Python list that should be empty when appending empty lists

Question

I wrote a small Python script below that is behaving in a way I did not anticipate, but I cannot pinpoint the error. Specifically, I have a variable xyz_all that I'd expect to be empty because I'm only ever appending an empty list to it, but in reality it contains the elements of xyz. How is this possible?

with open(filename,'r') as rf:
    xyz = []
    xyz_all = []
    for line in rf:
        if 'A' in line:
            line = line.split()
            xyz.append([float(j) for j in line[4:7]])
        elif 'B' in line:
            xyz = []
            xyz_all.append(xyz)
print(xyz_all)

user2357112 · Accepted Answer · 2018-02-16 05:22:17Z

3

You only ever append empty lists to xyz_all, but those lists don't stay empty. When you do

xyz.append([float(j) for j in line[4:7]])

you're appending to a list that may already be an element of xyz_all.

answered Feb 16, 2018 at 5:22

user2357112

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1 Comment

Argon Over a year ago

Oh, that makes a lot of sense. I totally missed that. Thank you.

My3 · Accepted Answer · 2018-02-16 05:26:02Z

-1

From your code, xyz will be an empty list of length zero but when you do xyz = [] xyz_all.append(xyz) you are appending an empty list (xyz) to list (xyz_all) which means now xyz_all has one element in it which is an empty list and its length is not zero but 1. xyz_all=[[]] is a list containing an empty list in it.

answered Feb 16, 2018 at 5:26

My3

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2 Comments

My3 Over a year ago

In the elif loop you reinitialized xyz to an empty list then how can xyz not be empty?

Argon Over a year ago

This is not correct. xyz_all contains numerical values at the end of the script.

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