I have a string representing a date like: `yymmdd` (for example, `170901` for the 1st of September of 2017), obtained from a `datetime` object.

I want to replace the leading zeros of each field by a blank space. `str.replace()` won't do it it because it replaces the trailing zeros as well.

What would be the Pythonic way of substituting a character for another in a string but only in certain positions (in this case in the even indexes of the string)?