Partition each element in a around sep.
Calls str.partition element-wise.
For each element in a, split the element as the first occurrence of sep, and return 3 strings containing the part before the separator, the separator itself, and the part after the separator. If the separator is not found, return 3 strings containing the string itself, followed by two empty strings.
Separator to split each string element in a.
Output array of str or unicode, depending on input type. The output array will have an extra dimension with 3 elements per input element.