MNT: simplify detecting when to do the initial backend switch

tacaswell · tacaswell · commit e6a9e4d87c8d · 2021-12-30T22:22:16.000-05:00
ht @anntzer for the implementation suggestion.
diff --git a/lib/matplotlib/pyplot.py b/lib/matplotlib/pyplot.py
@@ -211,6 +211,20 @@ def _get_required_interactive_framework(backend_mod):
     return getattr(
         backend_mod.FigureCanvas, "required_interactive_framework", None)
 
+_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:
+        # this will set the global!
+        _initial_switch_backend()
+    return _backend_mod
+
 
 def switch_backend(newbackend):
     """
@@ -298,14 +312,10 @@ class backend_mod(matplotlib.backend_bases._Backend):
     # Need to keep a global reference to the backend for compatibility reasons.
     # See https://github.com/matplotlib/matplotlib/issues/6092
     matplotlib.backends.backend = newbackend
-    switch_backend._ever_called = True
-switch_backend._ever_called = False
 
 
 def _warn_if_gui_out_of_main_thread():
-    if not switch_backend._ever_called:
-        _initial_switch_backend()
-    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 "
@@ -316,7 +326,7 @@ 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.
@@ -329,9 +339,7 @@ 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.
     """
-    if not switch_backend._ever_called:
-        _initial_switch_backend()
-    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.
@@ -380,7 +388,7 @@ 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():
diff --git a/lib/matplotlib/tests/test_backends_interactive.py b/lib/matplotlib/tests/test_backends_interactive.py
@@ -271,10 +271,10 @@ def _impl():
         # just importing pyplot should not be enough to trigger resolution
         bk = dict.__getitem__(matplotlib.rcParams, 'backend')
         assert not isinstance(bk, str)
-        assert not plt.switch_backend._ever_called
+        assert plt._backend_mod is None
         # but actually plotting should
         plt.plot(5)
-        assert plt.switch_backend._ever_called
+        assert plt._backend_mod is not None
         bk = dict.__getitem__(matplotlib.rcParams, 'backend')
         assert isinstance(bk, str)
 
