$ cat > engine.py << EOF
>
> from mercurial import templater
>
> class mytemplater(object):
> def __init__(self, loader, filters, defaults, resources, aliases):
> self.loader = loader
> self._defaults = defaults
> self._resources = resources
>
> def symbol(self, mapping, key):
> return mapping[key]
>
> def resource(self, mapping, key):
> v = self._resources[key]
> if v is None:
> v = mapping[key]
> return v
>
> def process(self, t, map):
> tmpl = self.loader(t)
> props = self._defaults.copy()
> props.update(map)
> for k, v in props.items():
> if k in ('templ', 'ctx', 'repo', 'revcache', 'cache', 'troubles'):
> continue
> if callable(v) and getattr(v, '_requires', None) is None:
> props = self._resources.copy()
> props.update(map)
> v = v(**props)
> elif callable(v):
> v = v(self, props)
> v = templater.stringify(v)
> tmpl = tmpl.replace('{{%s}}' % k, v)
> yield tmpl
>
> templater.engines['my'] = mytemplater
> EOF
$ hg init test
$ echo '[extensions]' > test/.hg/hgrc
$ echo "engine = `pwd`/engine.py" >> test/.hg/hgrc
$ cd test
$ cat > mymap << EOF
> changeset = my:changeset.txt
> EOF
$ cat > changeset.txt << EOF
> {{rev}} {{node}} {{author}}
> EOF
$ hg ci -Ama
adding changeset.txt
adding mymap
$ hg log --style=./mymap
0 97e5f848f0936960273bbf75be6388cd0350a32b test
$ cat > changeset.txt << EOF
> {{p1rev}} {{p1node}} {{p2rev}} {{p2node}}
> EOF
$ hg ci -Ama
$ hg log --style=./mymap
0 97e5f848f0936960273bbf75be6388cd0350a32b -1 0000000000000000000000000000000000000000
-1 0000000000000000000000000000000000000000 -1 0000000000000000000000000000000000000000
invalid engine type:
$ echo 'changeset = unknown:changeset.txt' > unknownenginemap
$ hg log --style=./unknownenginemap
abort: invalid template engine: unknown
[255]
$ cd ..