$ cat > engine.py << EOF

  > 

  > from mercurial import (

  >     pycompat,

  >     templater,

  >     templateutil,

  > )

  > 

  > class mytemplater(templater.engine):

  >     def _load(self, t):

  >         return self._loader(t)

  > 

  >     def process(self, t, map):

  >         tmpl = self._load(t)

  >         props = self._defaults.copy()

  >         props.update(map)

  >         for k, v in props.items():

  >             if b'{{%s}}' % k not in tmpl:

  >                 continue

  >             if callable(v) and getattr(v, '_requires', None) is None:

  >                 props = self._resources.copy()

  >                 props.update(map)

  >                 v = v(**pycompat.strkwargs(props))

  >             elif callable(v):

  >                 v = v(self, props)

  >             v = templateutil.stringify(v)

  >             tmpl = tmpl.replace(b'{{%s}}' % k, v)

  >         yield tmpl

  > 

  > templater.engines[b'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 ..