Typing on cassiobotaro

The write protocol

Fri, 15 May 2026 00:00:00 +0000

The problem Link to heading

When a function only needs to write to a stream, typing the argument as IO[str] is overkill, since you are demanding read, seek, close and the rest of the interface, even though you will never use them.

The tip Link to heading

Starting with Python 3.14, the io module exposes the Writer[T] and Reader[T] protocols. They describe only the minimum contract (write() or read()), so any object that implements just that method already satisfies the type.

The count and index pitfall in NamedTuple

Sat, 17 Jan 2026 00:00:00 +0000

The Problem Link to heading

You are using NamedTuple to create your data classes and decide to add a field called count or index. Everything works perfectly at runtime, but when you run mypy or another type checker, you get strange errors. What is going on?

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from typing import NamedTuple

class Item(NamedTuple):
 name: str
 count: int # Danger!

item = Item(name="apple", count=5)
print(item.count) # Works at runtime

Running mypy: