Question

I am getting an error in Django saying Caught TypeError while rendering: sequence item 1: expected string or Unicode, Property found. Here is my code:

def __unicode__( self ) :
    return "{} : {}".format( self.name, self.location )

I even tried

def __unicode__( self ) :
    return unicode( "{} : {}".format( self.name, self.location ) )

but the same error.

From what I know "this is x = {}".format( x ) returns a string right? Why is Python saying it’s a Property?

Full code:

class Item( models.Model ) :
    def __unicode__( self ) :
        return "{} : {}".format( self.name, self.location )

    name       = models.CharField( max_length = 135 )
    comment    = models.TextField( blank = True )
    item_type  = models.ForeignKey( ItemType )
    location   = models.ForeignKey( Location )
    t_created  = models.DateTimeField( auto_now_add = True, verbose_name = 'created' )
    t_modified = models.DateTimeField( auto_now = True, verbose_name = 'modified' )

class Location( models.Model ) :
    def __unicode__( self ) :
        locations = filter( None, [ self.room, self.floor, self.building ] )
        locations.append( self.prop )

        return ", ".join( locations ) # This will look in the form of like "room, floor, building, property"

    comment    = models.TextField( blank = True )
    room       = models.CharField( max_length = 135, blank = True )
    floor      = models.CharField( max_length = 135, blank = True )
    building   = models.CharField( max_length = 135, blank = True )
    prop       = models.ForeignKey( Property )
    t_created  = models.DateTimeField( auto_now_add = True, verbose_name = 'created' )
    t_modified = models.DateTimeField( auto_now = True, verbose_name = 'modified' )

class Property( models.Model ) :
    def __unicode__( self ) :
        return self.name

    name = models.CharField( max_length = 135 )

Answer 1

Property isn’t referring to a Python property, but to your Property class. What’s probably happening is this:

1. Item.__unicode__ gets called.
2. It grabs self.name and self.location.
3. self.name returns a unicode string from its __unicode__ method.
4. self.location is a foreign key, so Location.__unicode__ gets called.
5. That gets self.room, self.floor, and self.building, which all have __unicode__ methods that return unicode strings.
6. filter sees that these strings are all empty, so locations is set to [].
7. self.prop, which is a Property, gets appended to locations.
8. ", ".join( locations ) Throws a TypeError because a Property isn’t a string.
9. the str.format call in Item.__unicode__ catches that exception and throws its own, which is what you see.

Solution: change

locations.append( self.prop )

to

locations.append( unicode(self.prop) )

Moral: str.format calls str() on its arguments, but str.join does not.