Declare a list of custom class as argument of function in Python 3.6

user_10579819 0
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I want to type the inputs of a class method in Python 3. I want the argument to be a list of a custom class I created. The definition of the method is as follows:

def add_report_data(self, report_data: list[ReportData]):
    pass

ReportData is a regular class defined as follows:

class ReportData:
    def __init__(self, system: str, value: int):
        self.__system = system
        self.__value = value

When executing my code I receive the following error:

    def add_report_data(self, report_data: list[ReportData]):
TypeError: 'type' object is not subscriptable

However, changing the type of report_data to simply list is able to execute, but this is not exactly what I want to do. Any idea on what's going on? Thanks.

python-3.x python
3 Answers

You want to import typing.List and do List[ReportData]

List[ReportData] refers to the generic version of list used for typing instead of list[ReportData], which refers to the data type list

#Importing List from typing
from typing import List

class ReportData:
    def __init__(self, system: str, value: int):
        self.__system = system
        self.__value = value

class A:
    #Using List[ReportData]
    def add_report_data(self, report_data: List[ReportData]):
        pass
user_85914310

As Devesh answered, your issue is that in effect you aren't typing the report_data parameter with a list of type ReportData rather you are trying to get the element from list at index ReportData, and that is giving you the error as ReportData is not a valid index.

When trying to type as a list of type MyClass you should use the typing.List and pass that the element types.

user_81032070

On Python 3.9+ and Python 3.7+ if you do this

from __future__ import annotations

you can use list as a type annotation. Your code should work as you've written it.

user_30645380
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