1.写一个简单的(+-*/)的计算器，确保输入的都是数字（如果不是数字，让他重新输入）
可以得到结果
2.while单层循环完成9*9乘法表
递归
计算所有i(1-9)与j(1-9)的乘法再加条件限制显示
改进后，不必计算重复的值
3.str字符串中的部分方法
| center(self, width, fillchar=' ', /)
| Return a centered string of length width.
| 返回一个长宽居中的字符串
| Padding is done using the specified fill character (default is a space).
| 填充是使用指定的填充字符完成的（默认为空格）
| expandtabs(self, /, tabsize=8)
| Return a copy where all tab characters are expanded using spaces.
| 返回使用空格展开所有制表符的副本。
| If tabsize is not given, a tab size of 8 characters is assumed.
| 如果未给定tabsize，则假定tab大小为8个字符。
| join(self, iterable, /)
| Concatenate any number of strings.
| 串联任意数量的字符串。
| The string whose method is called is inserted in between each given string.
| The result is returned as a new string.
| 调用其方法的字符串插入到每个给定字符串之间。
| 结果将作为新字符串返回。
| ljust(self, width, fillchar=' ', /)
| Return a left-justified string of length width.
| 返回长度宽度为左对齐的字符串。
| rjust(self, width, fillchar=' ', /)
| Return a right-justified string of length width.
| 返回长度宽度为右对齐的字符串。
| Padding is done using the specified fill character (default is a space).
| 填充是使用指定的填充字符完成的（默认为空格）
| partition(self, sep, /)
| Partition the string into three parts using the given separator.
| 使用给定的分隔符将字符串分为三部分。
| This will search for the separator in the string. If the separator is found,
| returns a 3-tuple containing the part before the separator, the separator
| itself, and the part after it.
|
| If the separator is not found, returns a 3-tuple containing the original string
| and two empty strings.
| 如果未找到分隔符，则返回包含原始字符串的3元组和两个空字符串。
| replace(self, old, new, count=-1, /)
| Return a copy with all occurrences of substring old replaced by new.
| 返回一个副本，其中所有出现的子字符串旧替换为新。
| count
| Maximum number of occurrences to replace.
| -1 (the default value) means replace all occurrences.
| 要替换的最大出现次数。
| -1（默认值）表示替换所有引用。
| If the optional argument count is given, only the first count occurrences are
| replaced.
| 如果给定了可选的参数计数，则只会出现第一次计数已替换。
| split(self, /, sep=None, maxsplit=-1)
| Return a list of the words in the string, using sep as the delimiter string.
| 使用sep作为分隔符字符串，返回字符串中的单词列表。
| sep
| The delimiter according which to split the string.
| None (the default value) means split according to any whitespace,
| and discard empty strings from the result.
| 分割字符串所依据的分隔符。
| None（默认值）表示根据任何空格进行拆分，
| 并从结果中丢弃空字符串。
| maxsplit
| Maximum number of splits to do.
| -1 (the default value) means no limit.
| strip(self, chars=None, /)
| Return a copy of the string with leading and trailing whitespace removed.
| 返回删除前导和尾随空格的字符串副本。
| If chars is given and not None, remove characters in chars instead.
| 如果给定了chars而不是None，请删除chars中的字符。
4.格式化输出
格式化输出三种方式
"str".format()
"str".format_map()
f/F"str"
