setattr in class

[Bojan Mihelac]

Hi all - when trying to set some dynamic attributes in class, for
example:

class A:
for lang in ['1', '2']:
exec('title_%s = lang' % lang) #this work but is ugly
# setattr(A, "title_%s" % lang, lang) # this wont work

setattr(A, "title_1", "x") # this work when outside class

print A.title_1
print A.title_2

I guess A class not yet exists in line 4. Is it possible to achive
adding dynamic attributes without using exec?

thanks,
Bojan

 

[Fredrik Lundh]

Hi all - when trying to set some dynamic attributes in class, for
example:

class A:
for lang in ['1', '2']:
exec('title_%s = lang' % lang) #this work but is ugly
# setattr(A, "title_%s" % lang, lang) # this wont work

setattr(A, "title_1", "x") # this work when outside class

print A.title_1
print A.title_2

I guess A class not yet exists in line 4. Is it possible to achive
adding dynamic attributes without using exec?

Move the for-in loop out of the class definition:
.... pass
....

>>> for lang in ['1', '2']: .... setattr(A, "title_%s" % lang, lang)
>>> a = A()
>>> a.title_1

'1'

A truly dynamic solution (using __getattr__ and modification on access)
would probably give you a more "pythonic" solution.

</F>

 

[Bojan Mihelac]

Correct, the class doesn't exist until the end of the class body. You
can either do it outside the class definition or you can use a metaclass.

Christian

thanks, can you give example on using a metaclass?

 

[Bojan Mihelac]

Hi all - when trying to set some dynamic attributes in class, for
example:

class A:
    for lang in ['1', '2']:
        exec('title_%s = lang' % lang) #this work but is ugly
        # setattr(A, "title_%s" % lang, lang) # this wont work
I guess A class not yet exists in line 4. Is it possible to achive
adding dynamic attributes without using exec?

Yes, it is:

-------------
class A:
   for i in (1, 2):
      locals()['xxx%d' % (i)] = i

print A.xxx1
print A.xxx2
a = A()
print a.xxx1
print a.xxx2
-------------

And the output is:

-----
1
2
1
2

thnx Wojtek, this works!

 

[Arnaud Delobelle]

thanks, can you give example on using a metaclass?

class MoreMeta(type):
def __init__(self, name, bases, attrs):
more = attrs.get('moreattrs')
if more:
for attr, val in more.iteritems():
setattr(self, attr, val)

class MoreObject(object):
__metaclass__ = MoreMeta

class A(MoreObject):
moreattrs = {}
for i in '12':
moreattrs['title_' + i] = int(i)

 

[Michael Palmer]

Hi all - when trying to set some dynamic attributes in class, for
example:

class A:
for lang in ['1', '2']:
exec('title_%s = lang' % lang) #this work but is ugly
# setattr(A, "title_%s" % lang, lang) # this wont work

setattr(A, "title_1", "x") # this work when outside class

print A.title_1
print A.title_2

I guess A class not yet exists in line 4. Is it possible to achive
adding dynamic attributes without using exec?

thanks,
Bojan

Is it really worth it? If the names of the attributes are only known
at runtime, why not just use a dict - that's what they are for. If you
want a dict plus some special behaviour, just write a class that
inherits from dict, or use UserDict.DictMixin.