Merge pull request matplotlib#14471 from tacaswell/fix_dont_close_on_noop_switch

FIX: don't close figures if switch_backend is a no-op

Author: anntzer

lib/matplotlib/__init__.py

@@ -1124,6 +1124,10 @@ def use(backend, *, force=True):
     """
     Select the backend used for rendering and GUI integration.
 
+    If pyplot is already imported, `~matplotlib.pyplot.switch_backend` is used
+    and if the new backend is different than the current backend, all Figures
+    will be closed.
+
     Parameters
     ----------
     backend : str
@@ -1154,6 +1158,8 @@ def use(backend, *, force=True):
     --------
     :ref:`backends`
     matplotlib.get_backend
+    matplotlib.pyplot.switch_backend
+
     """
     name = validate_backend(backend)
     # don't (prematurely) resolve the "auto" backend setting

lib/matplotlib/pyplot.py

@@ -210,21 +210,24 @@ def _get_backend_mod():
 
 def switch_backend(newbackend):
     """
-    Close all open figures and set the Matplotlib backend.
+    Set the pyplot backend.
 
-    The argument is case-insensitive.  Switching to an interactive backend is
-    possible only if no event loop for another interactive backend has started.
-    Switching to and from non-interactive backends is always possible.
+    Switching to an interactive backend is possible only if no event loop for
+    another interactive backend has started.  Switching to and from
+    non-interactive backends is always possible.
+
+    If the new backend is different than the current backend then all open
+    Figures will be closed via ``plt.close('all')``.
 
     Parameters
     ----------
     newbackend : str
-        The name of the backend to use.
+        The case-insensitive name of the backend to use.
+
     """
     global _backend_mod
     # make sure the init is pulled up so we can assign to it later
     import matplotlib.backends
-    close("all")
 
     if newbackend is rcsetup._auto_backend_sentinel:
         current_framework = cbook._get_running_interactive_framework()
@@ -261,6 +264,8 @@ def switch_backend(newbackend):
             switch_backend("agg")
             rcParamsOrig["backend"] = "agg"
             return
+    # have to escape the switch on access logic
+    old_backend = dict.__getitem__(rcParams, 'backend')
 
     backend_mod = importlib.import_module(
         cbook._backend_module_name(newbackend))
@@ -325,6 +330,8 @@ def draw_if_interactive():
     # Need to keep a global reference to the backend for compatibility reasons.
     # See https://github.com/matplotlib/matplotlib/issues/6092
     matplotlib.backends.backend = newbackend
+    if not cbook._str_equal(old_backend, newbackend):
+        close("all")
 
     # make sure the repl display hook is installed in case we become
     # interactive

lib/matplotlib/tests/test_pyplot.py

@@ -429,3 +429,14 @@ def test_minor_ticks():
     tick_labels = ax.get_yticklabels(minor=True)
     assert np.all(tick_pos == np.array([3.5, 6.5]))
     assert [l.get_text() for l in tick_labels] == ['a', 'b']
+
+
+def test_switch_backend_no_close():
+    plt.switch_backend('agg')
+    fig = plt.figure()
+    fig = plt.figure()
+    assert len(plt.get_fignums()) == 2
+    plt.switch_backend('agg')
+    assert len(plt.get_fignums()) == 2
+    plt.switch_backend('svg')
+    assert len(plt.get_fignums()) == 0