How do you debug Mako templates?

Mako actually provides a VERY nice way to track down errors in a template:

from mako import exceptions

try:
    template = lookup.get_template(uri)
    print template.render()
except:
    print exceptions.html_error_template().render()

I break them down into pieces, and then reassemble the pieces when I've found the problem.

Not good, but it's really hard to tell what went wrong in a big, complex template.

My main frustration with Mako was that it was hard to see what was happening in the template. As the template code is a runnable object that is in-memory, no debugger can look into it.

One solution is to write the template code to file, and re-run the template using this file as a standard python module. Then you can debug to your hearts content.

An example:

import sys
from mako import exceptions, template 
from mako.template import DefTemplate
from mako.runtime import _render

<Do Great Stuff>

try:
    template.render(**arguments))
except:
    # Try to re-create the error using a proper file template
    # This will give a clearer error message.
    with open('failed_template.py', 'w') as out:
        out.write(template._code)
    import failed_template
    data = dict(callable=failed_template.render_body, **arguments)
    try:
        _render(DefTemplate(template, failed_template.render_body),
                failed_template.render_body,
                [],
                data)
    except:
        msg = '<An error occurred when rendering template for %s>\n'%arguments
        msg += exceptions.text_error_template().render()
        print(msg, file=sys.stderr)
        raise

Combining the two top answers with my own special sauce:

from flask.ext.mako import render_template as render_template_1
from mako import exceptions

app.config['MAKO_TRANSLATE_EXCEPTIONS'] = False    # seems to be necessary

def render_template(*args, **kwargs):
    kwargs2 = dict(**kwargs)
    kwargs2['config'] = app.config     # this is irrelevant, but useful
    try:
        return render_template_1(*args, **kwargs2)
    except:
        if app.config.get('DEBUG'):
            return exceptions.html_error_template().render()
        raise

It wraps the stock "render_template" function:

• catch exceptions, and
  • if debugging, render a backtrace
  • if not debugging, raise the exception again so it will be logged
• make config accessible from the page (irrelevant)