|   | 1 |  initial version  | 
As explained in the doc of the new_variable method, you should use the nonnegative option:
milp.new_variable(real=True, nonnegative=False)
 Copyright Sage, 2010. Some rights reserved under creative commons license. Content on this site is licensed under a Creative Commons Attribution Share Alike 3.0 license.
 
                
                Copyright Sage, 2010. Some rights reserved under creative commons license. Content on this site is licensed under a Creative Commons Attribution Share Alike 3.0 license.