FIX: delay resolving automatic backend until actually needed

tacaswell · tacaswell · commit 89159614b349 · 2022-03-03T17:08:04.000-05:00
This is to prevent the early importing of GUI bindings in the case where the user wants to use something later in our search list, but also has a toolkit higher in our list installed which we select. Thanks to @anntzer for the implementation suggestion.
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/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()
diff --git a/lib/matplotlib/tests/test_backends_interactive.py b/lib/matplotlib/tests/test_backends_interactive.py
@@ -9,6 +9,7 @@
 import sys
 import time
 import urllib.request
+import textwrap
 
 import pytest
 
@@ -255,6 +256,29 @@ def test_interactive_thread_safety(env):
     assert proc.stdout.count("CloseEvent") == 1
 
 
+def test_lazy_auto_backend_selection():
+
+    def _impl():
+        import matplotlib
+        import matplotlib.pyplot as plt
+        # just importing pyplot should not be enough to trigger resolution
+        bk = dict.__getitem__(matplotlib.rcParams, 'backend')
+        assert not isinstance(bk, str)
+        assert plt._backend_mod is None
+        # but actually plotting should
+        plt.plot(5)
+        assert plt._backend_mod is not None
+        bk = dict.__getitem__(matplotlib.rcParams, 'backend')
+        assert isinstance(bk, str)
+
+    proc = subprocess.run(
+        [sys.executable, "-c",
+         textwrap.dedent(inspect.getsource(_impl)) + "\n_impl()"],
+        env={**os.environ, "SOURCE_DATE_EPOCH": "0"},
+        timeout=_test_timeout, check=True,
+        stdout=subprocess.PIPE, universal_newlines=True)
+
+
 @pytest.mark.skipif('TF_BUILD' in os.environ,
                     reason="this test fails an azure for unknown reasons")
 @pytest.mark.skipif(os.name == "nt", reason="Cannot send SIGINT on Windows.")
diff --git a/lib/matplotlib/tests/test_rcparams.py b/lib/matplotlib/tests/test_rcparams.py
@@ -497,7 +497,8 @@ def test_backend_fallback_headless(tmpdir):
             [sys.executable, "-c",
              "import matplotlib;"
              "matplotlib.use('tkagg');"
-             "import matplotlib.pyplot"
+             "import matplotlib.pyplot;"
+             "matplotlib.pyplot.plot(42);"
              ],
             env=env, check=True, stderr=subprocess.DEVNULL)
 
