using % operator to print possibly unitialized data attributes

Leif K-Brooks eurleif at ecritters.biz
Fri Sep 9 22:08:36 CEST 2005


Adam Monsen wrote:
> class J:
>     name = ''
>     value = ''
>     def __str__(self):
>         vals = self.__class__.__dict__
>         vals.update(self.__dict__)
>         return 'name="%(name)s" value="%(value)s' % vals

This will update the class's attributes with instance attributes when
str() is called, which probably isn't what you want. For instance:

 >>> foo = J()
 >>> foo.name = "Joe Bloggs"
 >>> print foo
 name="Joe Bloggs" value="
 >>> bar = J()
 >>> print bar
 name="Joe Bloggs" value="

What's wrong with the obvious version:

class J(object):
    name = ''
    value = ''
    def __str__(self):
        return 'name=%r value=%r' % (self.name, self.value)



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