matplotlib · dstansby · Oct 20, 2019 · Sep 15, 2019 · Oct 20, 2019 · Oct 20, 2019
diff --git a/doc/api/next_api_changes/behaviour.rst b/doc/api/next_api_changes/behaviour.rst
@@ -17,3 +17,11 @@ Previously, it did not necessarily have such an attribute.  A check for
 ``hasattr(figure.canvas, "manager")`` should now be replaced by
 ``figure.canvas.manager is not None`` (or ``getattr(figure.canvas, "manager", None) is not None``
 for back-compatibility).
+
+`.cbook.CallbackRegistry` now propagates exceptions when no GUI event loop is running
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+`.cbook.CallbackRegistry` now defaults to propagating exceptions thrown by
+callbacks when no interactive GUI event loop is running.  If a GUI event loop
+*is* running, `.cbook.CallbackRegistry` still defaults to just printing a
+traceback, as unhandled exceptions can make the program completely ``abort()``
+in that case.
diff --git a/lib/matplotlib/backend_bases.py b/lib/matplotlib/backend_bases.py
@@ -45,7 +45,6 @@
 
 import numpy as np
 
-import matplotlib as mpl
 from matplotlib import (
     backend_tools as tools, cbook, colors, textpath, tight_bbox, transforms,
     widgets, get_backend, is_interactive, rcParams)
@@ -2552,7 +2551,7 @@ def show(self):
         warning in `.Figure.show`.
         """
         # This should be overridden in GUI backends.
-        if mpl.backends._get_running_interactive_framework() != "headless":
+        if cbook._get_running_interactive_framework() != "headless":
             raise NonGuiException(
                 f"Matplotlib is currently using {get_backend()}, which is "
                 f"a non-GUI backend, so cannot show the figure.")

diff --git a/lib/matplotlib/backends/__init__.py b/lib/matplotlib/backends/__init__.py
@@ -1,49 +1,2 @@
-import logging
-import os
-import sys
-
-_log = logging.getLogger(__name__)
-
-
 # NOTE: plt.switch_backend() (called at import time) will add a "backend"
 # attribute here for backcompat.
-
-
-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")
-    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
-    if 'matplotlib.backends._macosx' in sys.modules:
-        if sys.modules["matplotlib.backends._macosx"].event_loop_is_running():
-            return "macosx"
-    if sys.platform.startswith("linux") and not os.environ.get("DISPLAY"):
-        return "headless"
-    return None
diff --git a/lib/matplotlib/cbook/__init__.py b/lib/matplotlib/cbook/__init__.py
@@ -36,8 +36,51 @@
     MatplotlibDeprecationWarning, mplDeprecation)
 
 
+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")
+    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
+    if 'matplotlib.backends._macosx' in sys.modules:
+        if sys.modules["matplotlib.backends._macosx"].event_loop_is_running():
+            return "macosx"
+    if sys.platform.startswith("linux") and not os.environ.get("DISPLAY"):
+        return "headless"
+    return None
+
+
 def _exception_printer(exc):
-    traceback.print_exc()
+    if _get_running_interactive_framework() in ["headless", None]:
+        raise exc
+    else:
+        traceback.print_exc()
 
 
 class _StrongRef:

diff --git a/lib/matplotlib/figure.py b/lib/matplotlib/figure.py
@@ -17,7 +17,7 @@
 import numpy as np
 
 from matplotlib import rcParams
-from matplotlib import backends, docstring, projections
+from matplotlib import docstring, projections
 from matplotlib import __version__ as _mpl_version
 from matplotlib import get_backend
 

diff --git a/lib/matplotlib/pyplot.py b/lib/matplotlib/pyplot.py
@@ -66,7 +66,6 @@
     FormatStrFormatter, ScalarFormatter, LogFormatter, LogFormatterExponent,
     LogFormatterMathtext, Locator, IndexLocator, FixedLocator, NullLocator,
     LinearLocator, LogLocator, AutoLocator, MultipleLocator, MaxNLocator)
-from matplotlib.backends import _get_running_interactive_framework
 
 _log = logging.getLogger(__name__)
 
@@ -226,8 +225,7 @@ def switch_backend(newbackend):
     required_framework = getattr(
         Backend.FigureCanvas, "required_interactive_framework", None)
     if required_framework is not None:
-        current_framework = \
-            matplotlib.backends._get_running_interactive_framework()
+        current_framework = cbook._get_running_interactive_framework()
         if (current_framework and required_framework
                 and current_framework != required_framework):
             raise ImportError(
@@ -2250,7 +2248,7 @@ def getname_val(identifier):
 # is compatible with the current running interactive framework.
 if (rcParams["backend_fallback"]
         and dict.__getitem__(rcParams, "backend") in _interactive_bk
-        and _get_running_interactive_framework()):
+        and cbook._get_running_interactive_framework()):
     dict.__setitem__(rcParams, "backend", rcsetup._auto_backend_sentinel)
 # Set up the backend.
 switch_backend(rcParams["backend"])

diff --git a/lib/matplotlib/tests/test_cbook.py b/lib/matplotlib/tests/test_cbook.py
@@ -233,22 +233,33 @@ def test_pickling(self):
                        "callbacks")
 
 
+def test_callbackregistry_default_exception_handler(monkeypatch):
+    cb = cbook.CallbackRegistry()
+    cb.connect("foo", lambda: None)
+    monkeypatch.setattr(
+        cbook, "_get_running_interactive_framework", lambda: None)
+    with pytest.raises(TypeError):
+        cb.process("foo", "argument mismatch")
+    monkeypatch.setattr(
+        cbook, "_get_running_interactive_framework", lambda: "not-none")
+    cb.process("foo", "argument mismatch")  # No error in that case.
+
+
 def raising_cb_reg(func):
     class TestException(Exception):
         pass
 
     def raising_function():
         raise RuntimeError
 
+    def raising_function_VE():
+        raise ValueError
+
     def transformer(excp):
         if isinstance(excp, RuntimeError):
             raise TestException
         raise excp
 
-    # default behavior
-    cb = cbook.CallbackRegistry()
-    cb.connect('foo', raising_function)
-
     # old default
     cb_old = cbook.CallbackRegistry(exception_handler=None)
     cb_old.connect('foo', raising_function)
@@ -257,18 +268,21 @@ def transformer(excp):
     cb_filt = cbook.CallbackRegistry(exception_handler=transformer)
     cb_filt.connect('foo', raising_function)
 
+    # filter
+    cb_filt_pass = cbook.CallbackRegistry(exception_handler=transformer)
+    cb_filt_pass.connect('foo', raising_function_VE)
+
     return pytest.mark.parametrize('cb, excp',
-                                   [[cb, None],
-                                    [cb_old, RuntimeError],
-                                    [cb_filt, TestException]])(func)
+                                   [[cb_old, RuntimeError],
+                                    [cb_filt, TestException],
+                                    [cb_filt_pass, ValueError]])(func)
 
 
 @raising_cb_reg
-def test_callbackregistry_process_exception(cb, excp):
-    if excp is not None:
-        with pytest.raises(excp):
-            cb.process('foo')
-    else:
+def test_callbackregistry_custom_exception_handler(monkeypatch, cb, excp):
+    monkeypatch.setattr(
+        cbook, "_get_running_interactive_framework", lambda: None)
+    with pytest.raises(excp):
         cb.process('foo')