code: def _replace_match(m, env):
      
      # ditch the curly braces
      s = m.group()[1:-1].strip()
  
      try:
          # get the env attributes e.g. envpython or toxinidir.
          # Note: if you ask for a env methodname this will raise
          # later on... so don't do that.
          return getattr(env, s)
      except AttributeError:
          pass
  
      for r in [_replace_envvar, _replace_config, _replace_posargs]:
          try:
              return r(s, env)
          except ValueError:
              pass
  
      raise NotImplementedError("{%s} not understood in tox.ini file." % s)
docs: Given a match object, having matched something inside curly braces,
      replace the contents if matches one of the supported tox-substitutions.