Is there a way I can make a list of class instances?

August 11, 2024

I wanted to make a list of class instances. I’m using Python 3.12.4.

I tried:

class MyClass:
    def __init__ (self,MyClassData):
        self.MyClassData = MyClassData
MyClassList = [""]
MyClassList.append(MyClass("hello"))
def ReadClassData () :
    for i in MyClassList:
        return MyClassList[i].MyClassData
print(ReadClassData())

and it returns:

Traceback (most recent call last):
  File "C:/Users/malac/OneDrive/Documents/Python/Tests/Instance In List.py", line 9, in <module>
    print(ReadClassData())
  File "C:/Users/malac/OneDrive/Documents/Python/Tests/Instance In List.py", line 8, in ReadClassData
    return MyClassList[i].MyClassData
TypeError: list indices must be integers or slices, not str

I’m confused on that last part. I thought [i] was an integer?

>Solution :

Here’s a walkthrough of what happens:

class MyClass:
    def __init__ (self,MyClassData):
        self.MyClassData = MyClassData

You defined the class MyClass. So far, so good

MyClassList = [""]
MyClassList.append(MyClass("hello"))

MyClassList is a list initiated with a single element (empty string ""), and is immediately appended with the object MyClass("hello"). MyClassList is now ["", MyClass("hello")]

def ReadClassData () :
    for i in MyClassList:
        return MyClassList[i].MyClassData
print(ReadClassData())

The for loop iterates through MyClassList, got its first element, i.e. "", and assign it to i, so i is now "". That’s why it shows an error because you’re trying to access an element of the list MyClassList with a str index