When should we use unwrap vs expect in Rust

Question

I'm new to rust and trying to understand when should we use unwrap vs expect.

Here's a sample code:

use std::env;

fn main() {
    let args: Vec<String> = env::args().collect();

    let query = args.get(1).unwrap();
    println!("query from unwrawp: {}", query);

    let query = args.get(1).expect("insufficient arguments");
    println!("query from expect: {}", query);

    //$ cargo run hello
    //OUTPUT:
    //query from expect: hello
    //query from unwrawp: hello

}

Only difference I observed is there's a custom panic message in expect.
Are these two interchangeable or are there any special scenarios where we should use one over the other?

Answer 1

Rust doesn't have function overloading, so there should be a way to declare "unwrap with a message", and that is expect.

1. expect == unwrap with a message
2. expect_err == unwrap_err with a message

About usage scenarios of "unwrap vs expect" Rust Book (Ch 9) says:

Using expect instead of unwrap and providing good error messages can convey your intent and make tracking down the source of a panic easier. Because this error message starts with the text we specified... it will be easier to find where in the code this error message is coming from.