Sane way to define default variable values from within a jinja template?

Question

I'd like to set default values for variables used in my Jinja template inside of the template itself. Looking at the Jinja2 documentation, I don't see any way to do this. Have I missed something? I see the "default" filter, but I want to set the value template wide instead of a use-by-use basis.

I spent an hour or so trying to teach myself enough about the Jinja2 extension writing process to write an extension tag setdefault, which could look like this:

{% setdefault animal = 'wumpas' %}

The desired effect would be equivalent to the set tag if the assigned-to name was undefined, but to have no effect if the assigned-to name was defined. I have thusfar failed to get this to work.

My work around is to circumvent jinja entirely and make a compound file; the area before a special marker is a (yaml) mapping of default values, and the area after a marker is the jinja template. An proof of concept implementation of this that seems to work fine is:

skel_text = """\
animal: wumpas
%%
The car carried my {{animal}} to the vet.
"""
class Error(Exception): pass
_skel_rx = re.compile(
    r"""((?P<defaults>.*?)^%%[ \t]*\n)?(?P<template>.*)""",
    re.MULTILINE|re.DOTALL)
_env = jinja2.Environment(trim_blocks=True)
def render(skel, **context):
    m = _skel_rx.match(skel_text)
    if not m:
        raise Error('skel split failed')
    defaults = yaml.load(m.group('defaults') or '{}')
    template = _env.from_string(m.group('template') or '')
    template.globals.update(defaults)
    return template.render(**context)

print render(skel_text)
print render(skel_text, animal='cat')

So, is there a way to do the equivalent in stock Jinja2, or how might one write an extension to accomplish the desired effect?

Answer 1

What worked for me was to use a filter:

t = '''Hello {{name | default('John Doe')}}'''