Accessing list items with getattr/setattr in Python

Question

Trying to access/assign items in a list with getattr and setattr funcions in Python. Unfortunately there seems to be no way of passing the place in the list index along with the list name.
Here's some of my tries with some example code:

class Lists (object):
  def __init__(self):
    self.thelist = [0,0,0]

Ls = Lists()

# trying this only gives 't' as the second argument.  Python error results.
# Interesting that you can slice a string to in the getattr/setattr functions
# Here one could access 'thelist' with with [0:7]
print getattr(Ls, 'thelist'[0])


# tried these two as well to no avail.  
# No error message ensues but the list isn't altered. 
# Instead a new variable is created Ls.'' - printed them out to show they now exist.
setattr(Lists, 'thelist[0]', 3)
setattr(Lists, 'thelist\[0\]', 3)
print Ls.thelist
print getattr(Ls, 'thelist[0]')
print getattr(Ls, 'thelist\[0\]')

Also note in the second argument of the attr functions you can't concatenate a string and an integer in this function.

Cheers

Answer 1

getattr(Ls, 'thelist')[0] = 2
getattr(Ls, 'thelist').append(3)
print getattr(Ls, 'thelist')[0]

If you want to be able to do something like getattr(Ls, 'thelist[0]'), you have to override __getattr__ or use built-in eval function.

Answer 2

You could do:

l = getattr(Ls, 'thelist')
l[0] = 2  # for example
l.append("bar")
l is getattr(Ls, 'thelist')  # True
# so, no need to setattr, Ls.thelist is l and will thus be changed by ops on l

getattr(Ls, 'thelist') gives you a reference to the same list that can be accessed with Ls.thelist.

Answer 3

As you discovered, __getattr__ doesn't work this way. If you really want to use list indexing, use __getitem__ and __setitem__, and forget about getattr() and setattr(). Something like this:

class Lists (object):

    def __init__(self):
        self.thelist = [0,0,0]

    def __getitem__(self, index):
        return self.thelist[index]

    def __setitem__(self, index, value):
        self.thelist[index] = value

    def __repr__(self):
        return repr(self.thelist)

Ls = Lists()
print Ls
print Ls[1]
Ls[2] = 9
print Ls
print Ls[2]