指定想要得到字符串的长度，(可选，没有指定则返回从指定开始位置到原字符串结束位置)。

The number of characters in the substring. If this argument is omitted, the returned substring includes all characters from the starting position to the end of the string.

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返回一个新字符串，该字符串通过在此字符串中的字符右侧填充空格来达到指定的总长度，从而使这些字符左对齐。

Returns a new string of a specified length in which the end of the current string is padded with Spaces or with a specified Unicode character.

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