Handling specific exceptions from Jinja2 and futures?

David Rueter

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Jan 30, 2016, 11:34:16 PM1/30/16

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I am using Tornado (4.3) under Python (3.4) with Jinja2 (2.8) and things work well--except I am having trouble catching exceptions that Jinja2 raises while processing templates.

Since I am using @tornado.gen.coroutine and @run_on_executor decorators, I can't catch the exceptions with something like this:

try:

buf = template.render(data=this_data)

except Exception as e:

buf = '<html><body>Error rendering template.</body></html>'

When an exception is raised by Jinja2 while rendering, it is not caught by the except block. Instead, the exception bubbles up with a traceback like this:

Can't convert 'NoneType' object to str implicitly

['Traceback (most recent call last):\n', ' File "C:/Projects/OsPy/OsPy.py", line 1020, in background_process_post_authenticated\n buf = template.render(data=this_data)\n', ' File "C:\\Prog\\Python34\\lib\\site-packages\\jinja2\\environment.py", line 989, in render\n return self.environment.handle_exception(exc_info, True)\n', ' File "C:\\Prog\\Python34\\lib\\site-packages\\jinja2\\environment.py", line 754, in handle_exception\n reraise(exc_type, exc_value, tb)\n', ' File "C:\\Prog\\Python34\\lib\\site-packages\\jinja2\\_compat.py", line 37, in reraise\n raise value.with_traceback(tb)\n', ' File "

I do have a write_error method defined, so I can catch the exception that Tornado raises, but I don't have any visibility into detailed template errors encountered by Jinja2. This makes debugging difficult.

Can anyone share a pattern for handling detailed exceptions such as template errors thrown by Jinja2 when rendering from the template inside a @run_on_executor routine called from a @tornado.gen.coroutine method?

A. Jesse Jiryu Davis

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Jan 31, 2016, 5:34:07 AM1/31/16

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Can we see your actual code where you use run_on_executor and Jinja2's "render" method?

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David Rueter

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Jan 31, 2016, 7:51:06 PM1/31/16

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My code goes something like this:

#!/usr/bin/env python

import xxxx

...import a bunch of stuff, including jinja2, tornado

#create Jinja2 environment

jinja_env = Environment()

jinja_env.undefined = SilentUndefined

class OSHandler(tornado.web.RequestHandler):

executor = ThreadPoolExecutor(max_workers=MAX_WORKERS)

...

#various methods here

...

@run_on_executor

def background_process_post_authenticated(self):

# Do everything that is needed to process an HTTP post on an authenticated session

...Code here to implement needed functionality, retrieve a template and data

...On a background thread because the request may take several seconds to service as there is a complex

...SQL database to access, complex business rules to process, etc

...Note: this is a corporate application for dozens or hundreds of users--not a public-facing website

#create Jinja2 template object from the template_str we retrieved above

template = jinja_env.from_string(template_str)

#render output using template and data

try:

buf = template.render(data=this_data)

except Exception as e:

buf = '<html><body>Error rendering template (' + next_page + '). ' + str(e)

return buf

@tornado.gen.coroutine

def post(self):

#MAIN ENTRY POINT FOR HTTP POST REQUEST

...code here to manage application-level session, database connection, etc.

buf = yield self.background_process_post_authenticated()

self.write(buf)

self.finish()

def run():

...code here to load settings, get ready to start server

application = tornado.web.Application([

(r"/", OSHandler),

],

debug=True,

xsrf_cookies=True,

cookie_secret='txxxxx secret stuff =')

http_server = tornado.httpserver.HTTPServer(application)

http_server.listen(options.port)

ioloop = tornado.ioloop.IOLoop.instance()

tornado.ioloop.PeriodicCallback(do_periodic_callback, 1000).start()

ioloop.start()

ioloop.close()

if __name__ == "__main__":

run()

A. Jesse Jiryu Davis

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Jan 31, 2016, 9:41:37 PM1/31/16

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That appears, to me, like it should work. It may not be useful to render the template on a thread, since Python can't take advantage of multithreading for computational tasks like template-rendering. But I don't see why your code wouldn't catch rendering exceptions using that pattern, yet the traceback does appear to show an uncaught exception from render().

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