old_contents: import re
  
  
  # regular expression for finding assignments
  _quotes = "['|\"|\"\"\"]"
  BASE_ASSIGN_PATTERN = r'({}\s*=\s*[ubr]?' + _quotes + r')(.*?)(' +\
                        _quotes + r')'
  
  
  def find_assign(data, varname):
      """Finds a substring that looks like an assignment.
  
      :param data: Source to search in.
      :param varname: Name of the variable for which an assignment should be
                      found.
      """
      ASSIGN_RE = re.compile(BASE_ASSIGN_PATTERN.format(varname))
  
      if len(ASSIGN_RE.findall(data)) > 1:
          raise ValueError('Found multiple {}-strings.'.format(varname))
  
      if len(ASSIGN_RE.findall(data)) < 1:
          raise ValueError('No version assignment ("{}") found.'.format(varname))
  
      return ASSIGN_RE.search(data).group(2)
  
  
  def replace_assign(data, varname, new_value):
      ASSIGN_RE = re.compile(BASE_ASSIGN_PATTERN.format(varname))
  
      def repl(m):
          return m.group(1) + new_value + m.group(3)
  
      return ASSIGN_RE.sub(repl, data)
  
new_contents: from unleash.exc import PluginError
  
  import re
  
  
  # regular expression for finding assignments
  _quotes = "['|\"|\"\"\"]"
  BASE_ASSIGN_PATTERN = r'({}\s*=\s*[ubr]?' + _quotes + r')(.*?)(' +\
                        _quotes + r')'
  
  
  def find_assign(data, varname):
      """Finds a substring that looks like an assignment.
  
      :param data: Source to search in.
      :param varname: Name of the variable for which an assignment should be
                      found.
      """
      ASSIGN_RE = re.compile(BASE_ASSIGN_PATTERN.format(varname))
  
      if len(ASSIGN_RE.findall(data)) > 1:
          raise PluginError('Found multiple {}-strings.'.format(varname))
  
      if len(ASSIGN_RE.findall(data)) < 1:
          raise PluginError('No version assignment ("{}") found.'
                            .format(varname))
  
      return ASSIGN_RE.search(data).group(2)
  
  
  def replace_assign(data, varname, new_value):
      ASSIGN_RE = re.compile(BASE_ASSIGN_PATTERN.format(varname))
  
      def repl(m):
          return m.group(1) + new_value + m.group(3)
  
      return ASSIGN_RE.sub(repl, data)
  
subject: Raise PluginErrors instead of ValueErrors in versions.