
+        # Note that the NSApp event loop is also running when a non-native
+        # toolkit (e.g. Qt5) is active, but in that case we want to report the
+        # other toolkit; thus, this check comes after the other toolkits.
+        if _macosx.event_loop_is_running():
+            return "macosx"
+    if sys.platform.startswith("linux") and not os.environ.get("DISPLAY"):
+        return "headless"
+    return None
+
+
 def pylab_setup(name=None):
     """
     Return new_figure_manager, draw_if_interactive and show for pyplot.

diff --git a/lib/matplotlib/backends/_backend_tk.py b/lib/matplotlib/backends/_backend_tk.py
@@ -994,6 +994,7 @@ def trigger(self, *args):
 
 @_Backend.export
 class _BackendTk(_Backend):
+    required_interactive_framework = "tk"
     FigureManager = FigureManagerTk
 
     @classmethod

diff --git a/lib/matplotlib/backends/backend_gtk3.py b/lib/matplotlib/backends/backend_gtk3.py
@@ -976,6 +976,7 @@ def error_msg_gtk(msg, parent=None):
 
 @_Backend.export
 class _BackendGTK3(_Backend):
+    required_interactive_framework = "gtk3"
     FigureCanvas = FigureCanvasGTK3
     FigureManager = FigureManagerGTK3
 

diff --git a/lib/matplotlib/backends/backend_macosx.py b/lib/matplotlib/backends/backend_macosx.py
@@ -185,6 +185,7 @@ def set_message(self, message):
 
 @_Backend.export
 class _BackendMac(_Backend):
+    required_interactive_framework = "macosx"
     FigureCanvas = FigureCanvasMac
     FigureManager = FigureManagerMac
 

diff --git a/lib/matplotlib/backends/backend_qt4.py b/lib/matplotlib/backends/backend_qt4.py
@@ -7,4 +7,4 @@
 
 @_BackendQT5.export
 class _BackendQT4(_BackendQT5):
-    pass
+    required_interactive_framework = "qt4"
diff --git a/lib/matplotlib/backends/backend_qt5.py b/lib/matplotlib/backends/backend_qt5.py
@@ -1109,6 +1109,7 @@ def exception_handler(type, value, tb):
 
 @_Backend.export
 class _BackendQT5(_Backend):
+    required_interactive_framework = "qt5"
     FigureCanvas = FigureCanvasQT
     FigureManager = FigureManagerQT
 

diff --git a/lib/matplotlib/backends/backend_wx.py b/lib/matplotlib/backends/backend_wx.py
@@ -1930,6 +1930,7 @@ def OnPrintPage(self, page):
 
 @_Backend.export
 class _BackendWx(_Backend):
+    required_interactive_framework = "wx"
     FigureCanvas = FigureCanvasWx
     FigureManager = FigureManagerWx
     _frame_class = FigureFrameWx

diff --git a/src/_macosx.m b/src/_macosx.m
@@ -2789,6 +2789,16 @@ - (int)index
 }
 @end
 
+static PyObject*
+event_loop_is_running(PyObject* self)
+{
+    if ([NSApp isRunning]) {
+        Py_RETURN_TRUE;
+    } else {
+        Py_RETURN_FALSE;
+    }
+}
+
 static PyObject*
 show(PyObject* self)
 {
@@ -3038,6 +3048,11 @@ static bool verify_framework(void)
 }
 
 static struct PyMethodDef methods[] = {
+   {"event_loop_is_running",
+    (PyCFunction)event_loop_is_running,
+    METH_NOARGS,
+    "Return whether the NSApp main event loop is currently running."
+   },
    {"show",
     (PyCFunction)show,
     METH_NOARGS,