def mergeTxt(txtFiles):
    with open('result.txt', 'w') as fp:
        with open(txtFiles[0]) as fp1, open(txtFiles[1]) as fp2:
            while True:
                # 交替读取文件1和文件2中的行，写入结果文件
                line1 = fp1.readline()
                if line1:
                    fp.write(line1)
                else:
                    # 如果文件1结束，结束循环
                    flag = False
                    break
                line2 = fp2.readline()
                if line2:
                    fp.write(line2)
                else:
                    # 如果文件2结束，结束循环
                    flag = True
                    break
            # 获取尚未结束的文件对象
            fp3 = fp1 if flag else fp2
            # 把剩余内容写入结果文件
            for line in fp3:
                fp.write(line)

txtFiles = ['1.txt', '2.txt']
mergeTxt(txtFiles)