Make it possible to use rc_context as a decorator.

anntzer · anntzer · commit e4d63b163b2d · 2020-03-24T10:07:59.000+01:00
See changelog note.

There are also other advantages with contextmanager, e.g. reusing a
context multiple times works

```
ctx = rc_context(...)
with ctx: ...  # in the context
...  # out of the context
with ctx: ...  # in the context again
```

(with a test for the decorator form, which is equivalent) but in
practice this will often run into the issue of early/late resolution of
rcParams, so I didn't mention it in the changelog.

xkcd() still needs to be implemented manually in terms of
`__enter__`/`__exit__` but at least we know that creation of the
contextmanager cannot fail, so things are simpler.

Also we don't need to check for whether the backend has been
resolved because rcParams now just prevent re-setting the backend to
auto_backend_sentinel.
diff --git a/doc/users/next_whats_new/2018-09-19-AL.rst b/doc/users/next_whats_new/2018-09-19-AL.rst
@@ -0,0 +1,11 @@
+:orphan:
+
+`matplotlib.rc_context` is now a `contextlib.contextmanager`
+````````````````````````````````````````````````````````````
+
+`matplotlib.rc_context` can now be used as a decorator (technically, it is now
+implemented as a `contextlib.contextmanager`), e.g. ::
+
+    @rc_context({"lines.linewidth": 2})
+    def some_function(...):
+        ...
diff --git a/doc/users/prev_whats_new/whats_new_1.3.rst b/doc/users/prev_whats_new/whats_new_1.3.rst
@@ -86,7 +86,7 @@ New plotting features
 To give your plots a sense of authority that they may be missing,
 Michael Droettboom (inspired by the work of many others in
 :ghpull:`1329`) has added an `xkcd-style <http://xkcd.com/>`__ sketch
-plotting mode.  To use it, simply call :func:`matplotlib.pyplot.xkcd`
+plotting mode.  To use it, simply call `matplotlib.pyplot.xkcd`
 before creating your plot. For really fine control, it is also possible
 to modify each artist's sketch parameters individually with
 :meth:`matplotlib.artist.Artist.set_sketch_params`.
diff --git a/lib/matplotlib/__init__.py b/lib/matplotlib/__init__.py
@@ -1035,7 +1035,8 @@ def rc_file(fname, *, use_default_template=True):
                          if k not in STYLE_BLACKLIST})
 
 
-class rc_context:
+@contextlib.contextmanager
+def rc_context(rc=None, fname=None):
     """
     Return a context manager for managing rc settings.
 
@@ -1052,49 +1053,24 @@ class rc_context:
         with mpl.rc_context(rc={'text.usetex': True}, fname='screen.rc'):
             plt.plot(x, a)
 
-    The 'rc' dictionary takes precedence over the settings loaded from
-    'fname'.  Passing a dictionary only is also valid. For example a
-    common usage is::
+    The *rc* dictionary takes precedence over the settings loaded from *fname*.
+    Passing a dictionary only is also valid.  For example, a common usage is::
 
-        with mpl.rc_context(rc={'interactive': False}):
+        with mpl.rc_context({'interactive': False}):
             fig, ax = plt.subplots()
             ax.plot(range(3), range(3))
             fig.savefig('A.png', format='png')
             plt.close(fig)
     """
-    # While it may seem natural to implement rc_context using
-    # contextlib.contextmanager, that would entail always calling the finally:
-    # clause of the contextmanager (which restores the original rcs) including
-    # during garbage collection; as a result, something like `plt.xkcd();
-    # gc.collect()` would result in the style being lost (as `xkcd()` is
-    # implemented on top of rc_context, and nothing is holding onto context
-    # manager except possibly circular references.
-
-    def __init__(self, rc=None, fname=None):
-        self._orig = rcParams.copy()
-        try:
-            if fname:
-                rc_file(fname)
-            if rc:
-                rcParams.update(rc)
-        except Exception:
-            self.__fallback()
-            raise
-
-    def __fallback(self):
-        # If anything goes wrong, revert to the original rcs.
-        updated_backend = self._orig['backend']
-        dict.update(rcParams, self._orig)
-        # except for the backend.  If the context block triggered resolving
-        # the auto backend resolution keep that value around
-        if self._orig['backend'] is rcsetup._auto_backend_sentinel:
-            rcParams['backend'] = updated_backend
-
-    def __enter__(self):
-        return self
-
-    def __exit__(self, exc_type, exc_value, exc_tb):
-        self.__fallback()
+    orig = rcParams.copy()
+    try:
+        if fname:
+            rc_file(fname)
+        if rc:
+            rcParams.update(rc)
+        yield
+    finally:
+        dict.update(rcParams, orig)  # Revert to the original rcs.
 
 
 def use(backend, *, force=True):
diff --git a/lib/matplotlib/pyplot.py b/lib/matplotlib/pyplot.py
@@ -408,12 +408,11 @@ def setp(obj, *args, **kwargs):
 
 def xkcd(scale=1, length=100, randomness=2):
     """
-    Turn on `xkcd <https://xkcd.com/>`_ sketch-style drawing mode.
-    This will only have effect on things drawn after this function is
-    called.
+    Turn on `xkcd <https://xkcd.com/>`_ sketch-style drawing mode.  This will
+    only have effect on things drawn after this function is called.
 
     For best results, the "Humor Sans" font should be installed: it is
-    not included with matplotlib.
+    not included with Matplotlib.
 
     Parameters
     ----------
@@ -440,29 +439,46 @@ def xkcd(scale=1, length=100, randomness=2):
         # This figure will be in regular style
         fig2 = plt.figure()
     """
-    if rcParams['text.usetex']:
-        raise RuntimeError(
-            "xkcd mode is not compatible with text.usetex = True")
-
-    from matplotlib import patheffects
-    return rc_context({
-        'font.family': ['xkcd', 'xkcd Script', 'Humor Sans', 'Comic Neue',
-                        'Comic Sans MS'],
-        'font.size': 14.0,
-        'path.sketch': (scale, length, randomness),
-        'path.effects': [patheffects.withStroke(linewidth=4, foreground="w")],
-        'axes.linewidth': 1.5,
-        'lines.linewidth': 2.0,
-        'figure.facecolor': 'white',
-        'grid.linewidth': 0.0,
-        'axes.grid': False,
-        'axes.unicode_minus': False,
-        'axes.edgecolor': 'black',
-        'xtick.major.size': 8,
-        'xtick.major.width': 3,
-        'ytick.major.size': 8,
-        'ytick.major.width': 3,
-    })
+    return _xkcd(scale, length, randomness)
+
+
+class _xkcd:
+    # This cannot be implemented in terms of rc_context() because this needs to
+    # work as a non-contextmanager too.
+
+    def __init__(self, scale, length, randomness):
+        self._orig = rcParams.copy()
+
+        if rcParams['text.usetex']:
+            raise RuntimeError(
+                "xkcd mode is not compatible with text.usetex = True")
+
+        from matplotlib import patheffects
+        rcParams.update({
+            'font.family': ['xkcd', 'xkcd Script', 'Humor Sans', 'Comic Neue',
+                            'Comic Sans MS'],
+            'font.size': 14.0,
+            'path.sketch': (scale, length, randomness),
+            'path.effects': [
+                patheffects.withStroke(linewidth=4, foreground="w")],
+            'axes.linewidth': 1.5,
+            'lines.linewidth': 2.0,
+            'figure.facecolor': 'white',
+            'grid.linewidth': 0.0,
+            'axes.grid': False,
+            'axes.unicode_minus': False,
+            'axes.edgecolor': 'black',
+            'xtick.major.size': 8,
+            'xtick.major.width': 3,
+            'ytick.major.size': 8,
+            'ytick.major.width': 3,
+        })
+
+    def __enter__(self):
+        return self
+
+    def __exit__(self, *args):
+        dict.update(rcParams, self._orig)
 
 
 ## Figures ##
diff --git a/lib/matplotlib/tests/test_rcparams.py b/lib/matplotlib/tests/test_rcparams.py
@@ -54,6 +54,14 @@ def test_rcparams(tmpdir):
         assert mpl.rcParams['lines.linewidth'] == 44
     assert mpl.rcParams['lines.linewidth'] == linewidth
 
+    # test context as decorator (and test reusability, by calling func twice)
+    @mpl.rc_context({'lines.linewidth': 44})
+    def func():
+        assert mpl.rcParams['lines.linewidth'] == 44
+
+    func()
+    func()
+
     # test rc_file
     mpl.rc_file(rcpath)
     assert mpl.rcParams['lines.linewidth'] == 33
