Deprecate add_subplot(<no positional args>) silently doing nothing.

anntzer · anntzer · commit b73a94a4db2a · 2019-01-07T10:42:29.000+01:00
To avoid head-scratching when `add_subplot(projection="3d")` does
nothing (the correct call is `add_subplot(111, projection="3d")`).
diff --git a/doc/api/next_api_changes/2018-01-09-AL.rst b/doc/api/next_api_changes/2018-01-09-AL.rst
@@ -0,0 +1,5 @@
+Deprecations
+````````````
+
+Calling `.Figure.add_subplot()` with no positional arguments used to do
+nothing; this is deprecated.
diff --git a/lib/matplotlib/figure.py b/lib/matplotlib/figure.py
@@ -1311,14 +1311,14 @@ def add_subplot(self, *args, **kwargs):
         --------
         ::
 
-            fig=plt.figure()
+            fig = plt.figure()
             fig.add_subplot(221)
 
             # equivalent but more general
-            ax1=fig.add_subplot(2, 2, 1)
+            ax1 = fig.add_subplot(2, 2, 1)
 
             # add a subplot with no frame
-            ax2=fig.add_subplot(222, frameon=False)
+            ax2 = fig.add_subplot(222, frameon=False)
 
             # add a polar subplot
             fig.add_subplot(223, projection='polar')
@@ -1333,6 +1333,9 @@ def add_subplot(self, *args, **kwargs):
             fig.add_subplot(ax2)
         """
         if not len(args):
+            cbook.warn_deprecated("3.1", message=
+                "Support for calling add_subplot() with no arguments is "
+                "deprecated and will be removed %(removal)s.")
             return
 
         if len(args) == 1 and isinstance(args[0], Integral):
