code: def _inherit_from(context, uri, calling_uri):
      
  
      if uri is None:
          return None
      template = _lookup_template(context, uri, calling_uri)
      self_ns = context['self']
      ih = self_ns
      while ih.inherits is not None:
          ih = ih.inherits
      lclcontext = context._locals({'next': ih})
      ih.inherits = TemplateNamespace("self:%s" % template.uri,
                                  lclcontext,
                                  template=template,
                                  populate_self=False)
      context._data['parent'] = lclcontext._data['local'] = ih.inherits
      callable_ = getattr(template.module, '_mako_inherit', None)
      if callable_ is not None:
          ret = callable_(template, lclcontext)
          if ret:
              return ret
  
      gen_ns = getattr(template.module, '_mako_generate_namespaces', None)
      if gen_ns is not None:
          gen_ns(context)
      return (template.callable_, lclcontext)
docs: called by the _inherit method in template modules to set
      up the inheritance chain at the start of a template's
      execution.