Question

I’m new to Python and I’m having a problem viewing error messages: I have a javascript function making a call like:

$.getJSON('test.py', {data: 'somedata'}, function(return){alert(return.echo)})

And a test.py file successfully echoing that message:

def application(environ, start_response):
  import json
  status = '200 OK'
  response_headers = [('Content-type', 'application/json')]
  start_response(status, response_headers)
  data = environ['QUERY_STRING'].split('=')[1])   
  return json.dumps({'echo': data}, separators=(',',':'))

NB: I just retyped that script from memory so if you notice a syntactical error it’s me keying it wrong, the base script is working fine.

If i run the page with the call, I get my alert, and then if I visit the Network tab in Chrome, I see

Request URL:http://localhost/test.py?data=somedata
Request Method:GET
Status Code:200 OK

and on the Preview and Response tabs, i see:

{"echo":"testdata"}

However, if I then throw an actual error into the script (which is more often than not during this learning period!) and test the page out in Chrome, the network tab shows the call being made

Request URL:http://localhost/test.py?data=somedata
Request Method:GET
Status Code:500 Internal Server Error

Query String Parameters: (view URL encoded)
    data:'somedata'

But, if I click on preview, It shows:

Internal Server Error

The server encountered an internal error or misconfiguration and was unable to complete your request.

Please contact the server administrator, [e-mail] and inform them of the time the error occurred, and anything you might have done that may have caused the error.

More information about this error may be available in the server error log.

Apache/2.2.21 (Unix) DAV/2 mod_wsgi/3.3 Python/2.6.7 Server at localhost Port 80

My question is whether or not it’s possible to use this window to show a stack trace or any information that would help me diagnose problems. I’m doing all my debugging by trial-and-error at the moment, other than that loving every second of Python.

Would anybody be able to point me in the right direction?

EDIT: This page:

http://code.google.com/p/modwsgi/wiki/DebuggingTechniques

has a solution under the heading “Error Catching Middleware” but their example isn’t working for me.

Answer 1

Try writing your code to be tolerant of not being given a QUERY_STRING. The following will fail if there isn’t one.

data = environ['QUERY_STRING'].split('=')[1])

In general it is always better to use some sort of framework which ensures little things like this may not be a problem.

See Flask as a good example of a small micro framework.