FIX: delay resolving automatic backend until actually needed

tacaswell · tacaswell · commit fa6e9380cd91 · 2021-12-18T21:06:01.000-05:00
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.
diff --git a/lib/matplotlib/pyplot.py b/lib/matplotlib/pyplot.py
@@ -104,6 +104,15 @@ def _copy_docstring_and_deprecators(method, func=None):
 
 ## Global ##
 
+def _initial_switch_backend():
+    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 respect `mpl.is_interactive()`
+    # to determine if they should trigger a draw.
+    install_repl_displayhook()
+
 
 _IP_REGISTERED = None
 _INSTALL_FIG_OBSERVER = False
@@ -289,9 +298,13 @@ 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)
             and threading.current_thread() is not threading.main_thread()):
         _api.warn_external(
@@ -316,6 +329,8 @@ 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)
 
 
@@ -2229,15 +2244,6 @@ 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_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 not plt.switch_backend._ever_called
+        # but actually plotting should
+        plt.plot(5)
+        assert plt.switch_backend._ever_called
+        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.")
