Mysterious error in Python Urlobject called from Django Rest Framework

Question

I am using the Django Rest Framework to build an api, and have an implementation on my development machine. I wanted to quickly deploy it today and it is proving far more error prone than I expected. I have re-installed dependancies, re-installed Django, spent hours hunting down bugs, but this latest one has defeated me and Google.

On the development machine, when I call one of the views from the framework I get the correct response (minus the static files but I'll deal with that later.) On the deployment machine however, I get this error:

'SplitResult' object has no attribute '_replace'

The traceback goes through a template tag in the rest framework, but the error seems to be thrown in urlobject in this method:

def __replace(self, **replace):
        """Replace a field in the ``urlparse.SplitResult`` for this URL."""
        return type(self)(urlparse.urlunsplit(
            urlparse.urlsplit(self)._replace(**replace))) 

I cannot reproduce this on my development machine, and I cannot find any references to this error anywhere. Just in case I have tried it on the deployment server with Django 1.2, 1.3, and 1.4, but the error seems to be unrelated to Django itself.

Here is the full traceback:

Django Version: 1.3.1
Python Version: 2.5.2
Installed Applications:
['django.contrib.auth',
 'django.contrib.contenttypes',
 'django.contrib.sessions',
 'django.contrib.sites',
 'django.contrib.messages',
 'staticfiles',
 'django.contrib.admin',
 'django.contrib.admindocs',
 'char',
 'djangorestframework']
Installed Middleware:
('django.middleware.common.CommonMiddleware',
 'django.contrib.sessions.middleware.SessionMiddleware',
 'django.middleware.csrf.CsrfViewMiddleware',
 'django.contrib.auth.middleware.AuthenticationMiddleware',
 'django.contrib.messages.middleware.MessageMiddleware')


Traceback:
File "/usr/lib/python2.5/site-packages/django/core/handlers/base.py" in get_response
  111.                         response = callback(request, *callback_args, **callback_kwargs)
File "/usr/lib/python2.5/site-packages/django/views/generic/base.py" in view
  47.             return self.dispatch(request, *args, **kwargs)
File "/usr/lib/python2.5/site-packages/django/views/decorators/csrf.py" in wrapped_view
  39.         resp = view_func(*args, **kwargs)
File "/usr/lib/python2.5/site-packages/django/views/decorators/csrf.py" in wrapped_view
  52.         return view_func(*args, **kwargs)
File "/home/wfrp/wfrp-django/DjangoWfrp/external apps/django-rest-framework/djangorestframework/views.py" in dispatch
  243.         return self.final(request, response, *args, **kwargs)
File "/home/wfrp/wfrp-django/DjangoWfrp/external apps/django-rest-framework/djangorestframework/views.py" in final
  196.         return self.render(response)
File "/home/wfrp/wfrp-django/DjangoWfrp/external apps/django-rest-framework/djangorestframework/mixins.py" in render
  251.             content = renderer.render(response.cleaned_content, media_type)
File "/home/wfrp/wfrp-django/DjangoWfrp/external apps/django-rest-framework/djangorestframework/renderers.py" in render
  351.         ret = template.render(context)
File "/usr/lib/python2.5/site-packages/django/template/base.py" in render
  123.             return self._render(context)
File "/usr/lib/python2.5/site-packages/django/template/base.py" in _render
  117.         return self.nodelist.render(context)
File "/usr/lib/python2.5/site-packages/django/template/base.py" in render
  744.                 bits.append(self.render_node(node, context))
File "/usr/lib/python2.5/site-packages/django/template/base.py" in render_node
  757.         return node.render(context)
File "/usr/lib/python2.5/site-packages/django/template/loader_tags.py" in render
  127.         return compiled_parent._render(context)
File "/usr/lib/python2.5/site-packages/django/template/base.py" in _render
  117.         return self.nodelist.render(context)
File "/usr/lib/python2.5/site-packages/django/template/base.py" in render
  744.                 bits.append(self.render_node(node, context))
File "/usr/lib/python2.5/site-packages/django/template/base.py" in render_node
  757.         return node.render(context)
File "/usr/lib/python2.5/site-packages/django/template/defaulttags.py" in render
  311.             return self.nodelist_true.render(context)
File "/usr/lib/python2.5/site-packages/django/template/base.py" in render
  744.                 bits.append(self.render_node(node, context))
File "/usr/lib/python2.5/site-packages/django/template/base.py" in render_node
  757.         return node.render(context)
File "/usr/lib/python2.5/site-packages/django/template/defaulttags.py" in render
  227.                 nodelist.append(node.render(context))
File "/usr/lib/python2.5/site-packages/django/template/defaulttags.py" in render
  500.         output = self.nodelist.render(context)
File "/usr/lib/python2.5/site-packages/django/template/base.py" in render
  744.                 bits.append(self.render_node(node, context))
File "/usr/lib/python2.5/site-packages/django/template/base.py" in render_node
  757.         return node.render(context)
File "/usr/lib/python2.5/site-packages/django/template/base.py" in render
  792.             output = self.filter_expression.resolve(context)
File "/usr/lib/python2.5/site-packages/django/template/base.py" in resolve
  536.                 new_obj = func(obj, *arg_vals)
File "/home/wfrp/wfrp-django/DjangoWfrp/external apps/django-rest-framework/djangorestframework/templatetags/add_query_param.py" in add_query_param
  7.     return unicode(URLObject(url).with_query(param))
File "/home/wfrp/wfrp-django/DjangoWfrp/external apps/urlobject/lib/urlobject/urlobject.py" in with_query
  117.         return self.__replace(query=query)
File "/home/wfrp/wfrp-django/DjangoWfrp/external apps/urlobject/lib/urlobject/urlobject.py" in __replace
  181.             urlparse.urlsplit(self)._replace(**replace)))

Exception Type: AttributeError at /api/
Exception Value: 'SplitResult' object has no attribute '_replace'

This is my first time on Stack Overflow — thanks for the help.

Answer 1

I suspect your development and deployment machines are running different Python versions. The Python docs for SplitResult don't mention anything about the internal _replace method, but the leading underscore suggests that it is an internal API. This is a Django Rest Framework bug, I suggest filing a bug report with them.

If you're curious, SplitResult is a subclass of namedtuple (introduced in Python 2.6). It takes kwargs and returns a new SplitResult with the relevant values replaced.

In [16]: sr = urlparse.urlsplit("http:///www.google.com")

In [17]: sr
Out[17]: SplitResult(scheme='http', netloc='', path='/www.google.com', query='', fragment='')

In [18]: sr._replace(scheme='https')
Out[18]: SplitResult(scheme='https', netloc='', path='/www.google.com', query='', fragment='')

I believe the version of SplitResult in Python < 2.6 was a subclass of tuple, which does not have a _replace method, which would explain the error that you are seeing