old_contents: #!/usr/bin/env python
  import os
  import sys
  if not os.path.exists('.tox/configure'):
      import virtualenv
      import subprocess
      print("Bootstrapping ...")
      virtualenv.create_environment('.tox/configure')
      print("Installing `jinja2` and `matrix` into bootstrap environment ...")
      if sys.platform == 'win32':
          subprocess.check_call(['.tox/configure/Scripts/pip', 'install', 'jinja2', 'matrix'])
      else:
          subprocess.check_call(['.tox/configure/bin/pip', 'install', 'jinja2', 'matrix'])
  if sys.platform == 'win32':
      execfile('.tox/configure/Scripts/activate_this.py', dict(__file__='.tox/configure/Scripts/activate_this.py'))
  else:
      execfile('.tox/configure/bin/activate_this.py', dict(__file__='.tox/configure/bin/activate_this.py'))
  import jinja2
  import matrix
  
  jinja = jinja2.Environment(
      loader=jinja2.FileSystemLoader('conf'),
      trim_blocks=True,
      lstrip_blocks=True,
      keep_trailing_newline=True
  )
  tox_environments = {}
  for alias, conf in matrix.from_file('setup.cfg').items():
      python = conf['python_versions']
      deps = conf['dependencies']
      cover = {'false': False, 'true': True}[conf['coverage_flags'].lower()]
      env_vars = conf['environment_variables']
  
      tox_environments[alias] = {
          'python': 'python' + python if 'py' not in python else python,
          'deps': deps.split(),
          'cover': cover,
          'env_vars': env_vars.split(),
      }
  
  for name in os.listdir('conf'):
      with open(name, 'w') as fh:
          fh.write(jinja.get_template(name).render(tox_environments=tox_environments))
      print("Wrote %s" % name)
  
  print("DONE.")
new_contents: #!/usr/bin/env python
  import os
  import sys
  if not os.path.exists('.tox/configure'):
      import virtualenv
      import subprocess
      print("Bootstrapping ...")
      virtualenv.create_environment('.tox/configure')
      print("Installing `jinja2` and `matrix` into bootstrap environment ...")
      if sys.platform == 'win32':
          subprocess.check_call(['.tox/configure/Scripts/pip', 'install', 'jinja2', 'matrix'])
      else:
          subprocess.check_call(['.tox/configure/bin/pip', 'install', 'jinja2', 'matrix'])
  if sys.platform == 'win32':
      exec(compile(open('.tox/configure/Scripts/activate_this.py').read(), '.tox/configure/Scripts/activate_this.py', 'exec'), dict(__file__='.tox/configure/Scripts/activate_this.py'))
  else:
      exec(compile(open('.tox/configure/bin/activate_this.py').read(), '.tox/configure/bin/activate_this.py', 'exec'), dict(__file__='.tox/configure/bin/activate_this.py'))
  import jinja2
  import matrix
  
  jinja = jinja2.Environment(
      loader=jinja2.FileSystemLoader('conf'),
      trim_blocks=True,
      lstrip_blocks=True,
      keep_trailing_newline=True
  )
  tox_environments = {}
  for alias, conf in matrix.from_file('setup.cfg').items():
      python = conf['python_versions']
      deps = conf['dependencies']
      cover = {'false': False, 'true': True}[conf['coverage_flags'].lower()]
      env_vars = conf['environment_variables']
  
      tox_environments[alias] = {
          'python': 'python' + python if 'py' not in python else python,
          'deps': deps.split(),
          'cover': cover,
          'env_vars': env_vars.split(),
      }
  
  for name in os.listdir('conf'):
      with open(name, 'w') as fh:
          fh.write(jinja.get_template(name).render(tox_environments=tox_environments))
      print("Wrote %s" % name)
  
  print("DONE.")
subject: Update from template.