commit 644f7c23531edb407171fbe9befc8636c25f6f0f
Author: Thomas Kluyver <thomas@kluyver.me.uk>
Date:   Wed Mar 3 10:16:48 2021 +0000

    Deprecate setting default_file_mode, only allow setting to 'r'

diff --git a/h5py/_hl/files.py b/h5py/_hl/files.py
index 6fdb767e..0d32d65e 100644
--- a/h5py/_hl/files.py
+++ b/h5py/_hl/files.py
@@ -315,7 +315,7 @@ class File(Group):
         else:
             raise RuntimeError('SWMR support is not available in HDF5 version {}.{}.{}.'.format(*hdf5_version))
 
-    def __init__(self, name, mode=None, driver=None,
+    def __init__(self, name, mode='r', driver=None,
                  libver=None, userblock_size=None, swmr=False,
                  rdcc_nslots=None, rdcc_nbytes=None, rdcc_w0=None,
                  track_order=None, fs_strategy=None, fs_persist=False, fs_threshold=1,
@@ -424,8 +424,6 @@ class File(Group):
 
             if track_order is None:
                 track_order = h5.get_config().track_order
-            if mode is None:
-                mode = h5.get_config().default_file_mode  # default: 'r'
 
             if fs_strategy and mode not in ('w', 'w-', 'x'):
                 raise ValueError("Unable to set file space strategy of an existing file")
diff --git a/h5py/h5.pyx b/h5py/h5.pyx
index 387ec619..74bd5c80 100644
--- a/h5py/h5.pyx
+++ b/h5py/h5.pyx
@@ -169,13 +169,18 @@ cdef class H5PYConfig:
             return self._default_file_mode
 
         def __set__(self, val):
-            if val in {'r', 'r+', 'x', 'w-', 'w', 'a'}:
-                if val != 'r':
-                    warn("Using default_file_mode other than 'r' is deprecated. "
-                         "Pass the mode to h5py.File() instead.",
-                         category=H5pyDeprecationWarning,
-                    )
-                self._default_file_mode = val
+            if val == 'r':
+                warn(
+                    "Setting h5py.default_file_mode is deprecated. "
+                    "'r' (read-only) is the default from h5py 3.0.",
+                    category=H5pyDeprecationWarning,
+                )
+            elif val in {'r+', 'x', 'w-', 'w', 'a'}:
+                raise ValueError(
+                    "Using default_file_mode other than 'r' is no longer "
+                    "supported. Pass the mode to h5py.File() instead."
+
+                )
             else:
                 raise ValueError("Invalid mode; must be one of r, r+, w, w-, x, a")