Python list appending issue

Question

I ran into a rather weird problem with python list appending today. I was trying to create an array whose each element would be like a C struct. One of these elements was a list by itself. This is the problematic code:

class players:
    name='placeholder'
    squad=list()

teams=list()
teams.append(players())
teams.append(players())
teams[0].name="abc"
teams[1].name="xyz"
teams[0].squad.append("Joe")

for w in teams:
    print(w.name)
    print(w.squad)

The output I expected is:

abc
['Joe']
xyz
[]

Since I only added a member to squad for teams[0]. But the output I get is:

abc
['Joe']
xyz
['Joe']

The name is set fine but the .append appended it to both elements of teams!

What causes this and how can I work around this?

chepner · Accepted Answer · 2016-04-21 19:49:10Z

12

The reason is that in your class definition, squad and name are class variables, not instance variables. When a new player object is initialized, it is essentially sharing the same squad variable across all instances of the player. Instead, you want to define an __init__ method for the player class that explicitly separates the instance-specific variables.

class players:
    def __init__(self):
        self.name = 'placeholder'
        self.squad = []

Then, when you initialize your new player object, it has its own squad variable. The rest of your code should work just fine now, with only the correct object's squad being appended to.

edited Apr 21, 2016 at 19:49

chepner

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answered Apr 21, 2016 at 19:45

mprat

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1 Comment

HasanAlyazidi Over a year ago

And some people say JavaScript is weird

Abdulelah Al-Wainany · Accepted Answer · 2024-10-30 09:34:36Z

0

Here's the full code [Correct One]

class players:
def __init__(self):
    self.name = 'placeholder'
    self.squad = []

teams = []
teams.append(players())
teams.append(players())
teams[0].name = "abc"
teams[1].name = "xyz"
teams[0].squad.append("Joe")

for w in teams:
    print(w.name)
    print(w.squad)

answered Oct 30, 2024 at 9:34

Abdulelah Al-Wainany

12 bronze badges

1 Comment

Community Over a year ago

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