Add is_widget_backend() to the widget code path

src/plopp/backends/matplotlib/figure.py

@@ -7,7 +7,7 @@
 
 from ...graphics import BaseFig
 from .canvas import Canvas
-from .utils import fig_to_bytes, is_interactive_backend
+from .utils import fig_to_bytes, is_widget_backend
 
 try:
     from ipywidgets import VBox
@@ -218,7 +218,7 @@ def to_widget(self):
 
 
 def Figure(*args, **kwargs):
-    if is_interactive_backend():
+    if is_widget_backend():
         return InteractiveFigure(*args, **kwargs)
     else:
         return StaticFigure(*args, **kwargs)

src/plopp/backends/matplotlib/utils.py

@@ -51,6 +51,17 @@ def is_interactive_backend() -> bool:
     )
 
 
+def is_widget_backend() -> bool:
+    """
+    Return ``True`` if the current backend used by Matplotlib renders figures as
+    Jupyter widgets (e.g. ipympl). Note that this is different from
+    :func:`is_interactive_backend`: GUI backends such as Qt or Tk are interactive
+    but do not render figures as Jupyter widgets.
+    """
+    backend = mpl.get_backend().lower()
+    return any(b in backend for b in ('ipympl', 'nbagg', 'notebook', 'widget'))
+
+
 def make_figure(*args, **kwargs) -> plt.Figure:
     """
     Create a new figure.

tests/backends/matplotlib/mpl_utils_test.py

@@ -0,0 +1,44 @@
+# SPDX-License-Identifier: BSD-3-Clause
+# Copyright (c) 2023 Scipp contributors (https://github.com/scipp)
+
+import matplotlib
+import pytest
+
+from plopp.backends.matplotlib.figure import StaticFigure
+from plopp.backends.matplotlib.utils import is_interactive_backend, is_widget_backend
+from plopp.data.testing import data_array
+
+
+@pytest.mark.parametrize(
+    'backend', ['qtagg', 'tkagg', 'macosx', 'gtk4agg', 'wxagg', 'webagg']
+)
+def test_gui_backends_are_interactive_but_not_widget(monkeypatch, backend):
+    monkeypatch.setattr(matplotlib, 'get_backend', lambda: backend)
+    assert is_interactive_backend()
+    assert not is_widget_backend()
+
+
+@pytest.mark.parametrize('backend', ['module://ipympl.backend_nbagg', 'nbagg'])
+def test_widget_backends_are_interactive_and_widget(monkeypatch, backend):
+    monkeypatch.setattr(matplotlib, 'get_backend', lambda: backend)
+    assert is_interactive_backend()
+    assert is_widget_backend()
+
+
+@pytest.mark.parametrize(
+    'backend', ['agg', 'module://matplotlib_inline.backend_inline', 'pdf', 'svg']
+)
+def test_static_backends_are_neither_interactive_nor_widget(monkeypatch, backend):
+    monkeypatch.setattr(matplotlib, 'get_backend', lambda: backend)
+    assert not is_interactive_backend()
+    assert not is_widget_backend()
+
+
+@pytest.mark.parametrize('backend', ['qtagg', 'tkagg', 'macosx'])
+def test_gui_backend_makes_static_figure(monkeypatch, backend):
+    # GUI backends are used when plotting from a terminal, where ipywidgets-based
+    # interactive figures cannot be displayed. See issue #589.
+    monkeypatch.setattr(matplotlib, 'get_backend', lambda: backend)
+    fig = data_array(ndim=1).plot()
+    assert isinstance(fig, StaticFigure)
+    assert not fig.interactive