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properties and formatting with self.__dict__

Michele Simionato mis6 at pitt.edu
Wed Feb 19 15:05:41 EST 2003 
"Mark McEahern" <mark at mceahern.com> wrote in message news:<mailman.1045664347.7149.python-list at python.org>...
> I hope the following example is not too obtuse, but it shows something that
> I thought was surprising about properties.  Attempting to format a string
> (traceback follows, with code at the end of the message) with named
> parameters and expecting the property to show up naively via self.__dict__
> generates this error:
> 
>   Traceback (most recent call last):
>     File "./junk.py", line 23, in ?
>       print p
>     File "./junk.py", line 19, in __repr__
>       return template % self.__dict__
>   KeyError: full_name
> 
> I'm not asking for workarounds because there are many and they are obvious.
> I guess I'm wondering whether this is a likely stumbling block in the
> intersection of string formatting and the use of properties?
> 
> Thanks,
> 
> // mark
> 
> #!/usr/bin/env python
> 
> class Person(object):
> 
>     def __init__(self, first_name, last_name):
>         self.first_name = first_name
>         self.last_name = last_name
> 
>     def get_full_name(self):
>         return '%s %s' % (self.first_name, self.last_name)
> 
>     full_name = property(get_full_name)
> 
>     def __repr__(self):
>         template = '<Person ' \
>                    'first=%(first_name)s ' \
>                    'last=%(last_name)s ' \
>                    'full=%(full_name)s'
>         return template % self.__dict__
> 
> p = Person('mark', 'mceahern')
> 
> print p
> 
> -

The problem is that writing "full_name = property(get_full_name)" in
the class scope, as you do, put "full_name" in the class dictionary, 
not in the instance dictionary (this is the standard behaviour).
You should write in the __init__ method something like

"self.full_name=property(self.get_full_name)"

This will give you as output

<Person first=mark last=mceahern full=<property object at 0x8108a7c>

and from there you should be able yourself to figure out where to go ;)


                                Michele
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