__unicode__() works, unicode() blows up.

[Roy Smith]

Environment:
Python-2.7.3
Ubuntu Precise
mongoengine 0.6.20

I have a class which includes a __unicode__() method:

class User(mongoengine.Document):
def __unicode__(self):
return self.username

If I create an instance of this class by calling the constructor
directly, self.username is None. When I pass that to unicode(), it
blows up. However, calling __unicode__() directly, works as expected:

None
None

Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: coercing to Unicode: need string or buffer, NoneType found

What's going on here? I thought
(http://docs.python.org/2/library/functions.html#unicode) the latter two
calls should be identical, but obviously they're not.

 

[Roy Smith]

None

Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: coercing to Unicode: need string or buffer, NoneType found

What's going on here? I thought
(http://docs.python.org/2/library/functions.html#unicode) the latter two
calls should be identical, but obviously they're not.[/QUOTE]

Why is it, that no matter how long you stare at a problem, the answer
comes to you moments after you hit the Post button?

The problem is that __unicode__() is supposed to return a Unicode
object, and unicode() enforces that. The fix is to change:

def __unicode__(self):
return self.username

to be:

def __unicode__(self):
return unicode(self.username)

This never got noticed before because normally, self.username already is
a unicode string, so it just works.

 

[Aahz]

Why is it, that no matter how long you stare at a problem, the answer
comes to you moments after you hit the Post button?

The problem is that __unicode__() is supposed to return a Unicode
object, and unicode() enforces that. The fix is to change:

def __unicode__(self):
return self.username

to be:

def __unicode__(self):
return unicode(self.username)

This never got noticed before because normally, self.username already is
a unicode string, so it just works.

You apparently need more coffee when programming after waking up! (Or
even worse, staying up all night.)

 

[Terry Reedy]

Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: coercing to Unicode: need string or buffer, NoneType found

What's going on here? I thought
(http://docs.python.org/2/library/functions.html#unicode) the latter two
calls should be identical, but obviously they're not.

Why is it, that no matter how long you stare at a problem, the answer
comes to you moments after you hit the Post button?

The problem is that __unicode__() is supposed to return a Unicode
object, and unicode() enforces that. The fix is to change:

def __unicode__(self):
return self.username

to be:

def __unicode__(self):
return unicode(self.username)

This never got noticed before because normally, self.username already is
a unicode string, so it just works.[/QUOTE]

The same principle applies to some of the other special methods that sit
behind builtin functions.
def __len__(self): return '42' # whoops
Traceback (most recent call last):
File "<pyshell#9>", line 1, in <module>
len(C())
TypeError: 'str' object cannot be interpreted as an integer
def __len__(self): return -42 # whoops again
Traceback (most recent call last):
File "<pyshell#12>", line 1, in <module>
len(C())
ValueError: __len__() should return >= 0