Python 3.10.8 TypeError: TypedDict does not support instance and class checks

2 min read 05-10-2024

Python 3.10.8 TypeError: TypedDict does not support instance and class checks

Understanding Python 3.10.8 TypeError: TypedDict Doesn't Support Instance and Class Checks

The Problem Explained

You're likely encountering the error "TypeError: TypedDict does not support instance and class checks" when working with Python's TypedDict in version 3.10.8 or later. This error occurs because you're trying to use an instance of a TypedDict in a way that expects a regular dictionary, or you're attempting to check its class using typical methods like isinstance or type. This error might seem confusing at first, so let's break it down.

Scenario and Code Example

Imagine you're creating a function to process user data. You've defined a TypedDict to represent the expected structure of the user data:

from typing import TypedDict

class User(TypedDict):
  name: str
  age: int

def process_user(user_data: User):
  print(f"User {user_data['name']} is {user_data['age']} years old.")

user_info = User(name='Alice', age=30)

process_user(user_info)

This code snippet will throw the error mentioned earlier. This happens because the User instance is treated differently than a regular dictionary, even though it appears to be a dictionary.

Understanding the Issue

TypedDict in Python is a special type that provides static type checking for dictionary-like structures. It ensures that the data stored in a TypedDict adheres to a specific structure and type. However, TypedDict is not a regular dictionary in Python's type system.

Here's why the error occurs:

Instance Check: When you use isinstance(user_info, dict), the result is False because TypedDict is its own distinct type.
Class Check: Similarly, type(user_info) == dict will also return False because the type of user_info is __main__.User (or a similar name depending on your code structure), not dict.

Solving the Issue

The solution is to work with the TypedDict as it is intended.

Accessing Values: You can access individual values in the TypedDict instance using key-value pairs, as demonstrated in the original code snippet.
Type Checking: Python's type checker will validate the data against the specified types in your TypedDict definition, ensuring data consistency.

Alternatives for Flexible Usage

If you need to treat your TypedDict instance like a regular dictionary while still preserving type information, consider these approaches:

Casting to dict: You can cast the TypedDict instance to a regular dictionary using dict(user_info) before processing. However, this loses the type information and the type checker will no longer validate your data.
Using dict directly: If you don't require the specific typing behavior of TypedDict, you can use a regular dict for flexibility. You can add type hints to your function parameters using the dict type for type checking.

Conclusion

While the "TypeError: TypedDict does not support instance and class checks" error might seem confusing, it's due to the specific nature of TypedDict as a distinct type in Python. By understanding its limitations and leveraging its strengths, you can effectively utilize TypedDict to achieve type safety and data validation in your Python applications.