Debugging Applications — Werkzeug documentation
Depending on the WSGI gateway/server, exceptions are handled differently. Most of the time, exceptions go to stderr or the error log, and a generic “500 Internal Server Error” message is displayed.
Since this is not the best debugging environment, Werkzeug provides a WSGI middleware that renders nice tracebacks, optionally with an interactive debug console to execute code in any frame.
The debugger allows the execution of arbitrary code which makes it a major security risk. The debugger must never be used on production machines. We cannot stress this enough. Do not enable the debugger in production.
The interactive debugger does not work in forking environments, such as a server that starts multiple processes. Most such environments are production servers, where the debugger should not be enabled anyway.
Enabling the Debugger
Enable the debugger by wrapping the application with the
DebuggedApplication middleware. Alternatively, you can pass
run_simple() and it will do that for you.
Using the Debugger
Once enabled and an error happens during a request you will see a detailed traceback instead of a generic “internal server error”. The traceback is still output to the terminal as well.
The error message is displayed at the top. Clicking it jumps to the bottom of the traceback. Frames that represent user code, as opposed to built-ins or installed packages, are highlighted blue. Clicking a frame will show more lines for context, clicking again will hide them.
If you have the
evalex feature enabled you can get a console for every frame in the traceback by hovering over a frame and clicking the console icon that appears at the right. Once clicked a console opens where you can execute Python code in:
class=align-center|a screenshot of the interactive debugger Inside the interactive consoles you can execute any kind of Python code. Unlike regular Python consoles the output of the object reprs is colored and stripped to a reasonable size by default. If the output is longer than what the console decides to display a small plus sign is added to the repr and a click will expand the repr.
To display all variables that are defined in the current frame you can use the
dump() function. You can call it without arguments to get a detailed list of all variables and their values, or with an object as argument to get a detailed list of all the attributes it has.
Starting with Werkzeug 0.11 the debug console is protected by a PIN. This is a security helper to make it less likely for the debugger to be exploited if you forget to disable it when deploying to production. The PIN based authentication is enabled by default.
The first time a console is opened, a dialog will prompt for a PIN that is printed to the command line. The PIN is generated in a stable way that is specific to the project. An explicit PIN can be provided through the environment variable
WERKZEUG_DEBUG_PIN. This can be set to a number and will become the PIN. This variable can also be set to the value
off to disable the PIN check entirely.
If an incorrect PIN is entered too many times the server needs to be restarted.
This feature is not meant to entirely secure the debugger. It is intended to make it harder for an attacker to exploit the debugger. Never enable the debugger in production.
If you click on the “Traceback (most recent call last)” header, the view switches to a traditional text-based traceback. You can copy and paste this in order to provide information when asking a question or reporting an issue.