$ cat > unix2mac.py <<EOF

  > import sys

  > 

  > for path in sys.argv[1:]:

  >     data = open(path, 'rb').read()

  >     data = data.replace(b'\n', b'\r')

  >     open(path, 'wb').write(data)

  > EOF

  $ cat > print.py <<EOF

  > import sys

  > print(sys.stdin.read().replace('\n', '<LF>').replace('\r', '<CR>').replace('\0', '<NUL>'))

  > EOF

  $ hg init

  $ echo '[hooks]' >> .hg/hgrc

  $ echo 'pretxncommit.cr = python:hgext.win32text.forbidcr' >> .hg/hgrc

  $ echo 'pretxnchangegroup.cr = python:hgext.win32text.forbidcr' >> .hg/hgrc

  $ cat .hg/hgrc

  [hooks]

  pretxncommit.cr = python:hgext.win32text.forbidcr

  pretxnchangegroup.cr = python:hgext.win32text.forbidcr

  $ echo hello > f

  $ hg add f

  $ hg ci -m 1

  $ "$PYTHON" unix2mac.py f

  $ hg ci -m 2

  attempt to commit or push text file(s) using CR line endings

  in dea860dc51ec: f

  transaction abort!

  rollback completed

  abort: pretxncommit.cr hook failed

  [255]

  $ hg cat f | "$PYTHON" print.py

  hello<LF>

  $ cat f | "$PYTHON" print.py

  hello<CR>