Issue #28847: dbm.dumb now supports reading read-only files and no longer

writes the index file when it is not changed.

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)
@@ -91,8 +92,9 @@ class _Database(collections.MutableMapping):
         try:
             f = _io.open(self._dirfile, 'r', encoding="Latin-1")
         except OSError:
-            pass
+            self._modified = not self._readonly
         else:
+            self._modified = False
             with f:
                 for line in f:
                     line = line.rstrip()
@@ -107,7 +109,7 @@ class _Database(collections.MutableMapping):
         # CAUTION:  It's vital that _commit() succeed, and _commit() can
         # be called from __del__().  Therefore we must never reference a
         # global in this routine.
-        if self._index is None:
+        if self._index is None or not self._modified:
             return  # nothing to do
 
         try:
@@ -187,6 +189,7 @@ class _Database(collections.MutableMapping):
         elif not isinstance(val, (bytes, bytearray)):
             raise TypeError("values must be bytes or strings")
         self._verify_open()
+        self._modified = True
         if key not in self._index:
             self._addkey(key, self._addval(val))
         else:
@@ -215,6 +218,7 @@ class _Database(collections.MutableMapping):
         if isinstance(key, str):
             key = key.encode('utf-8')
         self._verify_open()
+        self._modified = True
         # The blocks used by the associated value are lost.
         del self._index[key]
         # XXX It's unclear why we do a _commit() here (the code always