Python string formatting returning a Property instead of a string (unicode)?

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 )

Looks like it's saying self.name is something it can't turn into a string. Could you show how self.name is defined? — Mike DeSimone
– Mike DeSimone, Commented Mar 8, 2012 at 22:59
@hobbes3: That is obviously not the full code (most importantly, the module models is missing). — Philipp
– Philipp, Commented Mar 8, 2012 at 23:23

Mike DeSimone · Accepted Answer · 2012-03-09 13:48:09Z

1

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

Item.__unicode__ gets called.
It grabs self.name and self.location.
self.name returns a unicode string from its __unicode__ method.
self.location is a foreign key, so Location.__unicode__ gets called.
That gets self.room, self.floor, and self.building, which all have __unicode__ methods that return unicode strings.
filter sees that these strings are all empty, so locations is set to [].
self.prop, which is a Property, gets appended to locations.
", ".join( locations ) Throws a TypeError because a Property isn't a string.
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.

edited Mar 9, 2012 at 13:48

answered Mar 8, 2012 at 23:24

Mike DeSimone

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1 Comment

hobbes3 Over a year ago

Great explanation! I am a little confused about one part though. If you say that self.prop calls Property.__unicode__, then isn't Property.__unicode__ also a return a unicode string since it just returns self.name? Why do I need to do unicode( self.prop )? I'm assuming I always have to do that with a ForeignKey, but a little clarification would be nice :-).

Gerard · Accepted Answer · 2012-03-08 23:18:41Z

0

Have you tried?:

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

or

def __unicode__( self ):
    return "{0}: {1}".format(self.name, self.location)

or

def __unicode__( self ):
    return "%s: %s" % (self.name, self.location)

Hope it helps :)

answered Mar 8, 2012 at 23:18

Gerard

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1 Comment

Daenyth Over a year ago

None of those have any real semantic difference from his code, so if the behavior changed, it would be due to a bug in python.

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