Stop at the line where a variable gets changed

Currently pycharm does not have a default built-in feature that tracks variable modification in real time. Alternatively you can do this:

1. run debug
2. From debugger pane -> Variables, right click the variable you would like to track and and add it to Watches.
3. In Watches pane, right click the variable and select referring objects.

The feature you are talking about is, I believe, called watchpoint support and according to this article:
http://sourceforge.net/blog/watchpoints-in-python/
Eric and PyScriptor has the feature but not pycharm.