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__unicode__() works, unicode() blows up.

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  __unicode__() works, unicode() blows up. Roy Smith <roy@panix.com> - 2012-11-04 08:32 -0500
    Re: __unicode__() works, unicode() blows up. (Never mind!) Roy Smith <roy@panix.com> - 2012-11-04 08:41 -0500
      Re: __unicode__() works, unicode() blows up. (Never mind!) aahz@pythoncraft.com (Aahz) - 2012-11-04 09:13 -0800
      Re: __unicode__() works, unicode() blows up. (Never mind!) Terry Reedy <tjreedy@udel.edu> - 2012-11-04 13:10 -0500

#32731 — __unicode__() works, unicode() blows up.

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:

>>> u = User()
>>> print u.username
None

>>> print u.__unicode__()
None

>>> print unicode(u)
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.

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#32732 — Re: __unicode__() works, unicode() blows up. (Never mind!)

In article <roy-90D9A2.08321804112012@news.panix.com>,
 Roy Smith <roy@panix.com> wrote:

> >>> print u.__unicode__()
> None
> 
> >>> print unicode(u)
> 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.

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#32736 — Re: __unicode__() works, unicode() blows up. (Never mind!)

In article <roy-30BA92.08410804112012@news.panix.com>,
Roy Smith  <roy@panix.com> wrote:
>In article <roy-90D9A2.08321804112012@news.panix.com>,
> Roy Smith <roy@panix.com> wrote:
>>
>> >>> print u.__unicode__()
>> None
>> 
>> >>> print unicode(u)
>> 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.

You apparently need more coffee when programming after waking up!  (Or
even worse, staying up all night.)
-- 
Aahz (aahz@pythoncraft.com)           <*>         http://www.pythoncraft.com/

"....Normal is what cuts off your sixth finger and your tail..."  --Siobhan

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#32739 — Re: __unicode__() works, unicode() blows up. (Never mind!)

On 11/4/2012 8:41 AM, Roy Smith wrote:
> In article <roy-90D9A2.08321804112012@news.panix.com>,
>   Roy Smith <roy@panix.com> wrote:
>
>>>>> print u.__unicode__()
>> None
>>
>>>>> print unicode(u)
>> 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.

The same principle applies to some of the other special methods that sit 
behind builtin functions.

 >>> class C:
	def __len__(self): return '42'  # whoops
	
 >>> len(C())
Traceback (most recent call last):
   File "<pyshell#9>", line 1, in <module>
     len(C())
TypeError: 'str' object cannot be interpreted as an integer

 >>> class C:
	def __len__(self): return -42  # whoops again
	
 >>> len(C())
Traceback (most recent call last):
   File "<pyshell#12>", line 1, in <module>
     len(C())
ValueError: __len__() should return >= 0


-- 
Terry Jan Reedy

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