from IPython.core.magic import magics_class, Magics, line_magic, cell_magic

@magics_class
class MyMagics(Magics):
    foo = 0
    
    @line_magic
    def setfoo(self, line):
        self.foo = int(line)
    
    @cell_magic
    def foocell(self, line, cell):
        print cell.format(foo=self.foo)
    
ip = get_ipython()
ip.magics_manager.register(MyMagics)

%%foocell

foo = {foo}

%setfoo 5

%%foocell
foo was 0, but now foo is {foo}