new file mode 100644
--- /dev/null
+++ b/svnexternals.py
@@ -0,0 +1,59 @@
+import cStringIO
+
+from mercurial import util as merc_util
+
+class externalsfile(dict):
+    """Map svn directories to lists of externals entries.
+    """
+    def __init__(self):
+        super(externalsfile, self).__init__()
+        self.encoding = 'utf-8'
+
+    def __setitem__(self, key, value):
+        if value is None:
+            value = []
+        elif isinstance(value, basestring):
+            value = value.splitlines()
+        if key == '.':
+            key = ''
+        if not value:
+            if key in self:
+                del self[key]
+        else:
+            super(externalsfile, self).__setitem__(key, value)
+
+    def write(self):
+        fp = cStringIO.StringIO()
+        for target in merc_util.sort(self):
+            lines = self[target]
+            if not lines:
+                continue
+            if not target:
+                target = '.'
+            fp.write('[%s]\n' % target)
+            for l in lines:
+                l = ' ' + l + '\n'
+                fp.write(l)
+        return fp.getvalue()
+
+    def read(self, data):
+        self.clear()
+        fp = cStringIO.StringIO(data)
+        dirs = {}
+        target = None
+        for line in fp.readlines():
+            if not line.strip():
+                continue
+            if line.startswith('['):
+                line = line.strip()
+                if line[-1] != ']':
+                    raise merc_util.Abort('invalid externals section name: %s' % line)
+                target = line[1:-1]
+                if target == '.':
+                    target = ''
+            elif line.startswith(' '):
+                line = line.rstrip('\n')
+                if target is None or not line:
+                    continue
+                self.setdefault(target, []).append(line[1:])
+