Be a bit more stringent on invalid inputs.

and give the user fewer ways to shoot themselves in the foot.

The change in backend_cairo is on private API; _save is only ever called
by the various print_foo methods, with a valid fmt.

doc/api/next_api_changes/2018-10-25-AL.rst

@@ -0,0 +1,13 @@
+Invalid inputs
+``````````````
+
+Passing invalid locations to `legend` and `table` used to fallback on a default
+location.  This behavior is deprecated and will throw an exception in a future
+version.
+
+`offsetbox.AnchoredText` is unable to handle the ``horizontalalignment`` or
+``verticalalignment`` kwargs, and used to ignore them with a warning.  This
+behavior is deprecated and will throw an exception in a future version.
+
+Passing steps less than 1 or greater than 10 to `MaxNLocator` used to result in
+undefined behavior.  It now throws a ValueError.

lib/matplotlib/backends/backend_cairo.py

@@ -8,7 +8,6 @@
 
 import copy
 import gzip
-import warnings
 
 import numpy as np
 
@@ -602,8 +601,7 @@ def _save(self, fo, fmt, **kwargs):
                     fo = gzip.GzipFile(None, 'wb', fileobj=fo)
             surface = cairo.SVGSurface(fo, width_in_points, height_in_points)
         else:
-            warnings.warn("unknown format: %s" % fmt, stacklevel=2)
-            return
+            raise ValueError("Unknown format: {!r}".format(fmt))
 
         # surface.set_dpi() can be used
         renderer = RendererCairo(self.figure.dpi)

lib/matplotlib/cbook/deprecation.py

@@ -24,7 +24,7 @@ def _generate_deprecation_message(
         obj_type='attribute', addendum='', *, removal=''):
 
     if removal == "":
-        removal = {"2.2": "in 3.1", "3.0": "in 3.2"}.get(
+        removal = {"2.2": "in 3.1", "3.0": "in 3.2", "3.1": "in 3.3"}.get(
             since, "two minor releases later")
     elif removal:
         if pending:

lib/matplotlib/legend.py

@@ -491,22 +491,26 @@ def __init__(self, parent, handles, labels,
         if isinstance(loc, str):
             if loc not in self.codes:
                 if self.isaxes:
-                    warnings.warn('Unrecognized location "%s". Falling back '
-                                  'on "best"; valid locations are\n\t%s\n'
-                                  % (loc, '\n\t'.join(self.codes)))
+                    cbook.warn_deprecated(
+                        "3.1", message="Unrecognized location {!r}. Falling "
+                        "back on 'best'; valid locations are\n\t{}\n"
+                        "This will raise an exception %(removal)s."
+                        .format(loc, '\n\t'.join(self.codes)))
                     loc = 0
                 else:
-                    warnings.warn('Unrecognized location "%s". Falling back '
-                                  'on "upper right"; '
-                                  'valid locations are\n\t%s\n'
-                                  % (loc, '\n\t'.join(self.codes)))
+                    cbook.warn_deprecated(
+                        "3.1", message="Unrecognized location {!r}. Falling "
+                        "back on 'upper right'; valid locations are\n\t{}\n'
+                        "This will raise an exception %(removal)s."
+                        .format(loc, '\n\t'.join(self.codes)))
                     loc = 1
             else:
                 loc = self.codes[loc]
         if not self.isaxes and loc == 0:
-            warnings.warn('Automatic legend placement (loc="best") not '
-                          'implemented for figure legend. '
-                          'Falling back on "upper right".')
+            cbook.warn_deprecated(
+                "3.1", message="Automatic legend placement (loc='best') not "
+                "implemented for figure legend. Falling back on 'upper "
+                "right'. This will raise an exception %(removal)s.")
             loc = 1
 
         self._mode = mode

lib/matplotlib/offsetbox.py

@@ -1249,8 +1249,10 @@ def __init__(self, s, loc, pad=0.4, borderpad=0.5, prop=None, **kwargs):
             prop = {}
         badkwargs = {'ha', 'horizontalalignment', 'va', 'verticalalignment'}
         if badkwargs & set(prop):
-            warnings.warn("Mixing horizontalalignment or verticalalignment "
-                          "with AnchoredText is not supported.")
+            cbook.warn_deprecated(
+                "3.1", "Mixing horizontalalignment or verticalalignment with "
+                "AnchoredText is not supported, deprecated since %(version)s, "
+                "and will raise an exception %(removal)s.")
 
         self.txt = TextArea(s, textprops=prop, minimumdescent=False)
         fp = self.txt._text.get_fontproperties()

lib/matplotlib/table.py

@@ -17,9 +17,7 @@
 Author    : John Gill <jng@europe.renre.com>
 Copyright : 2004 John Gill and John Hunter
 License   : matplotlib license
-
 """
-import warnings
 
 from . import artist, cbook, docstring
 from .artist import Artist, allow_rasterization
@@ -243,9 +241,11 @@ def __init__(self, ax, loc=None, bbox=None, **kwargs):
 
         if isinstance(loc, str):
             if loc not in self.codes:
-                warnings.warn('Unrecognized location %s. Falling back on '
-                              'bottom; valid locations are\n%s\t' %
-                              (loc, '\n\t'.join(self.codes)))
+                cbook.warn_deprecated(
+                    "3.1", message="Unrecognized location {!r}. Falling back "
+                    "on 'bottom'; valid locations are\n\t{}\n"
+                    "This will raise an exception %(removal)s."
+                    .format(loc, '\n\t'.join(self.codes)))
                 loc = 'bottom'
             loc = self.codes[loc]
         self.set_figure(ax.figure)

lib/matplotlib/ticker.py

@@ -1878,16 +1878,12 @@ def __init__(self, *args, **kwargs):
     @staticmethod
     def _validate_steps(steps):
         if not np.iterable(steps):
-            raise ValueError('steps argument must be a sequence of numbers '
-                             'from 1 to 10')
+            raise ValueError('steps argument must be an increasing sequence '
+                             'of numbers between 1 and 10 inclusive')
         steps = np.asarray(steps)
-        if np.any(np.diff(steps) <= 0):
-            raise ValueError('steps argument must be uniformly increasing')
-        if steps[-1] > 10 or steps[0] < 1:
-            warnings.warn('Steps argument should be a sequence of numbers\n'
-                          'increasing from 1 to 10, inclusive. Behavior with\n'
-                          'values outside this range is undefined, and will\n'
-                          'raise a ValueError in future versions of mpl.')
+        if np.any(np.diff(steps) <= 0) or steps[-1] > 10 or steps[0] < 1:
+            raise ValueError('steps argument must be an increasing sequence '
+                             'of numbers between 1 and 10 inclusive')
         if steps[0] != 1:
             steps = np.hstack((1, steps))
         if steps[-1] != 10: