CODEWITHSUNDEEP
pythonlist
Argon
Argon

Reputation: 435

Python list that should be empty when appending empty lists

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)

Upvotes: 1

Views: 70

Answers (2)

My3
My3

Reputation: 140

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.

Upvotes: -1

user2357112
user2357112

Reputation: 280564

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.

Upvotes: 3

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