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 (
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
@dataclass(eq=True, frozen=True) class HashLiteral(Expression): pairs: Mapping[Expression, Expression] ...
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.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
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
Problem solved! 💥
Remember, in the world of Python, every error is a new learning opportunity! Keep coding, keep learning, and stay Pythonic! 🐍✌️
(Stay tuned for more Python wisdom. Hit the like button, comment, and share if you enjoyed the post! 😃)
Emily Rosemary Collins is a tech enthusiast with a strong background in computer science, always staying up-to-date with the latest trends and innovations. Apart from her love for technology, Emily enjoys exploring the great outdoors, participating in local community events, and dedicating her free time to painting and photography. Her interests and passion for personal growth make her an engaging conversationalist and a reliable source of knowledge in the ever-evolving world of technology.