How to save a "self" variable in python?

Question

In python, inside a class, it appears that when I save a "self" variable into another one, if I edit the new variable, the "self" is also edited:

    undropped_lines = self.flagged_lines
    print self.flagged_lines
    del undropped_lines[0]
    print self.flagged_lines

How should one avoid this trait in the code?

score 4 · Accepted Answer · answered Oct 15 '15 at 13:55

4

This is because lists are mutable and when you say undropped_lines = self.flagged_lines you are just pointing a new name at the same instance.

If you want a copy use undropped_lines = self.flagged_lines[:]

answered Oct 15 '15 at 13:55

Holloway

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score 3 · Answer 2 · answered Oct 15 '15 at 13:55

3

This is because undropped_lines and self.flagged_lines are pointing to the same data. Think of it as two different "names" pointing to the same entity.

You can get around this by creating a shallow copy of the list when assigning to undropped_lines. Something like:

undropped_lines = list( self.flagged_lines )

answered Oct 15 '15 at 13:55

Sanjay T. Sharma

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Almost word for word 3 seconds apart +1 – Holloway Oct 15 '15 at 13:55
@Trengot: Indeed mate, +1 to you too :) – Sanjay T. Sharma Oct 15 '15 at 13:57

How to save a "self" variable in python?

2 Answers2