此答案应归因于@JFSebastian。感谢您的评论!
以下代码获得了我的预期行为:
import pexpect
analyzer = pexpect.spawn('hfst-lookup analyser-gt-desc.hfstol', encoding='utf-8')
analyzer.expect('> ')
for word in ['слово', 'сработай']:
print('Trying', word, '...')
analyzer.sendline(word)
analyzer.expect('> ')
print(analyzer.before)