code: def is_changed(fpath, prev_version_fpath):
      
      if not (fpath and os.path.isfile(fpath)):
          raise ValueError("Requirements file {0!r} doesn't exist.".format(fpath))
  
      # Compile the list of new requirements.
      new_requirements = parse_pip_requirements(fpath)
  
      # Hash them.
      new_requirements_hash = _str_to_sha1hex(str(new_requirements))
  
      # Read the hash of the previous requirements if any.
      previous_requirements_hash = 0
      if os.path.exists(prev_version_fpath):
          with open(prev_version_fpath) as fd:
              previous_requirements_hash = fd.read()
  
      # Create/Update the file with the hash of the new requirements.
      dirname = os.path.dirname(prev_version_fpath)
      # First time when running tox in the project .tox directory is missing.
      if not os.path.isdir(dirname):
          os.makedirs(dirname)
      with open(prev_version_fpath, 'w+') as fd:
          fd.write(new_requirements_hash)
  
      # Compare the hash of the new requirements with the hash of the previous
      # requirements.
      return previous_requirements_hash != new_requirements_hash
docs: Check requirements file is updated relatively to prev. version of the file.
  
      :param str fpath: Path to the requirements file.
      :param str prev_version_fpath: Path to the prev. version requirements file.
      :rtype: bool
      :raise ValueError: Requirements file doesn't exist.