Editing symbology according to attribute with python?(ArcGIS10.0)

Hello, everyone.

I am wondering if I can create a code to change how existing line features are displayed according to their attributes.

I have line features for rivers, and the table has an attribute called [type] and each river has an attribute like dry, perennial, intermittent, but all are displayed with the same line symbol.

Of course I can set symbology in layer properties,
but I am hoping to create a code that can do this to keep all the maps consistent.

I did some research for sample codes, but could not find one to change symbology..
I do not know if it is even possible.

If someone could give me any idea or just even a clue, I would really appreciate it.

Thanks for reading my post:)