astropy/astropy — pull request #2879 by embray (2014-08-20 22:00:19+00:00)
Title: Fix reference cycle between Table and Column

As discovered while looking into #2877 there is a reference cycle that goes:

`Table` -> `TableColumns` -> `Column` -> `Table`

by way of the `parent_table` attribute on columns. This fixes it by making `parent_table` a weakref (hidden away via a `property` allowing minimal code changes elsewhere). A test demonstrating the fix is included.