Series.str.casefold
`Series.str.lower` implements `str.lower`, as expected. There are also corresponding `Series` methods for the other Python 2 string casing methods. However, Python 3's `str.casefold` is missing. [Casefold](https://docs.python.org/3/library/stdtypes.html#str.casefold) improves string equality and other comparisons, because it handles a greater variety of characters, as per the Unicode Standard. It'd be nice to have `Series.str.casefold` in Pandas.

The current alternative is more verbose and is slower than `Series.str.lower`.

    pd.Series(s.casefold() if isinstance(s, str) else s for s in series)

Further, this alternative encourages a frustrating mistake -- forgetting to keep the original Series' index, which causes trouble if the new Series needs to be inserted into the same DataFrame as the original.


Apologies for double-posting. I used the wrong account for #25404 .