Why does changing a Python list created by assigning another modify both?

Asked Aug 25 '16 at 20:17

Active Mar 17 '18 at 15:57

Viewed 67 times

Can someone explain to me what the hell is going on here:

>>> listOne=[1,2,3,4]
>>> listOne
[1, 2, 3, 4]
>>> listTwo=listOne
>>> listTwo
[1, 2, 3, 4]
>>> listTwo[0]=0
>>> listTwo
[0, 2, 3, 4]
>>> listOne
[0, 2, 3, 4]

I've been quite clear I only want to change the first element of the list listTwo, but clearly listOne has remained linked to it and is edited also. There is nothing in the documentation about the equals operator that I can find that explains this...

I can of course fix it by turning listOne into a tuple after the assignment to listTwo.

edited Aug 25 '16 at 21:52

Charles Duffy

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asked Aug 25 '16 at 20:17

eatYourGreens

7

May I recommend [The Python Tutorial](https://docs.python.org/3/tutorial/)? As to your specific question, see https://docs.python.org/3/reference/datamodel.html – Robᵩ Aug 25 '16 at 20:19
2

`listTwo=listOne`: you have copied the reference: they both point to the same object. – Jean-François Fabre Aug 25 '16 at 20:22
2

The answer to this question explains it nicely: http://stackoverflow.com/questions/6793872/variable-assignment-and-modification-in-python – Edward Carney Aug 25 '16 at 20:23
Very good - thanks all, that is clear – eatYourGreens Aug 25 '16 at 20:57
BTW -- in the future, try to choose titles that make it clear what your question is, not just what data structure or operator that question revolves around. – Charles Duffy Aug 25 '16 at 21:52

Why does changing a Python list created by assigning another modify both?

0 Answers0