Using a variable as part of a variable name

Question

I have object1 which has many sub-objects in it. These sub-objects are accessed in the form object1.subobject. I have a function which returns a list of sub-objects of the original object. All I would like to do is iterate through the list and access each sub-object. Something like this:

temp_list = listSubChildren(object1)  #Get list of sub-objects
for sub_object in temp_list:          #Iterate through list of sub-objects
    blah = object1.sub-object         #This is where I need help 
    #Do something with blah           #So that I can access and use blah

I looked at similar questions where people used dictionaries and getattr but couldn't get either of those methods to work for this.

What is the type of the objects in temp_list? Also, you probably need to change sub-object to sub_object as the former is a SyntaxError I think ... — mgilson
– mgilson, Commented Jan 24, 2013 at 14:04
You need to provide an example, as I would use getattr for this problem. I cannot see why this should not work... — Alex
– Alex, Commented Jan 24, 2013 at 14:06
So you would use blah = object1.getattr(object1,sub_object) ? — BloonsTowerDefence
– BloonsTowerDefence, Commented Jan 24, 2013 at 14:07
@BloonsTowerDefence -- nope. getattr is a builtin function, not an instance method -- That one would be __getattr__, but don't call that directly here ... — mgilson
– mgilson, Commented Jan 24, 2013 at 14:09
What do you mean by 'sub-objects'? Do you mean instances of the object or a sub-class of the class? — dawg
– dawg, Commented Jan 24, 2013 at 14:13

mgilson · Accepted Answer · 2013-01-24 14:37:07Z

It seems to me that if your listSubChildren method is returning strings as you imply, you can use the builtin getattr function.

>>> class foo: pass
... 
>>> a = foo()
>>> a.bar = 1
>>> getattr(a,'bar')
1
>>> getattr(a,'baz',"Oops, foo doesn't have an attrbute baz")
"Oops, foo doesn't have an attrbute baz"

Or for your example:

for name in temp_list:
    blah = getattr(object1,name)

As perhaps a final note, depending on what you're actually doing with blah, you might also want to consider operator.attrgetter. Consider the following script:

import timeit
import operator

class foo(object):
    def __init__(self):
        self.a = 1
        self.b = 2
        self.c = 3

def abc(f):
    return [getattr(f,x) for x in ('a','b','c')]

abc2 = operator.attrgetter('a','b','c')

f = foo()
print abc(f)
print abc2(f)

print timeit.timeit('abc(f)','from __main__ import abc,f')
print timeit.timeit('abc2(f)','from __main__ import abc2,f')

Both functions (abc, abc2) do nearly the same thing. abc returns the list [f.a, f.b, f.c] whereas abc2 returns a tuple much faster, Here are my results -- the first 2 lines show the output of abc and abc2 respectively and the 3rd and 4th lines show how long the operations take:

[1, 2, 3]
(1, 2, 3)
0.781795024872
0.247200965881

Note that in your example, you could use getter = operator.attrgetter(*temp_list)

The thing is that I need blah to equal object1.name ... so I would have object1.first, object1.second, object1.third ...
Which brings us back to my original question. What does listSubChildren actually return? If it returns an iterable like ["first","second","third"] then this will set blah to object1.first on the first pass, then object1.second, then object1.third ...
Yes it returns strings. Sorry for the confusion, this works indeed.
@BloonsTowerDefence -- I don't know what you plan on doing with listSubChildren, but you might want to also consider operator.attrgetter. It's pretty neat.
debates internally ... Should I post about f.__dict__[x] too? It's faster than getattr, but a lot uglier and not all classes have __dict__ ...

cleg · Accepted Answer · 2013-01-24 14:07:54Z

0

It should look something like this:

temp_list = [] 
for property_name in needed_property_names:
    temp_list.append(getattr(object1, property_name))

So, getattr is what you need.

answered Jan 24, 2013 at 14:07

cleg

5,0525 gold badges38 silver badges52 bronze badges

Comments

PaulMcG · Accepted Answer · 2013-01-24 14:19:42Z

0

Add this to the class that object1 is an instance of:

def getSubObjectAttributes(self):
    childAttrNames = "first second third".split()
    return [getattr(self, attrname, None) for attrname in childAttrNames]

answered Jan 24, 2013 at 14:19

PaulMcG

64.1k16 gold badges98 silver badges135 bronze badges

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