strange behaviour of python format unicode/string method

Question

I'm having a problem displaying a variable in a template. I have a model with this __unicode__ method:

class MyClass(models.Model):

...

def __unicode__(self):
    return 'Admisión para {student}'.format(student=self.student.__unicode__())  # Look at that accented char (ó)

In the top of my models.py file, i have these two lines:

# coding=utf-8

from __future__ import unicode_literals

I'm using python 2.7.3

The template that should show what i want is like this: (just the three interesting lines)

<td>{{obj}}</td>
<td>{{obj.status}}</td>
<td>{{obj.stage.datetime|date:"SHORT_DATE_FORMAT"}}</td>

obj is an object of type "MyClass". The second and third line show what they were expected to, but the first line doesn't show anything. I know that when one do that ({{obj}}) the method called is "unicode", so i tried many things:

I try something similar but in the console: print obj. I didn't have any problem. It displayed as i expected.

I suspected that unicode method was raising an exception. So i change the method to be like this:

def __unicode__(self):
    try:
        txt = 'Admisión para {student}'.format(student=self.student.__unicode__())
    except Exception, e:
        txt = unicode(e)

    return txt

After that change, when the template is rendered, {{obj}} was shown as "'ascii' codec can't decode byte 0xc3 in position 25: ordinal not in range(128)", so as i supposed, it was raising an exception.

But the most strange behaviour of all was that, when i change unicode method to look like this:

def __unicode__(self):
    txt = self.student.__unicode__())
    return 'Admisión para {student}'.format(student=txt)

{{obj}} was shown as i expected to...

Can somebody tell me where is my error? is it a python/django template bug? or what?

Thank you in advance!!!