def expand(fn, col, inputtype=pd.DataFrame):
    """
    Wrap a function applying to a single column to make a function
    applying to a multi-dimensional dataframe or ndarray

    Parameters
    ----------
    fn : function
        Function that applies to a series or vector.

    col : str or int
        Index of column to which to apply `fn`.

    inputtype : class or type
        Type of input to be expected by the wrapped function.
        Normally pd.DataFrame or np.ndarray. Defaults to pd.DataFrame.

    Returns
    ----------
    wrapped : function
        Function that takes an input of type `inputtype` and applies
        `fn` to the specified `col`.
    """
    if inputtype == pd.DataFrame:
        if isinstance(col, int):
            def _wrapper(*args, **kwargs):
                return fn(args[0].iloc[:, col], *args[1:], **kwargs)
            return _wrapper
        def _wrapper(*args, **kwargs):
            return fn(args[0].loc[:, col], *args[1:], **kwargs)
        return _wrapper
    elif inputtype == np.ndarray:
        def _wrapper(*args, **kwargs):
            return fn(args[0][:, col], *args[1:], **kwargs)
        return _wrapper
    raise TypeError("invalid input type")
Wrap a function applying to a single column to make a function
    applying to a multi-dimensional dataframe or ndarray

    Parameters
    ----------
    fn : function
        Function that applies to a series or vector.

    col : str or int
        Index of column to which to apply `fn`.

    inputtype : class or type
        Type of input to be expected by the wrapped function.
        Normally pd.DataFrame or np.ndarray. Defaults to pd.DataFrame.

    Returns
    ----------
    wrapped : function
        Function that takes an input of type `inputtype` and applies
        `fn` to the specified `col`.