Document behavior of savefig("extensionless-name").

anntzer · anntzer · commit ece71a05e180 · 2020-03-12T20:19:56.000+01:00
When the filename has no extension *and* the `format` kwarg is unset,
the suffix derived from the savefig.format rcparam is appended to the
filename.

(I'm not sure the behavior is really that nice, but that's what it is
right now; this is just documenting it.)
diff --git a/lib/matplotlib/figure.py b/lib/matplotlib/figure.py
@@ -2044,12 +2044,16 @@ def savefig(self, fname, *, transparent=None, **kwargs):
             possibly some backend-dependent object such as
             `matplotlib.backends.backend_pdf.PdfPages`.
 
-            If *format* is not set, then the output format is inferred from
-            the extension of *fname*, if any, and from :rc:`savefig.format`
-            otherwise.  If *format* is set, it determines the output format.
-
-            Hence, if *fname* is not a path or has no extension, remember to
-            specify *format* to ensure that the correct backend is used.
+            If *format* is set, it determines the output format, and the file
+            is saved as *fname*.  Note that *fname* is used verbatim, and there
+            is no attempt to make the extension, if any, of *fname* match
+            *format*, and no extension is appended.
+
+            If *format* is not set, then the format is inferred from the
+            extension of *fname*, if there is one.  If *format* is not
+            set and *fname* has no extension, then the file is saved with
+            :rc:`savefig.format` and the appropriate extension is appended to
+            *fname*.
 
         Other Parameters
         ----------------
