Avoiding Python UnicodeDecodeError in Jinja's nl2br filter


I'm using Jinja2's nl2br filter, which looks like:

import re
from jinja2 import environmentfilter, Markup, escape

_paragraph_re = re.compile(r'(?:\r\n|\r|\n){2,}')

def nl2br(eval_ctx, value):
    result = u'\n\n'.join(u'<p>%s</p>' % p.replace('\n', '<br>\n')
                      for p in _paragraph_re.split(escape(value)))
    if eval_ctx.autoescape:
        result = Markup(result)
    return result

The problem is if "value" has anything but ascii characters (for example: "/mɒnˈtænə/" causes it to fail). I get this error:

Traceback (most recent call last):
  File "/usr/local/lib/python2.6/dist-packages/Flask-0.6.1-py2.6.egg/flask/app.py", line 889, in __call__
    return self.wsgi_app(environ, start_response)
  File "/usr/local/lib/python2.6/dist-packages/Flask-0.6.1-py2.6.egg/flask/app.py", line 879, in wsgi_app
    response = self.make_response(self.handle_exception(e))
  File "/usr/local/lib/python2.6/dist-packages/Flask-0.6.1-py2.6.egg/flask/app.py", line 876, in wsgi_app
    rv = self.dispatch_request()
  File "/usr/local/lib/python2.6/dist-packages/Flask-0.6.1-py2.6.egg/flask/app.py", line 695, in dispatch_request
    return self.view_functions[rule.endpoint](**req.view_args)
  File "/home/mcrittenden/Dropbox/Code/dropdo/dropdo.py", line 105, in view
    return render_template(template, src = url, data = content)
  File "/usr/local/lib/python2.6/dist-packages/Flask-0.6.1-py2.6.egg/flask/templating.py", line 85, in render_template
    context, ctx.app)
  File "/usr/local/lib/python2.6/dist-packages/Flask-0.6.1-py2.6.egg/flask/templating.py", line 69, in _render
    rv = template.render(context)
  File "/usr/local/lib/python2.6/dist-packages/Jinja2-2.5.5-py2.6.egg/jinja2/environment.py", line 891, in render
    return self.environment.handle_exception(exc_info, True)
  File "/home/mcrittenden/Dropbox/Code/dropdo/templates/text.html", line 1, in top-level template code
    {% extends "layout.html" %}
  File "/home/mcrittenden/Dropbox/Code/dropdo/templates/layout.html", line 25, in top-level template code
    {% block content %}{% endblock %}
  File "/home/mcrittenden/Dropbox/Code/dropdo/templates/text.html", line 8, in block "content"
    {{ data|nl2br }}
  File "/home/mcrittenden/Dropbox/Code/dropdo/dropdo.py", line 26, in nl2br
    for p in _paragraph_re.split(escape(value)))
UnicodeDecodeError: 'ascii' codec can't decode byte 0xc9 in position 12: ordinal not in range(128)

What's the best to prevent the error but not remove the problem characters altogether?

2/25/2011 5:09:30 PM

Accepted Answer

Use unicode literals everywhere.

"Unicode in Python, Completely Demystified"

2/25/2011 5:11:15 PM

If "value" has anything but ascii characters, you want it to be Unicode, and nothing but Unicode, throughout your entire app, except for a few places where you explicitly encode or decode it. Pass Unicode to your templates, too.

If you acquire the string "/mɒnˈtænə/" somehow, you probably know its encoding; use it: value = "/mɒnˈtænə/".decode(the_encoding).

How do you learn the encoding? A HTTP request knows its encoding. An XML file knows its encoding. A plain text file usually does not; you must know its encoding by some other means.

Note that UTF-8 is not Unicode though it is an encoding that can fully represent Unicode. It's still an encoding, and to get a Python Unicode string from it, you need to .decode("utf-8") it.

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