(Fix) TypeError: ‘ABCMeta’ object is not subscriptable

Problem Formulation 🧩

Picture this: you’re chugging along, writing some great Python code, leveraging the sheer power of dataclasses, and BAM! You hit an error you’ve never seen before: “TypeError: 'ABCMeta' object is not subscriptable“. 😵‍💫

Here’s the culprit, a scenario where you have a base dataclass Expression, inherited from the Node class and Python’s Abstract Base Class (ABC).

from abc import ABC
from collections.abc import Mapping
from dataclasses import dataclass, field

@dataclass(eq=True, order=True, frozen=True)
class Expression(Node, ABC):
    def node(self):
        raise NotImplementedError

You then derive a new dataclass, HashLiteral, from this base Expression class.

@dataclass(eq=True, frozen=True)
class HashLiteral(Expression):
    pairs: Mapping[Expression, Expression]
    ...

Here, the Node class is defined as:

@dataclass(eq=True, frozen=True)
class Node:
    def __str__(self) -> str:
        raise NotImplementedError

Sounds pretty standard, right? So, why the error when trying to use the HashLiteral class? 🤔

The Solution 💡

The core issue lies in the use of collections.abc.Mapping in the HashLiteral class. Python isn’t happy when you try to mix this with other classes, especially within the same class that hasn’t fully materialized in the interpreter’s understanding.

But fear not, my fellow Pythonistas! The typing module is here to save the day. 🦸‍♀️🦸‍♂️ You can use the typing.Dict or typing.Mapping class which are designed specifically for these cases. If any kind of mapping is okay, go with typing.Mapping; if you want a dictionary specifically, opt for typing.Dict.

from typing import Dict

@dataclass(eq=True, frozen=True)
class HashLiteral(Expression):
    pairs: Dict['Expression', 'Expression']
    ...

Voila! Notice the use of forward references (i.e., we put ‘Expression’ in quotes) to handle the not-yet-fully-defined Expression class.

Problem solved! 💥

Remember, in the world of Python, every error is a new learning opportunity! Keep coding, keep learning, and stay Pythonic! 🐍✌️

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