Experimenting with the 'with' statement for a Rails-like XML builder.

xmlbuilder.py

@@ -0,0 +1,74 @@
+__author__ = ('Jonas Galvez', 'jonas@codeazur.com.br', 'http://jonasgalvez.com.br')
+
+from __future__ import with_statement
+import sys
+
+class GeneratorContextManager(object):
+  def __init__(self, gen):
+    self.gen = gen
+  def __enter__(self):
+    try:
+      return self.gen.next()
+    except StopIteration:
+      raise RuntimeError("generator didn't yield")
+  def __exit__(self, type, value, traceback):
+    if type is None:
+      try:
+        self.gen.next()
+      except StopIteration:
+        return
+      else:
+        raise RuntimeError("generator didn't stop")
+    else:
+      try:
+        self.gen.throw(type, value, traceback)
+        raise RuntimeError("generator didn't stop after throw()")
+      except StopIteration:
+        return True
+      except:
+        if sys.exc_info()[1] is not value:
+          raise
+
+def contextmanager(ref, name, func):
+  def helper(*args, **kwds):
+    if len(args) > 0:
+      ref.write_element(name, args[0])
+    else:
+      return GeneratorContextManager(func(*args, **kwds))
+  return helper
+
+class XMLBuilder:
+  def __init__(self):
+    self.document = ""
+  def __getattr__(self, name):
+    return contextmanager(self, name, self._element_writer(name))
+  def _element_writer(self, name):
+    ref = self
+    def _write_element_generator(value=None):
+      try:
+        ref.document += '<%s>' % name
+        if value != None:
+          ref.document += value
+        yield
+      finally:
+        ref.document += '</%s>' % name
+    return _write_element_generator
+  def write_element(self, name, value):
+    self.document += '<%s>%s</%s>' % (name, value, name)
+
+##### USAGE #####
+
+xml = XMLBuilder()
+entries = [{'name': 'Entry #1'}, {'name': 'Entry #2'}]
+
+# pretty sweet, huh?
+with xml.entries():
+  for entry in entries:
+    xml.entry(entry['name'])
+
+print xml.document
+'''
+result:
+
+<entries><entry>Entry #1</entry><entry>Entry #2</entry></entries>
+'''