Backporting tojson filter from jinja2.9 to jinja2.6

jinja2_6_tojson_backport.py

def htmlsafe_json_dumps(obj, dumper=None, **kwargs):
  """Works exactly like :func:`dumps` but is safe for use in ``<script>``
  tags.  It accepts the same arguments and returns a JSON string.  Note that
  this is available in templates through the ``|tojson`` filter which will
  also mark the result as safe.  Due to how this function escapes certain
  characters this is safe even if used outside of ``<script>`` tags.
  The following characters are escaped in strings:
  -   ``<``
  -   ``>``
  -   ``&``
  -   ``'``
  This makes it safe to embed such strings in any place in HTML with the
  notable exception of double quoted attributes.  In that case single
  quote your attributes or HTML escape it in addition.
  """   
  if dumper is None:
    import json
    dumper = json.dumps
  rv = dumper(obj, **kwargs) \
    .replace(u'<', u'\\u003c') \
    .replace(u'>', u'\\u003e') \
    .replace(u'&', u'\\u0026') \
    .replace(u"'", u'\\u0027')
  return rv



def getJinja2Environment():
  env = jinja2.Environment(
      loader=jinja2.FileSystemLoader(os.path.dirname(__file__),encoding='utf-8'),
      extensions=['jinja2.ext.autoescape', 'jinja2.ext.with_'],
      autoescape=True)
  
  try:
    getattr(env, 'policies')
  except AttributeError as exception:
    import json
    policies = {
      'json.dumps_function': json.dumps,
      'json.dumps_kwargs': {'sort_keys': True},
    }
    setattr(env, 'policies', policies)
  
  try:
    from jinja2.filters import do_tojson
  except ImportError as e:
    # backport the tojson filter

    from jinja2.filters import evalcontextfilter
    @evalcontextfilter
    def do_tojson(eval_ctx, value, indent=None):
      """Dumps a structure to JSON so that it's safe to use in ``<script>``
      tags.  It accepts the same arguments and returns a JSON string.  Note that
      this is available in templates through the ``|tojson`` filter which will
      also mark the result as safe.  Due to how this function escapes certain
      characters this is safe even if used outside of ``<script>`` tags.

      The following characters are escaped in strings:
      -   ``<``
      -   ``>``
      -   ``&``
      -   ``'``

      This makes it safe to embed such strings in any place in HTML with the
      notable exception of double quoted attributes.  In that case single
      quote your attributes or HTML escape it in addition.

      The indent parameter can be used to enable pretty printing.  Set it to
      the number of spaces that the structures should be indented with.

      Note that this filter is for use in HTML contexts only.

      .. versionadded:: 2.9
      """
      policies = eval_ctx.environment.policies
      dumper = policies['json.dumps_function']
      options = policies['json.dumps_kwargs']
      if indent is not None:
        options = dict(options)
        options['indent'] = indent
      return htmlsafe_json_dumps(value, dumper=dumper, **options)
    env.filters['tojson'] = do_tojson
  return env