Should setting A to B then changing A affect B in python? - Stack Overflow

a = [0,0]
b = a
a[0] = 1
print(b) 

This code returns [1,0]

Shouldn't it return [0,0]?

Is there any way to bypass this bug/feature? (to have it return [0,0])

I probably have used code very similar to this in the past, there weren't any problems before...

a = [0,0]
b = a
a[0] = 1
print(b) 

This code returns [1,0]

Shouldn't it return [0,0]?

Is there any way to bypass this bug/feature? (to have it return [0,0])

I probably have used code very similar to this in the past, there weren't any problems before...

• python

Share Improve this question Follow edited Feb 21 at 2:18 wjandrea 33.2k1010 gold badges6969 silver badges9898 bronze badges asked Jan 30 at 19:37 Allen LanAllen Lan 1333 bronze badges 2

• 4 Not a bug. See How do I clone a list so that it doesn't change unexpectedly after assignment? – Guy Incognito Commented Jan 30 at 19:41
• 1 See Facts and muths about python names and values – Ahmed AEK Commented Jan 30 at 19:48

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1 Answer 1

Sorted by: Reset to default 1

this is not a bug, it's how Python handles mutable objects like lists.

since b is assigned to a, both variables point to the same list in memory. Changing a affects b because there is only one list.

you can use a.copy(),this creates a new list with the same values

a = [0,0]
b = a.copy()
a[0] = 1
print(b) 

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