matplotlib
diff --git a/‎doc/api/next_api_changes/2019-03-07-AL.rst
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diff --git a/‎lib/matplotlib/backend_bases.py
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diff --git a/‎lib/matplotlib/backends/backend_nbagg.py
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diff --git a/‎lib/matplotlib/backends/backend_template.py
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diff --git a/‎lib/matplotlib/cbook/__init__.py
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diff --git a/‎lib/matplotlib/cbook/deprecation.py
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diff --git a/‎lib/matplotlib/tests/test_cbook.py
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@@ -0,0 +1,9 @@
+API changes
+```````````
+
+Passing the ``block`` argument of ``plt.show`` positionally is deprecated; it
+should be passed by keyword.
+
+When using the nbagg backend, ``plt.show`` used to silently accept and ignore
+all combinations of positional and keyword arguments.  This behavior is
+deprecated.
@@ -3253,8 +3253,10 @@ def draw_if_interactive(cls):
                 cls.trigger_manager_draw(manager)
 
     @classmethod
+    @cbook._make_keyword_only("3.1", "block")
     def show(cls, block=None):
-        """Show all figures.
+        """
+        Show all figures.
 
         `show` blocks by calling `mainloop` if *block* is ``True``, or if it
         is ``None`` and we are neither in IPython's ``%pylab`` mode, nor in
 
@@ -17,7 +17,7 @@
     # Jupyter/IPython 3.x or earlier
     from IPython.kernel.comm import Comm
 
-from matplotlib import is_interactive
+from matplotlib import cbook, is_interactive
 from matplotlib._pylab_helpers import Gcf
 from matplotlib.backend_bases import (
     _Backend, FigureCanvasBase, NavigationToolbar2)
@@ -241,7 +241,13 @@ def trigger_manager_draw(manager):
         manager.show()
 
     @staticmethod
-    def show(*args, **kwargs):
+    def show(*args, block=None, **kwargs):
+        if args or kwargs:
+            cbook.warn_deprecated(
+                "3.1", message="Passing arguments to show(), other than "
+                "passing 'block' by keyword, is deprecated %(since)s, and "
+                "support for it will be removed %(removal)s.")
+
         ## TODO: something to do when keyword block==False ?
         from matplotlib._pylab_helpers import Gcf
 
 
@@ -173,7 +173,7 @@ def draw_if_interactive():
     """
 
 
-def show(block=None):
+def show(*, block=None):
     """
     For image backends - is not required.
     For GUI backends - show() is usually the last line of a pyplot script and
 
@@ -32,7 +32,8 @@
 
 import matplotlib
 from .deprecation import (
-    deprecated, warn_deprecated, _rename_parameter, _delete_parameter,
+    deprecated, warn_deprecated,
+    _rename_parameter, _delete_parameter, _make_keyword_only,
     _suppress_matplotlib_deprecation_warning,
     MatplotlibDeprecationWarning, mplDeprecation)
 
 
@@ -371,6 +371,50 @@ def wrapper(*args, **kwargs):
     return wrapper
 
 
+def _make_keyword_only(since, name, func=None):
+    """
+    Decorator indicating that passing parameter *name* (or any of the following
+    ones) positionally to *func* is being deprecated.
+
+    Note that this decorator **cannot** be applied to a function that has a
+    pyplot-level wrapper, as the wrapper always pass all arguments by keyword.
+    If it is used, users will see spurious DeprecationWarnings every time they
+    call the pyplot wrapper.
+    """
+
+    if func is None:
+        return functools.partial(_make_keyword_only, since, name)
+
+    signature = inspect.signature(func)
+    POK = inspect.Parameter.POSITIONAL_OR_KEYWORD
+    KWO = inspect.Parameter.KEYWORD_ONLY
+    assert (name in signature.parameters
+            and signature.parameters[name].kind == POK), (
+        f"Matplotlib internal error: {name!r} must be a positional-or-keyword "
+        f"parameter for {func.__name__}()")
+    names = [*signature.parameters]
+    kwonly = [name for name in names[names.index(name):]
+              if signature.parameters[name].kind == POK]
+    func.__signature__ = signature.replace(parameters=[
+        param.replace(kind=inspect.Parameter.KEYWORD_ONLY)
+        if param.name in kwonly
+        else param
+        for param in signature.parameters.values()])
+
+    @functools.wraps(func)
+    def wrapper(*args, **kwargs):
+        bound = signature.bind(*args, **kwargs)
+        if name in bound.arguments and name not in kwargs:
+            warn_deprecated(
+                since, message="Passing the %(name)s %(obj_type)s "
+                "positionally is deprecated since Matplotlib %(since)s; the "
+                "parameter will become keyword-only %(removal)s.",
+                name=name, obj_type=f"parameter of {func.__name__}()")
+        return func(*args, **kwargs)
+
+    return wrapper
+
+
 @contextlib.contextmanager
 def _suppress_matplotlib_deprecation_warning():
     with warnings.catch_warnings():
 
@@ -13,7 +13,8 @@
 
 import matplotlib.cbook as cbook
 import matplotlib.colors as mcolors
-from matplotlib.cbook import delete_masked_points as dmp
+from matplotlib.cbook import (
+    MatplotlibDeprecationWarning, delete_masked_points as dmp)
 
 
 def test_is_hashable():
@@ -545,3 +546,17 @@ def test_safe_first_element_pandas_series(pd):
     s = pd.Series(range(5), index=range(10, 15))
     actual = cbook.safe_first_element(s)
     assert actual == 0
+
+
+def test_make_keyword_only(recwarn):
+    @cbook._make_keyword_only("3.0", "arg")
+    def func(pre, arg, post=None):
+        pass
+
+    func(1, arg=2)
+    assert len(recwarn) == 0
+
+    with pytest.warns(MatplotlibDeprecationWarning):
+        func(1, 2)
+    with pytest.warns(MatplotlibDeprecationWarning):
+        func(1, 2, 3)