What does it mean when I get a TypeError ‘type’ object is not subscriptable while using typing?

While working on a Python project, I hit a puzzling error. It said: "TypeError: 'type' object is not subscriptable." This problem can be tricky, especially for those using Python's typing module. The typing module, which gives type hints, helps make your code clearer. At first, I had no clue what caused the error. After some digging and trial and error, I figured out the problem and how to fix it. In this article, I'll walk you through what this error means and how you can resolve it.

What Causes the ‘TypeError: ‘type’ object is not subscriptable’?

The error appears when you use square brackets with a type that cannot handle it. This issue occurs with Python’s typing module. It happens often when using type hints. It occurred when I tried to set a function parameter. I aimed to display the parameter as a list of strings. I wrote the code like this:

def process_items(items: list[str]) -> None:
    pass

To my surprise, Python threw the TypeError: ‘type’ object is not subscriptable. I found that I used list wrong. In Python before version 3.9, list cannot be subscripted with a type. Instead, use List from the typing module.

How to Resolve the Error

After spotting the error, I took these steps:

1. Import the Correct Type from typing:
   Make sure you import the correct type from the typing module. For Python versions under 3.9, use List from typing. Here’s the correct way:

from typing import List

def process_items(items: List[str]) -> None:
    return ['a', 'b', 'c']

2. Upgrade Python Version (If Possible):
   If you can upgrade to Python 3.9 or newer, use the built-in list with square brackets. This approach is simpler and keeps your code clean.

3. Use Correct Type Hinting:
   Always make sure that the type hinting is correct. If you’re not sure whether a type supports subscripting, check the Python documentation or test it in a simple script. This helps avoid the TypeError and keeps your code clean.

Common Pitfalls to Avoid

While fixing my code, I hit traps which cause this error. One trap is not importing types. Types like List, Dict, and Tuple. These come from the typing module, which is key in Python below 3.9. Another mistake is using the wrong brackets, which can lead to problems. It’s key to use square brackets [] for subscripting, not parentheses () or curly braces {}. Catching these small details, which seem minor, can save time and stop errors.

Final Thoughts

The TypeError: ‘type’ object is not subscriptable may confuse those who don’t know Python’s types. I found that Python versions, which handle type hinting, matter a lot. When you understand the error’s cause, the fix is clear. Use correct type hints, and update Python as needed. This way, you write stronger code, which avoids errors. My experience, which I share here, may help you steer clear of this issue, making your Python coding much easier.