Index: Lib/ConfigParser.py
===================================================================
--- Lib/ConfigParser.py	(revision 41859)
+++ Lib/ConfigParser.py	(working copy)
@@ -85,9 +85,14 @@
 
     write(fp)
         write the configuration state in .ini format
+
+    update_file(fp)
+        update the .ini format file to match the current configuration
+        state
 """
 
 import re
+import StringIO
 
 __all__ = ["NoSectionError", "DuplicateSectionError", "NoOptionError",
            "InterpolationError", "InterpolationDepthError",
@@ -373,6 +378,128 @@
                              (key, str(value).replace('\n', '\n\t')))
             fp.write("\n")
 
+    def update_file(self, fp, add_missing=True):
+        """Update the specified configuration file to match the current
+        configuration data.
+
+        Ordering (including blank lines) and comments are preserved.
+
+        Minor whitespace normalisation (that preceding continuation lines
+        and inline comments) does occur.
+
+        If add_missing is True, then new options are added to the end of
+        sections (if a section is in a file twice, the last section will be
+        used); new sections are added to the end of the file.  This
+        does mean that if the configuration was read from multiple files,
+        the file that is output to will contain the options from all of
+        those files.  To avoid this, use add_missing=False, and update each
+        of the input files.
+
+        Default values are not added; nor are the __name__ options.
+        """
+        sections = {}
+        current = StringIO.StringIO()
+        replacement = [current]
+        sect = None
+        opt = None
+        written = []
+        # Default to " = " to match write(), but use the most recent
+        # separator found if the file has any options.
+        vi = " = "
+        while True:
+            line = fp.readline()
+            if not line:
+                break
+            # Comment or blank line?
+            if line.strip() == '' or line[0] in '#;' or \
+               (line.split(None, 1)[0].lower() == 'rem' and \
+                line[0] in "rR"):
+                current.write(line)
+                continue
+            # Continuation line?
+            if line[0].isspace() and sect is not None and opt:
+                if ';' in line:
+                    # ';' is a comment delimiter only if it follows
+                    # a spacing character
+                    pos = line.find(';')
+                    if line[pos-1].isspace():
+                        comment = line[pos-1:]
+                        # Get rid of the newline, and put in the comment.
+                        current.seek(-1, 1)
+                        current.write(comment + "\n")
+                continue
+            # A section header or option header?
+            else:
+                # Is it a section header?
+                mo = self.SECTCRE.match(line)
+                if mo:
+                    # Remember the most recent section with this name,
+                    # so that any missing options can be added to it.
+                    if sect:
+                        sections[sect] = current
+                    sect = mo.group('header')
+                    current = StringIO.StringIO()
+                    replacement.append(current)
+                    if sect in self.sections():
+                        current.write(line)
+                    # So sections can't start with a continuation line:
+                    opt = None
+                # An option line?
+                else:
+                    mo = self.OPTCRE.match(line)
+                    if mo:
+                        opt, vi, value = mo.group('option', 'vi', 'value')
+                        comment = ""
+                        if vi in ('=', ':') and ';' in value:
+                            # ';' is a comment delimiter only if it follows
+                            # a spacing character
+                            pos = value.find(';')
+                            if value[pos-1].isspace():
+                                comment = value[pos-1:]
+                        opt = opt.rstrip().lower()
+                        if self.has_option(sect, opt):
+                            value = self.get(sect, opt)
+                            # Fix continuations.
+                            value = value.replace("\n", "\n\t")
+                            current.write("%s%s%s%s\n" % (opt, vi, value,
+                                                          comment))
+                            written.append((sect, opt))
+        if sect:
+            sections[sect] = current
+        if add_missing:
+            # Add any new sections.
+            sects = [DEFAULTSECT]
+            sects.extend(self.sections())
+            sects.sort()
+            for sect in sects:
+                if sect == DEFAULTSECT:
+                    opts = self._defaults.keys()
+                else:
+                    # Must use _section here to avoid defaults.
+                    opts = self._sections[sect].keys()
+                opts.sort()
+                if sect in sections:
+                    output = sections[sect] or current
+                else:
+                    output = current
+                    output.write("[%s]\n" % (sect,))
+                    sections[sect] = None
+                for opt in opts:
+                    if opt != "__name__" and not (sect, opt) in written:
+                        value = self.get(sect, opt)
+                        # Fix continuations.
+                        value = value.replace("\n", "\n\t")
+                        output.write("%s%s%s\n" % (opt, vi, value))
+                        written.append((sect, opt))
+                output.write("\n")
+        # Copy across the new file.
+        fp.seek(0)
+        fp.truncate()
+        for sect in replacement:
+            if sect is not None:
+                fp.write(sect.getvalue())
+        fp.write("\n")
+
     def remove_option(self, section, option):
         """Remove an option."""
         if not section or section == DEFAULTSECT: