Merge pull request #17796 from meeseeksmachine/auto-backport-of-pr-17764-on-v3.3.x

timhoffm · web-flow · commit f1778266d841 · 2020-06-30T01:14:36.000+02:00
Backport PR #17764 on branch v3.3.x (FIX: be more careful about not importing pyplot early)
diff --git a/lib/matplotlib/__init__.py b/lib/matplotlib/__init__.py
@@ -1136,16 +1136,34 @@ def use(backend, *, force=True):
     matplotlib.get_backend
     """
     name = validate_backend(backend)
+    # we need to use the base-class method here to avoid (prematurely)
+    # resolving the "auto" backend setting
     if dict.__getitem__(rcParams, 'backend') == name:
         # Nothing to do if the requested backend is already set
         pass
     else:
-        try:
-            from matplotlib import pyplot as plt
-            plt.switch_backend(name)
-        except ImportError:
-            if force:
-                raise
+        # if pyplot is not already imported, do not import it.  Doing
+        # so may trigger a `plt.switch_backend` to the _default_ backend
+        # before we get a chance to change to the one the user just requested
+        plt = sys.modules.get('matplotlib.pyplot')
+        # if pyplot is imported, then try to change backends
+        if plt is not None:
+            try:
+                # we need this import check here to re-raise if the
+                # user does not have the libraries to support their
+                # chosen backend installed.
+                plt.switch_backend(name)
+            except ImportError:
+                if force:
+                    raise
+        # if we have not imported pyplot, then we can set the rcParam
+        # value which will be respected when the user finally imports
+        # pyplot
+        else:
+            rcParams['backend'] = backend
+    # if the user has asked for a given backend, do not helpfully
+    # fallback
+    rcParams['backend_fallback'] = False
 
 
 if os.environ.get('MPLBACKEND'):
diff --git a/lib/matplotlib/tests/test_rcparams.py b/lib/matplotlib/tests/test_rcparams.py
@@ -473,7 +473,10 @@ def test_backend_fallback_headless(tmpdir):
     with pytest.raises(subprocess.CalledProcessError):
         subprocess.run(
             [sys.executable, "-c",
-             "import matplotlib; matplotlib.use('tkagg')"],
+             ("import matplotlib;" +
+              "matplotlib.use('tkagg');" +
+              "import matplotlib.pyplot")
+             ],
             env=env, check=True)
 
 
