How to increment numbers when value changes in another column?

How to add a new column before the PostDate column and enter a serial number where the PostDate column contains a value, and keep the space empty where the PostColumn contains nothing.

NewColumn                PostDate
      1                          01-04-2012
      2                          03-04-2012
      3                          05-04-2012
      4                          10-04-2012

Thanks in advance

More context is needed. Column of what? cvs table? html table? database table? pandas table?

Pandas table