class MalformedTemplateError(Exception): pass

def templateItem(channel, item, itemTemplate):
	"""
	Returns itemTemplate replaced with content from channel and item
	dictionaries. itemTemplate should have variables of the form
	$channel_variable and $variable, which will be replaced with
	appropriate content from channel and item respectively. The template
	may also contain conditional statements:
	
	$if_variable
		some content with $variable in it
	$else_variable
		some other content
	$endif_variable

	If a $variable is not present, it is replaced with the empty string.
	The $if construction is so you wont get <a href=''>comments</a> when
	$comments is not defined.
	"""

	tbits = []

	class Allow(object):
		def __init__(self, parent=None):
			self.parent = parent
		def __call__(self, msg):
			if self.parent: self.parent(msg)
			else: tbits.append(msg)
	class Disallow(object):
		def __init__(self, parent=None):
			self.parent = parent
		def __call__(self, msg): pass

	appender = Allow()

	tbitsIn = itemTemplate.split('$')
	appender(tbitsIn[0])
	del tbitsIn[0]

	for t in tbitsIn:
		cmdMO = re.search(r"^\w+", t)
		if not cmdMO:
			raise MalformedTemplateError("'$' followed by non-word character")
		cmd = cmdMO.group(0)

		if cmd == 'endif':
			if None == appender.parent:
				raise MalformedTemplateError("$endif without $if")

			appender = appender.parent

			appender(t[5:])
			continue  # next t

		if cmd == 'else':
			if None == appender.parent:
				raise MalformedTemplateError("$else without $if")

			if Allow == type(appender):
				newtype = Disallow
			else:
				newtype = Allow
			appender = newtype(appender.parent)

			appender(t[4:])
			continue  # next t

		if cmd == 'if':
			cmdMO = re.match(re.compile(r"if\s+(\w+)(.*)", re.DOTALL), t)
			if not cmdMO:
				raise MalformedTemplateError("Malformed $if")
			var = cmdMO.group(1)
			content = cmdMO.group(2)

			val = None
			if var.startswith('channel_'):
				val = channel.get(var[8:], None)
			else:
				val = item.get(var, None)

			if val:
				appender = Allow(appender)
			else:
				appender = Disallow(appender)

			appender(content)
			continue

		content = ''
		if cmd.startswith('channel_'):
			content = channel.get(cmd[8:], '')
		else:
			content = item.get(cmd, '')
		appender(content)
		appender(t[len(cmd):])

	return "".join(tbits)