Python unexpected interpretation of lines

Asked May 12 '18 at 20:55

Active May 12 '18 at 20:58

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I expect the code below to create x, then make a copy of x which i called y, change x and then output y. That is, I expect the lines of code to be run sequentially.

x = [10,20,30]
y = x
x[1] = 42
print(y)

Instead, y is changed when x is. Why is that, and how should I read the code? the output is:

[10, 42, 30]

edited May 12 '18 at 20:57

miradulo

28,857
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asked May 12 '18 at 20:55

Student tea

2

`x = y` is simply naming the **same** list `x`and `y`. To name a (shallow) **copy** of the list `y` use: `y = list(x)` or `y = x[:]` – Reut Sharabani May 12 '18 at 20:56
1

`y = x.copy()` works too... – Amaro Vita May 12 '18 at 21:05
`list.copy` is python3 specific – Reut Sharabani May 12 '18 at 21:21

Python unexpected interpretation of lines

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