Pylons mako templates: debugging "Internal Server Error" / "NoneType" errors

Question

I frequently encounter this error in Mako templates using Pylons 0.9.7:

AttributeError: 'NoneType' object has no attribute 'decode'

Usually I've entered a variable name that doesn't exist, tried to use a linbebreak within a code line, or some other minor error. Definitely my fault.

This results in a 'Internal Server Error' in the browser, same thing in the debug view, and a stack trace that starts in HTTPServer and ends with the AttributeError in mako/exceptions.py.

Is there anything I can do to make this easier to debug, like find out the line that exception is being generated on within the Mako template? Thanks!

Answer 1

In my case, it turns out there was a division by 0 error in my template. That was yielding an internal server error, and a very unhelpful stack trace in console output.

I know, it sounds like I shouldn't have had that logic in a template anyway, but in this case, I think it makes sense to do so. Here's the stack trace I get with division by 0:

Exception happened during processing of request from ('127.0.0.1', 50681)
Traceback (most recent call last):
  File "/Library/Python/2.6/site-packages/Paste-1.7.4-py2.6.egg/paste/httpserver.py", line 1068, in process_request_in_thread
    self.finish_request(request, client_address)
  File "/System/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/SocketServer.py", line 320, in finish_request
    self.RequestHandlerClass(request, client_address, self)
  File "/System/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/SocketServer.py", line 615, in __init__
    self.handle()
  File "/Library/Python/2.6/site-packages/Paste-1.7.4-py2.6.egg/paste/httpserver.py", line 442, in handle
    BaseHTTPRequestHandler.handle(self)
  File "/System/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/BaseHTTPServer.py", line 329, in handle
    self.handle_one_request()
  File "/Library/Python/2.6/site-packages/Paste-1.7.4-py2.6.egg/paste/httpserver.py", line 437, in handle_one_request
    self.wsgi_execute()
  File "/Library/Python/2.6/site-packages/Paste-1.7.4-py2.6.egg/paste/httpserver.py", line 287, in wsgi_execute
    self.wsgi_start_response)
  File "/Library/Python/2.6/site-packages/Paste-1.7.4-py2.6.egg/paste/cascade.py", line 130, in __call__
    return self.apps[-1](environ, start_response)
  File "/Library/Python/2.6/site-packages/Paste-1.7.4-py2.6.egg/paste/registry.py", line 375, in __call__
    app_iter = self.application(environ, start_response)
  File "/Library/Python/2.6/site-packages/Pylons-0.9.7-py2.6.egg/pylons/middleware.py", line 201, in __call__
    self.app, environ, catch_exc_info=True)
  File "/Library/Python/2.6/site-packages/Pylons-0.9.7-py2.6.egg/pylons/util.py", line 94, in call_wsgi_application
    app_iter = application(environ, start_response)
  File "/Library/Python/2.6/site-packages/WebError-0.10.2-py2.6.egg/weberror/evalexception.py", line 235, in __call__
    return self.respond(environ, start_response)
  File "/Library/Python/2.6/site-packages/WebError-0.10.2-py2.6.egg/weberror/evalexception.py", line 483, in respond
    return debug_info.content()
  File "/Library/Python/2.6/site-packages/WebError-0.10.2-py2.6.egg/weberror/evalexception.py", line 545, in content
    result = tmpl_formatter(self.exc_value)
  File "/Library/Python/2.6/site-packages/Pylons-0.9.7-py2.6.egg/pylons/error.py", line 43, in mako_html_data
    css=False)
  File "/Library/Python/2.6/site-packages/Mako-0.3.2-py2.6.egg/mako/template.py", line 189, in render
    return runtime._render(self, self.callable_, args, data)
  File "/Library/Python/2.6/site-packages/Mako-0.3.2-py2.6.egg/mako/runtime.py", line 403, in _render
    _render_context(template, callable_, context, *args, **_kwargs_for_callable(callable_, data))
  File "/Library/Python/2.6/site-packages/Mako-0.3.2-py2.6.egg/mako/runtime.py", line 434, in _render_context
    _exec_template(inherit, lclcontext, args=args, kwargs=kwargs)
  File "/Library/Python/2.6/site-packages/Mako-0.3.2-py2.6.egg/mako/runtime.py", line 457, in _exec_template
    callable_(context, *args, **kwargs)
  File "memory:0x1040470d0", line 54, in render_body
  File "/Library/Python/2.6/site-packages/Mako-0.3.2-py2.6.egg/mako/exceptions.py", line 88, in __init__
    self.records = self._init(traceback)
  File "/Library/Python/2.6/site-packages/Mako-0.3.2-py2.6.egg/mako/exceptions.py", line 166, in _init
    line = line.decode('ascii', 'replace')
AttributeError: 'NoneType' object has no attribute 'decode'
----------------------------------------

Answer 2

I am not absolutely sure that it's the same issue, but as far as I remember this used to happen a lot when you do AJAX loads of page fragments. Then you don't get anything more useful than this message.

However if you try to load the address of the AJAX request itself in your browser (replacing post parameters with get parameters if needed), you will get a "normal" debug page.