Python Lists append mutable variable

Question

Still new to programming/scripting, and this one's been bothering me. I have a function that searches through a list of names, comparing it to a template list of names, and when it finds a match, it places it in my final list in the correct order. For some later functions to work correctly, I need to be able to append some of these names as arrays/lists with. I'm running into the problem that every time I need to add a list to the final list, as soon as I change the variable, the final list updates with it. How do I fix this?

 light = ['template of names in here in correct order']
 listUser = ['names gathered from user input']

 for userChan in listUser: 
      for channelName in light:
         #check if channelName is a list or string

         if isinstance(channelName, basestring):
         #search for matches in userchan
             print channelName, 'is a string'
             if channelName in userChan.lower():
                 matchFound = True
                 listLight.append(userChan)
         else:
             print channelName, 'is a list'
             for piece in channelName:
                 print 'searching %s in %s' %(piece, userChan.lower())

                 if piece in userChan.lower():
                     print "found %s in %s" %(piece, userChan.lower())
                     lightMultList.append(piece)
                     matchFound = True
                     if len(lightMultList) == 2:
                        listLight.append(lightMultList)
                        del lightMultList[:]

So my problem is with the lightMultList. It's always going to be limited to 2 elements, but it changes. Hopefully this wasn't worded too horribly..

I can't quiet tell what your trying to do, but my guess is you where ever you append a list you'll want to make a copy of it instead of appending the actual list. So `listLight.append(lightMultList[:])` — Hoopdady, Oct 22 '13 at 18:10
@CG7Son: There's a better solution in this case. Just stop reusing `lightMultList` by clearing it out (`del lightMultList[:]`) and re-filling it. — abarnert, Oct 22 '13 at 18:16
I agree with @Hoopdady. `del lightMultList[:]` clears the list that you just appended to listLight - if you look at listLight[-1] right after the del, you'll see the problem. An alternate is to just create a new list `lightMultList = []` instead. — tdelaney, Oct 22 '13 at 18:17

score 1 · Answer 1 · answered Oct 22 '13 at 18:18

The problem is that you're only ever creating one lightMultList. You repeatedly clear it out (with del lightMultList[:]) and re-fill it, and append the same thing over and over to lightList.

The simple fix is to just create a new lightMultList each time. Which you can do by changing this line:

del lightMultList[:]

… to:

lightMultList = []

This kind of problem is often a result of trying to directly porting C or C++ code, or just thinking in C++. If you were expecting lightList.append(lightMultList) to call a "copy constructor", that's the root problem: there is no such thing in Python. Assigning a value to a variable, appending it to a list, etc., doesn't copy anything; it just binds another reference to the same value.

Also, a C++ programmer might try to optimize performance by avoiding the wasteful creation of all those temporary objects by trying to reuse the same one, but in Python, the cost of creating a new list is about the same as the cost of iterating one step over listUser in the first place. If it's slow enough to worry about, you're going to have to reorganize your code or move the whole thing to C or Cython anyway; this isn't going to help. (That being said, it's rarely a useful optimization in C++ either; the right thing to do there, on the rare occasions where it matters, is to construct the new vector in-place within the containing vector…)

Python Lists append mutable variable

1 Answers1