id	summary	reporter	owner	description	type	status	component	version	severity	resolution	keywords	cc	stage	has_patch	needs_docs	needs_tests	needs_better_patch	easy	ui_ux
25423	Using custom tag without {% load %} result in confusing traceback	Yuchen Ying	Tim Graham <timograham@…>	"In the following sample code, the author clearly registered the new tag but forgot to load it in template.

{{{
from django import template
from django import test
from django.template import context

register = template.Library()
                                                                                
@register.simple_tag(name=""test"")
def simple_tag(*args):
  return ""nothing here""

class MyTest(test.TestCase):
  def test_forgot_to_load(self):
    TEMPLATE = """"""
    {% if True %}
      {% test %}
    {% endif %}
    """"""
    template.Template(TEMPLATE).render(context.Context())
}}}

This code will create the following traceback in Python 3:

{{{

Traceback (most recent call last):
  File ""/home/ych/.py3kvenv/lib/python3.4/site-packages/django/template/base.py"", line 337, in parse
    compile_func = self.tags[command]
KeyError: 'test'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File ""/home/ych/testproject/testproject/test.py"", line 25, in test_forgot_to_load
    template.Template(TEMPLATE).render(context.Context())
  File ""/home/ych/.py3kvenv/lib/python3.4/site-packages/django/template/base.py"", line 190, in __init__
    self.nodelist = engine.compile_string(template_string, origin)
  File ""/home/ych/.py3kvenv/lib/python3.4/site-packages/django/template/engine.py"", line 261, in compile_string
    return parser.parse()
  File ""/home/ych/.py3kvenv/lib/python3.4/site-packages/django/template/base.py"", line 341, in parse
    compiled_result = compile_func(self, token)
  File ""/home/ych/.py3kvenv/lib/python3.4/site-packages/django/template/defaulttags.py"", line 1014, in do_if
    nodelist = parser.parse(('elif', 'else', 'endif'))
  File ""/home/ych/.py3kvenv/lib/python3.4/site-packages/django/template/base.py"", line 339, in parse
    self.invalid_block_tag(token, command, parse_until)
  File ""/home/ych/.py3kvenv/lib/python3.4/site-packages/django/template/base.py"", line 394, in invalid_block_tag
    (command, get_text_list([""'%s'"" % p for p in parse_until])))
django.template.base.TemplateSyntaxError: Invalid block tag: 'test', expected 'elif', 'else' or 'endif'
}}}

And this is the traceback in Python2:


{{{
Traceback (most recent call last):
  File ""/home/ych/testproject/testproject/test.py"", line 25, in test_forgot_to_load
    template.Template(TEMPLATE).render(context.Context())
  File ""/home/ych/.py2kvenv/local/lib/python2.7/site-packages/django/template/base.py"", line 190, in __init__
    self.nodelist = engine.compile_string(template_string, origin)
  File ""/home/ych/.py2kvenv/local/lib/python2.7/site-packages/django/template/engine.py"", line 261, in compile_string
    return parser.parse()
  File ""/home/ych/.py2kvenv/local/lib/python2.7/site-packages/django/template/base.py"", line 341, in parse
    compiled_result = compile_func(self, token)
  File ""/home/ych/.py2kvenv/local/lib/python2.7/site-packages/django/template/defaulttags.py"", line 1014, in do_if
    nodelist = parser.parse(('elif', 'else', 'endif'))
  File ""/home/ych/.py2kvenv/local/lib/python2.7/site-packages/django/template/base.py"", line 339, in parse
    self.invalid_block_tag(token, command, parse_until)
  File ""/home/ych/.py2kvenv/local/lib/python2.7/site-packages/django/template/base.py"", line 394, in invalid_block_tag
    (command, get_text_list([""'%s'"" % p for p in parse_until])))
TemplateSyntaxError: Invalid block tag: 'test', expected 'elif', 'else' or 'endif'
}}}

It says invalid block tag, but invalid how? Does that mean I should not use it at the specified location (which was my understanding when encountered this traceback), or invalid in other ways (this is the case here)? If invalid in other ways, how should I fix it?

The traceback is slightly better in Python3 in that it let you know it's looking for 'test' in a dict but cannot find it, which will help you debugging this issue. But in both case it's unclear that this can be fixed with a simple {% load %} tag.

Without knowing the implementation detail, I find it confusing that after registering the tag I still need to load it in template. If this extra step is unavoidable, could you please improve the error message a little bit? Something like:


{{{
Encountered unknown tag %s when parsing template %s at line %d: expect %s. Did you forgot to register or load this tag?
}}}
"	Cleanup/optimization	closed	Template system	1.8	Normal	fixed			Ready for checkin	1	0	0	0	0	0