Add an object to a python list

Question

I am trying to add an object to a list but since I'm adding the actual object when I try to reset the list thereafter, all the values in the list are reset. Is there an actual way how I can add a monitor object to the list and change the values and not affect the ones I've already saved in the list?

Thanks

Code:

arrayList = []

for x in allValues:


        result = model(x)

        arrayList.append(wM)

        wM.reset()

where wM is a monitor class - which is being calculated / worked out in the model method

I don't understand your problem. Some simple code showing what's going wrong could help. — Chris Lutz
– Chris Lutz, Commented Feb 4, 2010 at 2:23

Chris Lutz · Accepted Answer · 2010-02-04 03:00:26Z

26

Is your problem similar to this:

l = [[0]] * 4
l[0][0] += 1
print l # prints "[[1], [1], [1], [1]]"

If so, you simply need to copy the objects when you store them:

import copy
l = [copy.copy(x) for x in [[0]] * 4]
l[0][0] += 1
print l # prints "[[1], [0], [0], [0]]"

The objects in question should implement a __copy__ method to copy objects. See the documentation for copy. You may also be interested in copy.deepcopy, which is there as well.

EDIT: Here's the problem:

arrayList = []
for x in allValues:
    result = model(x)
    arrayList.append(wM) # appends the wM object to the list
    wM.reset()           # clears  the wM object

You need to append a copy:

import copy
arrayList = []
for x in allValues:
    result = model(x)
    arrayList.append(copy.copy(wM)) # appends a copy to the list
    wM.reset()                      # clears the wM object

But I'm still confused as to where wM is coming from. Won't you just be copying the same wM object over and over, except clearing it after the first time so all the rest will be empty? Or does model() modify the wM (which sounds like a terrible design flaw to me)? And why are you throwing away result?

edited Feb 4, 2010 at 3:00

answered Feb 4, 2010 at 2:29

Chris Lutz

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2 Comments

Chris Lutz Over a year ago

@Lily - If you had read and thought about either my original answer or ghostdog74's answer, you could have done this on your own. You should consider reading the documentation, particularly about immutable vs. mutable objects and the copy module.

oneday Over a year ago

@ChrisLutz - Thank you for such a lucid answer - 6 years on ur answer is still helping people like me :)

jathanism · Accepted Answer · 2010-02-04 02:46:06Z

3

You need to create a copy of the list before you modify its contents. A quick shortcut to duplicate a list is this:

mylist[:]

Example:

>>> first = [1,2,3]
>>> second = first[:]
>>> second.append(4)
>>> first
[1, 2, 3]
>>> second
[1, 2, 3, 4]

And to show the default behavior that would modify the orignal list (since a name in Python is just a reference to the underlying object):

>>> first = [1,2,3]
>>> second = first
>>> second.append(4)
>>> first
[1, 2, 3, 4]
>>> second
[1, 2, 3, 4]

Note that this only works for lists. If you need to duplicate the contents of a dictionary, you must use copy.deepcopy() as suggested by others.

answered Feb 4, 2010 at 2:46

jathanism

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Comments

ghostdog74 · Accepted Answer · 2010-02-04 02:28:00Z

2

while you should show how your code looks like that gives the problem, i think this scenario is very common. See copy/deepcopy

answered Feb 4, 2010 at 2:28

ghostdog74

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Comments

KHowlett · Accepted Answer · 2013-06-14 19:16:08Z

0

If i am correct in believing that you are adding a variable to the array but when you change that variable outside of the array, it also changes inside the array but you don't want it to then it is a really simple solution.

When you are saving the variable to the array you should turn it into a string by simply putting str(variablename). For example:

array.append(str(variablename))

Using this method your code should look like this:

arrayList = []
for x in allValues:
    result = model(x)
    arrayList.append(str(wM))        #this is the only line that is changed.
    wM.reset()

answered Jun 14, 2013 at 19:16

KHowlett

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