Generate executable python source code from an ast node.
      
    :param ast: ast node
    :param file: file to write output to.

def python_source(ast, file=sys.stdout):
    '''
    Generate executable python source code from an ast node.
      
    :param ast: ast node
    :param file: file to write output to.
    '''
    gen = SourceGen()
    gen.visit(ast)
    gen.dump(file)