Path: ...!npeer.as286.net!npeer-ng0.as286.net!fu-berlin.de!uni-berlin.de!not-for-mail From: ram@zedat.fu-berlin.de (Stefan Ram) Newsgroups: comp.lang.python Subject: Re: Struggling to understand Callable type hinting Date: 18 Jan 2025 15:38:37 GMT Organization: Stefan Ram Lines: 80 Expires: 1 Jan 2026 11:59:58 GMT Message-ID: References: Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-Trace: news.uni-berlin.de GtVe3VyU5CLjbe+vK02suwM/yO4cBjwMXahfuEfKRMlocn Cancel-Lock: sha1:6Gp1VlZDHh9Z085G8BwulU3vM2k= sha256:tUo8uXNWCbT3QcKQrCpwVH6FPKKttJV/lWpykD29Ito= X-Copyright: (C) Copyright 2025 Stefan Ram. All rights reserved. Distribution through any means other than regular usenet channels is forbidden. It is forbidden to publish this article in the Web, to change URIs of this article into links, and to transfer the body without this notice, but quotations of parts in other Usenet posts are allowed. X-No-Archive: Yes Archive: no X-No-Archive-Readme: "X-No-Archive" is set, because this prevents some services to mirror the article in the web. But the article may be kept on a Usenet archive server with only NNTP access. X-No-Html: yes Content-Language: en-US Bytes: 4407 ram@zedat.fu-berlin.de (Stefan Ram) wrote or quoted: >T = TypeVar('T') Alright, so type variables in Python are like the Swiss Army knife of the coding world. They're part of this thing called the typing module, and they let you whip up functions and classes that can roll with pretty much any type you throw at them. Here's the skinny on when you'd need one of these bad boys instead of just sticking to one type: Python from typing import TypeVar, List T = TypeVar('T') def first_and_last(items: List[T]) -> List[T]: return [items[0], items[-1]] Check it out - this "first_and_last" function is like a food truck that can serve up any cuisine. It takes a list of whatever and spits out a list with the first and last items. Using this T type variable is like having a menu that changes based on what's fresh that day. It keeps everything kosher between what goes in and what comes out. If we'd used a fixed type, like "List[int]", it'd be like a taco truck that only serves carne asada. By using a type variable, we're cooking up a function that's as flexible as a yoga instructor but still gives the lowdown on types to those picky static checkers and other devs. It's the best of both worlds, like living in California - you get the beach /and/ the mountains! "TypeVar" is a factory for creating type variables. It's not a type itself, but a tool to create types. When you write "T = TypeVar('T')", you're basically brewing your own custom type. It's like creating a signature blend at Philz Coffee - unique and tailored to your needs. Once you've created this custom type "T", you can use it in generic constructions like "List[T]". This tells Python, "Hey, I want a list of whatever type T turns out to be". The string argument 'T' in TypeVar('T') serves two main purposes: 1. It provides a name for the type variable, which is used for debugging and error reporting. This name helps developers and type checkers identify and track the type variable throughout the code. 2. It's a convention to use the same string as the variable name to which the TypeVar is assigned. This practice enhances code readability and maintainability. While any string could technically be used, it's strongly recommended to follow the convention of using the same string as the variable name. For example: Python T = TypeVar('T') # Recommended AnyStr = TypeVar('AnyStr') # Also follows the convention Using a different string wouldn't break the functionality, but it could lead to confusion: Python X = TypeVar('Y') # Works, but confusing and not recommended The name is primarily for human readers and tools. It doesn't affect the runtime behavior of the program, but it's crucial for static type checking and code comprehension.