matplotlib · QuLogic · Mar 14, 2020 · Feb 16, 2020 · QuLogic · Mar 10, 2020
diff --git a/doc/api/next_api_changes/deprecations.rst b/doc/api/next_api_changes/deprecations.rst
@@ -235,6 +235,12 @@ Stricter rcParam validation
 (case-insensitive) to the option "line".  This is deprecated; in a future
 version only the exact string "line" (case-sensitive) will be supported.
 
+``add_subplot()`` validates its inputs
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+In particular, for ``add_subplot(rows, cols, index)``, all parameters must
+be integral. Previously strings and floats were accepted and converted to
+int. This will now emit a deprecation warning.
+
 Toggling axes navigation from the keyboard using "a" and digit keys
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 Axes navigation can still be toggled programmatically using

diff --git a/lib/matplotlib/figure.py b/lib/matplotlib/figure.py
@@ -27,7 +27,7 @@
 
 from matplotlib.axes import Axes, SubplotBase, subplot_class_factory
 from matplotlib.blocking_input import BlockingMouseInput, BlockingKeyMouseInput
-from matplotlib.gridspec import GridSpec
+from matplotlib.gridspec import GridSpec, SubplotSpec
 import matplotlib.legend as mlegend
 from matplotlib.patches import Rectangle
 from matplotlib.text import Text
@@ -1254,19 +1254,19 @@ def add_subplot(self, *args, **kwargs):
 
         Parameters
         ----------
-        *args, default: (1, 1, 1)
-            Either a 3-digit integer or three separate integers
-            describing the position of the subplot. If the three
-            integers are *nrows*, *ncols*, and *index* in order, the
-            subplot will take the *index* position on a grid with *nrows*
-            rows and *ncols* columns. *index* starts at 1 in the upper left
-            corner and increases to the right.
-
-            *pos* is a three digit integer, where the first digit is the
-            number of rows, the second the number of columns, and the third
-            the index of the subplot. i.e. fig.add_subplot(235) is the same as
-            fig.add_subplot(2, 3, 5). Note that all integers must be less than
-            10 for this form to work.
+        *args, int or (int, int, int) or `SubplotSpec`, default: (1, 1, 1)
+            The position of the subplot described by one of
+
+            - Three integers (*nrows*, *ncols*, *index*). The subplot will
+              take the *index* position on a grid with *nrows* rows and
+              *ncols* columns. *index* starts at 1 in the upper left corner
+              and increases to the right.
+            - A 3-digit integer. The digits are interpreted as if given
+              separately as three single-digit integers, i.e.
+              ``fig.add_subplot(235)`` is the same as
+              ``fig.add_subplot(2, 3, 5)``. Note that this can only be used
+              if there are no more than 9 subplots.
+            - A `.SubplotSpec`.
 
             In rare circumstances, `.add_subplot` may be called with a single
             argument, a subplot axes instance already created in the
@@ -1346,27 +1346,43 @@ def add_subplot(self, *args, **kwargs):
             ax1.remove()  # delete ax1 from the figure
             fig.add_subplot(ax1)  # add ax1 back to the figure
         """
-        if not len(args):
-            args = (1, 1, 1)
-
-        if len(args) == 1 and isinstance(args[0], Integral):
-            if not 100 <= args[0] <= 999:
-                raise ValueError("Integer subplot specification must be a "
-                                 "three-digit number, not {}".format(args[0]))
-            args = tuple(map(int, str(args[0])))
-
         if 'figure' in kwargs:
             # Axes itself allows for a 'figure' kwarg, but since we want to
             # bind the created Axes to self, it is not allowed here.
             raise TypeError(
                 "add_subplot() got an unexpected keyword argument 'figure'")
 
-        if isinstance(args[0], SubplotBase):
+        nargs = len(args)
+        if nargs == 0:
+            args = (1, 1, 1)
+        elif nargs == 1:
+            if isinstance(args[0], Integral):
+                if not 100 <= args[0] <= 999:
+                    raise ValueError(f"Integer subplot specification must be "
+                                     f"a three-digit number, not {args[0]}")
+                args = tuple(map(int, str(args[0])))
+            elif isinstance(args[0], (SubplotBase, SubplotSpec)):
+                pass  # no further validation or normalization needed
+            else:
+                raise TypeError('Positional arguments are not a valid '
+                                'position specification.')
+        elif nargs == 3:
+            for arg in args:
+                if not isinstance(arg, Integral):
+                    cbook.warn_deprecated(
+                        "3.3",
+                        message="Passing non-integers as three-element "
+                                "position specification is deprecated.")
+            args = tuple(map(int, args))
+        else:
+            raise TypeError(f'add_subplot() takes 1 or 3 positional arguments '
+                            f'but {nargs} were given')
 
+        if isinstance(args[0], SubplotBase):
             ax = args[0]
             if ax.get_figure() is not self:
-                raise ValueError(
-                    "The Subplot must have been created in the present figure")
+                raise ValueError("The Subplot must have been created in "
+                                 "the present figure")
             # make a key for the subplot (which includes the axes object id
             # in the hash)
             key = self._make_key(*args, **kwargs)

diff --git a/lib/matplotlib/tests/test_axes.py b/lib/matplotlib/tests/test_axes.py
@@ -4050,7 +4050,7 @@ def test_mixed_collection():
 
 
 def test_subplot_key_hash():
-    ax = plt.subplot(np.float64(5.5), np.int64(1), np.float64(1.2))
+    ax = plt.subplot(np.int32(5), np.int64(1), 1)
     ax.twinx()
     assert ax.get_subplotspec().get_geometry() == (5, 1, 0, 0)
 

diff --git a/lib/matplotlib/tests/test_figure.py b/lib/matplotlib/tests/test_figure.py
@@ -4,7 +4,7 @@
 import warnings
 
 import matplotlib as mpl
-from matplotlib import rcParams
+from matplotlib import cbook, rcParams
 from matplotlib.testing.decorators import image_comparison, check_figures_equal
 from matplotlib.axes import Axes
 from matplotlib.ticker import AutoMinorLocator, FixedFormatter, ScalarFormatter
@@ -172,15 +172,35 @@ def test_gca():
 
 def test_add_subplot_invalid():
     fig = plt.figure()
-    with pytest.raises(ValueError):
+    with pytest.raises(ValueError, match='Number of columns must be > 0'):
         fig.add_subplot(2, 0, 1)
-    with pytest.raises(ValueError):
+    with pytest.raises(ValueError, match='Number of rows must be > 0'):
         fig.add_subplot(0, 2, 1)
-    with pytest.raises(ValueError):
+    with pytest.raises(ValueError, match='num must be 1 <= num <= 4'):
         fig.add_subplot(2, 2, 0)
-    with pytest.raises(ValueError):
+    with pytest.raises(ValueError, match='num must be 1 <= num <= 4'):
         fig.add_subplot(2, 2, 5)
 
+    with pytest.raises(ValueError, match='must be a three-digit number'):
+        fig.add_subplot(42)
+    with pytest.raises(ValueError, match='must be a three-digit number'):
+        fig.add_subplot(1000)
+
+    with pytest.raises(TypeError, match='takes 1 or 3 positional arguments '
+                                        'but 2 were given'):
+        fig.add_subplot(2, 2)
+    with pytest.raises(TypeError, match='takes 1 or 3 positional arguments '
+                                        'but 4 were given'):
+        fig.add_subplot(1, 2, 3, 4)
+    with pytest.warns(cbook.MatplotlibDeprecationWarning,
+                      match='Passing non-integers as three-element position '
+                            'specification is deprecated'):
+        fig.add_subplot('2', 2, 1)
+    with pytest.warns(cbook.MatplotlibDeprecationWarning,
+                      match='Passing non-integers as three-element position '
+                            'specification is deprecated'):
+        fig.add_subplot(2.0, 2, 1)
+
 
 @image_comparison(['figure_suptitle'])
 def test_suptitle():