2. Debugging

2.1. Werkzeug debugger

Kay has werkzeug’s debugger integrated in it, and it is enabled in local dev environment by default. Unfortunately, it can not be used on appengine server environment.

This debugger works on developer’s web browser, and it will run when catching any exception. You can use an interactive console and view-source capability on each step of the tracebacks when the exception occurs.

Also, you can view tracebacks in a plain text format, and post these tracebacks to a paste service hosted on the internet.

2.1.1. Debugger display

When an exception raises on dev environment, you can get following display:


Lineno and source code of each step of tracebacks are displayed. Following icons are showed in the right side of the row when you put your mouse cursor onto the rows of source code.


If you click on the left icon, an interactive console will be displayed. If you click on the right iocn, you can view source code with the problematic line highlighted.

2.1.2. Interactive console

Here is a screenshot of an interactive console.


In this console, you can execute any python code with the frame information at the time when the exception occurs, so its very helpful for debugging.

For example, execute locals(), you can get the dictionary for local variables.

[console ready]
>>> locals()
{'request': <Request 'http://localhost:8080/' [GET]>}

Fixing TYPO and re-executing your code will give you the correct result.

[console ready]
>>> comments = Comment.all().order('-created').fetch(100)
>>> comments
[<myapp.models.Comment object at 0x104c6c8d0>]

If you click the console icon again, you can hide the interactive console.

2.1.3. View source

Here is the souce code view. You can see the problematic line is highlighted.


If you click on the title that shows View Source, you can hide the source code view.

2.1.4. View traceback as plaintext

If you want to paste the traceback onto e-mail or something like that, you can click on the title that shows Traceback as bellow.


If you click on this title, the way for displaying traceback will be changed into Debugger/Plaintext in turn. Here is the display for viewing traceback in Plaintext.


2.1.5. Post your traceback

When you view your traceback as Plaintext, there is a button with a text create paste. If you click on this button, Kay will post your traceback to a paste service hosted at: http://paste.shehas.net/. If the post succeeds, a link for that paste will be shown.


Here is a screenshot of the paste service.


2.1.6. Exception in Jinja2 template

If an exception occurs in Jinja2 template, you will see wired traceback on the debugger. That is because of the restriction of appengine( can not use ctypes). For a workaround, we can patch dev_appserver.py in appengine SDK.

After adding ‘gestalt’ and ‘_ctypes’ to the list _WHITE_LIST_C_MODULES, you can see normal tracebacks on the debugger.

Having said that, some python distribution has a broken ctypes(ex: recent python25 in macports), and above workaround won’t work with broken ctypes. In such a case, copying _speedups.so into the directory kay/lib/jinja2 from another jinja2 installation(not from bundled in Kay), and adding ‘_speedups’ to the list _WHITE_LIST_C_MODULES could be another workaround. If you’re using MacOSX, the easiest way to get compiled _speedups.so is to install py25-jinja2 with macports.

2.2. Using pdb

You can also use pdb for debugging in dev environment. If you invoke kay.utils.set_trace() anywhere on your code, the execution of your program will stop. You can see a pdb prompt on the console in which you invoked manage.py runserver.

For example, you can execute your program step by step with a command step. For more details how to use pdb, please refer to following URL: