Block based truncate tag for Jinja2

from jinja2 import nodes
from jinja2.ext import Extension

class TruncatedBlockExtension(Extension):
    tags = (['block_truncate_chars'])

    def parse(self, parser):
        lineno = parser.stream.next().lineno

        args = [parser.parse_expression()]

        body = parser.parse_statements(['name:endblock_truncate_chars'], drop_needle=True)

        return nodes.CallBlock(self.call_method('_do_truncate', args),
                               [], [], body).set_lineno(lineno)

    def _do_truncate(self, length, caller):
        from jinja2.filters import do_truncate
        rv = caller()
        return do_truncate(rv, length)
block_truncate_chars = TruncatedBlockExtension