matplotlib · QuLogic · Mar 3, 2022 · Dec 19, 2021 · Jan 12, 2022 · Dec 19, 2021
diff --git a/doc/api/backend_qt_api.rst b/doc/api/backend_qt_api.rst
@@ -1,15 +1,70 @@
 :mod:`.backend_qtagg`, :mod:`.backend_qtcairo`
 ==============================================
 
-**NOTE** These backends are not documented here, to avoid adding a dependency
-to building the docs.
+**NOTE** These backends are not (auto) documented here, to avoid adding a
+dependency to building the docs.
 
 .. redirect-from:: /api/backend_qt4agg_api
 .. redirect-from:: /api/backend_qt4cairo_api
 .. redirect-from:: /api/backend_qt5agg_api
 .. redirect-from:: /api/backend_qt5cairo_api
 
+.. module:: matplotlib.backends.qt_compat
+.. module:: matplotlib.backends.backend_qt
 .. module:: matplotlib.backends.backend_qtagg
 .. module:: matplotlib.backends.backend_qtcairo
 .. module:: matplotlib.backends.backend_qt5agg
 .. module:: matplotlib.backends.backend_qt5cairo
+
+.. _QT_bindings:
+
+Qt Bindings
+-----------
+
+There are currently 2 actively supported Qt versions, Qt5 and Qt6, and two
+supported Python bindings per version -- `PyQt5
+<https://www.riverbankcomputing.com/static/Docs/PyQt5/>`_ and `PySide2
+<https://doc.qt.io/qtforpython-5/contents.html>`_ for Qt5 and `PyQt6
+<https://www.riverbankcomputing.com/static/Docs/PyQt6/>`_ and `PySide6
+<https://doc.qt.io/qtforpython/contents.html>`_ for Qt6 [#]_.  While both PyQt
+and Qt for Python (aka PySide) closely mirror the underlying C++ API they are
+wrapping, they are not drop-in replacements for each other [#]_.  To account
+for this, Matplotlib has an internal API compatibility layer in
+`matplotlib.backends.qt_compat` which covers our needs.  Despite being a public
+module, we do not consider this to be a stable user-facing API and it may
+change without warning [#]_.
+
+Previously Matplotlib's Qt backends had the Qt version number in the name, both
+in the module and the :rc:`backend` value
+(e.g. ``matplotlib.backends.backend_qt4agg`` and
+``matplotlib.backends.backend_qt5agg``). However as part of adding support for
+Qt6 we were able to support both Qt5 and Qt6 with a single implementation with
+all of the Qt version and binding support handled in
+`~matplotlib.backends.qt_compat`.  A majority of the renderer agnostic Qt code
+is now in `matplotlib.backends.backend_qt` with specialization for AGG in
+``backend_qtagg`` and cairo in ``backend_qtcairo``.
+
+The binding is selected at run time based on what bindings are already imported
+(by checking for the ``QtCore`` sub-package), then by the :envvar:`QT_API`
+environment variable, and finally by the :rc:`backend`.  In all cases when we
+need to search, the order is ``PyQt6``, ``PySide6``, ``PyQt5``, ``PySide2``.
+See :ref:`QT_API-usage` for usage instructions.
+
+The ``backend_qt5``, ``backend_qt5agg``, and ``backend_qt5cairo`` are provided
+and force the use of a Qt5 binding for backwards compatibility.  Their use is
+discouraged (but not deprecated) and ``backend_qt``, ``backend_qtagg``, or
+``backend_qtcairo`` should be preferred instead.  However, these modules will
+not be deprecated until we drop support for Qt5.
+
+
+
+
+.. [#] There is also `PyQt4
+       <https://www.riverbankcomputing.com/static/Docs/PyQt4/>`_ and `PySide
+       <https://srinikom.github.io/pyside-docs/>`_ for Qt4 but these are no
+       longer supported by Matplotlib and upstream support for Qt4 ended
+       in 2015.
+.. [#] Despite the slight API differences, the more important distinction
+       between the PyQt and Qt for Python series of bindings is licensing.
+.. [#] If you are looking for a general purpose compatibility library please
+       see `qtpy <https://github.com/spyder-ide/qtpy>`_.
diff --git a/doc/users/explain/backends.rst b/doc/users/explain/backends.rst
@@ -244,7 +244,8 @@ The :envvar:`QT_API` environment variable can be set to override the search
 when nothing has already been loaded. It may be set to (case-insensitively)
 PyQt6, PySide6, PyQt5, or PySide2 to pick the version and binding to use. If
 the chosen implementation is unavailable, the Qt backend will fail to load
-without attempting any other Qt implementations.
+without attempting any other Qt implementations.  See :ref:`QT_bindings` for
+more details.
 
 Using non-builtin backends
 --------------------------

diff --git a/lib/matplotlib/backends/__init__.py b/lib/matplotlib/backends/__init__.py
@@ -1,2 +1,3 @@
 # NOTE: plt.switch_backend() (called at import time) will add a "backend"
 # attribute here for backcompat.
+_QT_FORCE_QT5_BINDING = False
diff --git a/lib/matplotlib/backends/backend_qt.py b/lib/matplotlib/backends/backend_qt.py
@@ -114,6 +114,28 @@ def _create_qApp():
                     QtCore.Qt.AA_EnableHighDpiScaling)
             except AttributeError:  # Only for Qt>=5.6, <6.
                 pass
+
+            # Check to make sure a QApplication from a different major version
+            # of Qt is not instantiated in the process
+            if QT_API in {'PyQt6', 'PySide6'}:
+                other_bindings = ('PyQt5', 'PySide2')
+            elif QT_API in {'PyQt5', 'PySide2'}:
+                other_bindings = ('PyQt6', 'PySide6')
+            else:
+                raise RuntimeError("Should never be here")
+
+            for binding in other_bindings:
+                mod = sys.modules.get(f'{binding}.QtWidgets')
+                if mod is not None and mod.QApplication.instance() is not None:
+                    other_core = sys.modules.get(f'{binding}.QtCore')
+                    _api.warn_external(
+                        f'Matplotlib is using {QT_API} which wraps '
+                        f'{QtCore.qVersion()} however an instantiated '
+                        f'QApplication from {binding} which wraps '
+                        f'{other_core.qVersion()} exists.  Mixing Qt major '
+                        'versions may not work as expected.'
+                    )
+                    break
             try:
                 QtWidgets.QApplication.setHighDpiScaleFactorRoundingPolicy(
                     QtCore.Qt.HighDpiScaleFactorRoundingPolicy.PassThrough)

diff --git a/lib/matplotlib/backends/backend_qt5.py b/lib/matplotlib/backends/backend_qt5.py
@@ -1,4 +1,9 @@
-from .backend_qt import (
+from .. import backends
+
+backends._QT_FORCE_QT5_BINDING = True
+
+
+from .backend_qt import (  # noqa
     backend_version, SPECIAL_KEYS,
     # Public API
     cursord, _create_qApp, _BackendQT, TimerQT, MainWindow, FigureCanvasQT,
@@ -9,8 +14,15 @@
     FigureCanvasBase,  FigureManagerBase, MouseButton, NavigationToolbar2,
     TimerBase, ToolContainerBase, figureoptions, Gcf
 )
+from . import backend_qt as _backend_qt  # noqa
 
 
 @_BackendQT.export
 class _BackendQT5(_BackendQT):
     pass
+
+
+def __getattr__(name):
+    if name == 'qApp':
+        return _backend_qt.qApp
+    raise AttributeError(f"module {__name__!r} has no attribute {name!r}")
diff --git a/lib/matplotlib/backends/backend_qt5agg.py b/lib/matplotlib/backends/backend_qt5agg.py
@@ -1,10 +1,11 @@
 """
 Render to qt from agg
 """
+from .. import backends
 
-from .backend_qtagg import _BackendQTAgg
-from .backend_qtagg import (  # noqa: F401 # pylint: disable=W0611
-    FigureCanvasQTAgg, FigureManagerQT, NavigationToolbar2QT,
+backends._QT_FORCE_QT5_BINDING = True
+from .backend_qtagg import (    # noqa: F401, E402 # pylint: disable=W0611
+    _BackendQTAgg, FigureCanvasQTAgg, FigureManagerQT, NavigationToolbar2QT,
     backend_version,  FigureCanvasAgg,  FigureCanvasQT
 )
 

diff --git a/lib/matplotlib/backends/backend_qt5cairo.py b/lib/matplotlib/backends/backend_qt5cairo.py
@@ -1,6 +1,9 @@
-from .backend_qtcairo import _BackendQTCairo
-from .backend_qtcairo import (  # noqa: F401 # pylint: disable=W0611
-    FigureCanvasQTCairo, FigureCanvasCairo, FigureCanvasQT)
+from .. import backends
+
+backends._QT_FORCE_QT5_BINDING = True
+from .backend_qtcairo import (  # noqa: F401, E402 # pylint: disable=W0611
+    _BackendQTCairo, FigureCanvasQTCairo, FigureCanvasCairo, FigureCanvasQT
+)
 
 
 @_BackendQTCairo.export

diff --git a/lib/matplotlib/backends/qt_compat.py b/lib/matplotlib/backends/qt_compat.py
@@ -25,6 +25,7 @@
 import matplotlib as mpl
 from matplotlib import _api
 
+from . import _QT_FORCE_QT5_BINDING
 
 QT_API_PYQT6 = "PyQt6"
 QT_API_PYSIDE6 = "PySide6"
@@ -57,10 +58,16 @@
 # requested backend actually matches).  Use dict.__getitem__ to avoid
 # triggering backend resolution (which can result in a partially but
 # incompletely imported backend_qt5).
-elif dict.__getitem__(mpl.rcParams, "backend") in ["Qt5Agg", "Qt5Cairo"]:
+elif (
+        isinstance(dict.__getitem__(mpl.rcParams, "backend"), str) and
+        dict.__getitem__(mpl.rcParams, "backend").lower() in [
+            "qt5agg", "qt5cairo"
+        ]
+):
     if QT_API_ENV in ["pyqt5", "pyside2"]:
         QT_API = _ETS[QT_API_ENV]
     else:
+        _QT_FORCE_QT5_BINDING = True  # noqa
         QT_API = None
 # A non-Qt backend was selected but we still got there (possible, e.g., when
 # fully manually embedding Matplotlib in a Qt app without using pyplot).
@@ -112,12 +119,19 @@ def _isdeleted(obj):
 if QT_API in [QT_API_PYQT6, QT_API_PYQT5, QT_API_PYSIDE6, QT_API_PYSIDE2]:
     _setup_pyqt5plus()
 elif QT_API is None:  # See above re: dict.__getitem__.
-    _candidates = [
-        (_setup_pyqt5plus, QT_API_PYQT6),
-        (_setup_pyqt5plus, QT_API_PYSIDE6),
-        (_setup_pyqt5plus, QT_API_PYQT5),
-        (_setup_pyqt5plus, QT_API_PYSIDE2),
-    ]
+    if _QT_FORCE_QT5_BINDING:
+        _candidates = [
+            (_setup_pyqt5plus, QT_API_PYQT5),
+            (_setup_pyqt5plus, QT_API_PYSIDE2),
+        ]
+    else:
+        _candidates = [
+            (_setup_pyqt5plus, QT_API_PYQT6),
+            (_setup_pyqt5plus, QT_API_PYSIDE6),
+            (_setup_pyqt5plus, QT_API_PYQT5),
+            (_setup_pyqt5plus, QT_API_PYSIDE2),
+        ]
+
     for _setup, QT_API in _candidates:
         try:
             _setup()

diff --git a/lib/matplotlib/pyplot.py b/lib/matplotlib/pyplot.py
@@ -104,7 +104,6 @@ def _copy_docstring_and_deprecators(method, func=None):
 
 ## Global ##
 
-
 _IP_REGISTERED = None
 _INSTALL_FIG_OBSERVER = False
 
@@ -207,6 +206,28 @@ def _get_required_interactive_framework(backend_mod):
     # Inline this once the deprecation elapses.
     return backend_mod.FigureCanvas.required_interactive_framework
 
+_backend_mod = None
+
+
+def _get_backend_mod():
+    """
+    Ensure that a backend is selected and return it.
+
+    This is currently private, but may be made public in the future.
+    """
+    if _backend_mod is None:
+        # Use __getitem__ here to avoid going through the fallback logic (which
+        # will (re)import pyplot and then call switch_backend if we need to
+        # resolve the auto sentinel)
+        switch_backend(dict.__getitem__(rcParams, "backend"))
+        # Just to be safe.  Interactive mode can be turned on without calling
+        # `plt.ion()` so register it again here.  This is safe because multiple
+        # calls to `install_repl_displayhook` are no-ops and the registered
+        # function respects `mpl.is_interactive()` to determine if it should
+        # trigger a draw.
+        install_repl_displayhook()
+    return _backend_mod
+
 
 def switch_backend(newbackend):
     """
@@ -297,7 +318,7 @@ class backend_mod(matplotlib.backend_bases._Backend):
 
 
 def _warn_if_gui_out_of_main_thread():
-    if (_get_required_interactive_framework(_backend_mod)
+    if (_get_required_interactive_framework(_get_backend_mod())
             and threading.current_thread() is not threading.main_thread()):
         _api.warn_external(
             "Starting a Matplotlib GUI outside of the main thread will likely "
@@ -308,7 +329,7 @@ def _warn_if_gui_out_of_main_thread():
 def new_figure_manager(*args, **kwargs):
     """Create a new figure manager instance."""
     _warn_if_gui_out_of_main_thread()
-    return _backend_mod.new_figure_manager(*args, **kwargs)
+    return _get_backend_mod().new_figure_manager(*args, **kwargs)
 
 
 # This function's signature is rewritten upon backend-load by switch_backend.
@@ -321,7 +342,7 @@ def draw_if_interactive(*args, **kwargs):
         End users will typically not have to call this function because the
         the interactive mode takes care of this.
     """
-    return _backend_mod.draw_if_interactive(*args, **kwargs)
+    return _get_backend_mod().draw_if_interactive(*args, **kwargs)
 
 
 # This function's signature is rewritten upon backend-load by switch_backend.
@@ -370,7 +391,7 @@ def show(*args, **kwargs):
     explicitly there.
     """
     _warn_if_gui_out_of_main_thread()
-    return _backend_mod.show(*args, **kwargs)
+    return _get_backend_mod().show(*args, **kwargs)
 
 
 def isinteractive():
@@ -2226,15 +2247,6 @@ def polar(*args, **kwargs):
             set(_interactive_bk) - {'WebAgg', 'nbAgg'})
         and cbook._get_running_interactive_framework()):
     dict.__setitem__(rcParams, "backend", rcsetup._auto_backend_sentinel)
-# Set up the backend.
-switch_backend(rcParams["backend"])
-
-# Just to be safe.  Interactive mode can be turned on without
-# calling `plt.ion()` so register it again here.
-# This is safe because multiple calls to `install_repl_displayhook`
-# are no-ops and the registered function respect `mpl.is_interactive()`
-# to determine if they should trigger a draw.
-install_repl_displayhook()
 
 
 ################# REMAINING CONTENT GENERATED BY boilerplate.py ##############

diff --git a/lib/matplotlib/testing/__init__.py b/lib/matplotlib/testing/__init__.py
@@ -1,12 +1,13 @@
 """
 Helper functions for testing.
 """
-
+from pathlib import Path
+from tempfile import TemporaryDirectory
 import locale
 import logging
+import os
 import subprocess
-from pathlib import Path
-from tempfile import TemporaryDirectory
+import sys
 
 import matplotlib as mpl
 from matplotlib import _api
@@ -49,6 +50,46 @@ def setup():
     set_reproducibility_for_testing()
 
 
+def subprocess_run_helper(func, *args, timeout, **extra_env):
+    """
+    Run a function in a sub-process
+
+    Parameters
+    ----------
+    func : function
+        The function to be run.  It must be in a module that is importable.
+
+    *args : str
+        Any additional command line arguments to be passed in
+        the first argument to subprocess.run
+
+    **extra_env : Dict[str, str]
+        Any additional envromental variables to be set for
+        the subprocess.
+
+    """
+    target = func.__name__
+    module = func.__module__
+    proc = subprocess.run(
+        [sys.executable,
+         "-c",
+         f"""
+from {module} import {target}
+{target}()
+""",
+         *args],
+        env={
+            **os.environ,
+            "SOURCE_DATE_EPOCH": "0",
+            **extra_env
+        },
+        timeout=timeout, check=True,
+        stdout=subprocess.PIPE,
+        stderr=subprocess.PIPE,
+        universal_newlines=True)
+    return proc
+
+
 def _check_for_pgf(texsystem):
     """
     Check if a given TeX system + pgf is available

diff --git a/lib/matplotlib/tests/test_backend_tk.py b/lib/matplotlib/tests/test_backend_tk.py
@@ -64,6 +64,7 @@ def test_func():
 def test_blit():  # pragma: no cover
     import matplotlib.pyplot as plt
     import numpy as np
+    import matplotlib.backends.backend_tkagg  # noqa
     from matplotlib.backends import _tkagg
 
     fig, ax = plt.subplots()