Issue #21708: Deprecated dbm.dumb behavior that differs from common dbm

behavior: creating a database in 'r' and 'w' modes and modifying a database
in 'r' mode.

1 files changed, 18 insertions, 0 deletions

diff --git a/Lib/dbm/dumb.py b/Lib/dbm/dumb.py
index 7777a7c..e7c6440 100644
--- a/Lib/dbm/dumb.py
+++ b/Lib/dbm/dumb.py
@@ -47,6 +47,7 @@ class _Database(collections.MutableMapping):
 
     def __init__(self, filebasename, mode, flag='c'):
         self._mode = mode
+        self._readonly = (flag == 'r')
 
         # The directory file is a text file.  Each line looks like
         #    "%r, (%d, %d)\n" % (key, pos, siz)
@@ -80,6 +81,11 @@ class _Database(collections.MutableMapping):
         try:
             f = _io.open(self._datfile, 'r', encoding="Latin-1")
         except OSError:
+            if flag not in ('c', 'n'):
+                import warnings
+                warnings.warn("The database file is missing, the "
+                              "semantics of the 'c' flag will be used.",
+                              DeprecationWarning, stacklevel=4)
             with _io.open(self._datfile, 'w', encoding="Latin-1") as f:
                 self._chmod(self._datfile)
         else:
@@ -178,6 +184,10 @@ class _Database(collections.MutableMapping):
             f.write("%r, %r\n" % (key.decode("Latin-1"), pos_and_siz_pair))
 
     def __setitem__(self, key, val):
+        if self._readonly:
+            import warnings
+            warnings.warn('The database is opened for reading only',
+                          DeprecationWarning, stacklevel=2)
         if isinstance(key, str):
             key = key.encode('utf-8')
         elif not isinstance(key, (bytes, bytearray)):
@@ -212,6 +222,10 @@ class _Database(collections.MutableMapping):
             # (so that _commit() never gets called).
 
     def __delitem__(self, key):
+        if self._readonly:
+            import warnings
+            warnings.warn('The database is opened for reading only',
+                          DeprecationWarning, stacklevel=2)
         if isinstance(key, str):
             key = key.encode('utf-8')
         self._verify_open()
@@ -300,4 +314,8 @@ def open(file, flag='c', mode=0o666):
     else:
         # Turn off any bits that are set in the umask
         mode = mode & (~um)
+    if flag not in ('r', 'w', 'c', 'n'):
+        import warnings
+        warnings.warn("Flag must be one of 'r', 'w', 'c', or 'n'",
+                      DeprecationWarning, stacklevel=2)
     return _Database(file, mode, flag=flag)