Emit explanatory exception when no temporary cachedir can be created.

anntzer · anntzer · commit a3fd2d7c43a8 · 2023-05-09T17:21:22.000+02:00
A warning is already emitted when a temporary cachedir is created; it
seems reasonable to also explain the situation when no cachedir can be
created at all.  Also note that in these cases the directory really only
serves as cache, not as configdir (there won't be user-defined configs
there), so adjust the message accordingly.
diff --git a/lib/matplotlib/__init__.py b/lib/matplotlib/__init__.py
@@ -530,15 +530,21 @@ def _get_config_or_cache_dir(xdg_base_getter):
             return str(configdir)
     # If the config or cache directory cannot be created or is not a writable
     # directory, create a temporary one.
-    tmpdir = os.environ["MPLCONFIGDIR"] = \
-        tempfile.mkdtemp(prefix="matplotlib-")
+    try:
+        tmpdir = tempfile.mkdtemp(prefix="matplotlib-")
+    except OSError as exc:
+        raise OSError(
+            f"Matplotlib requires access to a writable cache directory, but the "
+            f"default path ({configdir}) is not a writable directory, and a temporary "
+            f"directory could not be created; set the MPLCONFIGDIR environment "
+            f"variable to a writable directory") from exc
+    os.environ["MPLCONFIGDIR"] = tmpdir
     atexit.register(shutil.rmtree, tmpdir)
     _log.warning(
-        "Matplotlib created a temporary config/cache directory at %s because "
-        "the default path (%s) is not a writable directory; it is highly "
-        "recommended to set the MPLCONFIGDIR environment variable to a "
-        "writable directory, in particular to speed up the import of "
-        "Matplotlib and to better support multiprocessing.",
+        "Matplotlib created a temporary cache directory at %s because the default path "
+        "(%s) is not a writable directory; it is highly recommended to set the "
+        "MPLCONFIGDIR environment variable to a writable directory, in particular to "
+        "speed up the import of Matplotlib and to better support multiprocessing.",
         tmpdir, configdir)
     return tmpdir
 
