Why does list.append(obj) modify obj in Python?

Question

I have a strange problem with list.append(). I build a list joining some values from a dict, as follows:

In [3]: myDict = {'k1': u'value1', 'k2': [u'value2']}

In [4]: myList = myDict['k2']

In [5]: myList
Out[5]: [u'value2']

In [6]: myList.append(myDict['k1'])

In [7]: myList
Out[7]: [u'value2', u'value1']

In [8]: myDict 
Out[8]: {'k1': u'value1', 'k2': [u'value2', u'value1']}

I don't understand the reason why myDict is modified after append in In [6] (see the difference between In [2] and Out [8]).

`myList is myDict['k2']` - a better question might be *why did you expect something else to happen?* — jonrsharpe, Oct 23 '15 at 11:26
Assignment statements in Python do not copy objects, they create bindings between a target and an object. — Doon, Oct 23 '15 at 11:26
@Doon beautiful explanation should copy this for future use :) — The6thSense, Oct 23 '15 at 11:28
You may find this article helpful: [Facts and myths about Python names and values](http://nedbatchelder.com/text/names.html), which was written by SO veteran Ned Batchelder. — PM 2Ring, Oct 23 '15 at 11:36
this really boils down to "pass by reference" concept (not just in python but in many other languages)...see my answer in this post http://stackoverflow.com/questions/33222944/append-vs-redefining-variable-in-python/33223855#33223855 — labheshr, Oct 23 '15 at 11:40

score 3 · Accepted Answer · answered Oct 23 '15 at 11:25

Because when you assign myDict['k2'] to myList you are actually just creating a pointer, myList, to the value with key k2 in myDict. You need to create a copy of myDict['k2'] otherwise you are just modifying the original.

Try myList = list(myDict['k2']).

score 2 · Answer 2 · edited Oct 23 '15 at 11:26

2

myDict[k2] is a pointer to a list object. You then set myList to point to the same list object. when you modify the list, all of the pointers that point to it see the modified one.

edited Oct 23 '15 at 11:26

The6thSense

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answered Oct 23 '15 at 11:25

Paul Becotte

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Why does list.append(obj) modify obj in Python?

2 Answers2