# Python — exceptions can be thrown from anywhere
import json

def load_config(path: str) -> dict:
    try:
        with open(path) as f:
            data = json.load(f)     # Can raise JSONDecodeError
            if "version" not in data:
                raise ValueError("Missing version field")
            return data
    except FileNotFoundError:
        print(f"Config file not found: {path}")
        return {}
    except json.JSONDecodeError as e:
        print(f"Invalid JSON: {e}")
        return {}
    # What other exceptions can this throw?
    # IOError? PermissionError? UnicodeDecodeError?
    # You can't tell from the function signature!

// Rust — errors are return values, visible in the function signature
use std::fs;
use serde_json::Value;

fn load_config(path: &str) -> Result<Value, ConfigError> {
    let contents = fs::read_to_string(path)    // Returns Result
        .map_err(|e| ConfigError::FileError(e.to_string()))?;

    let data: Value = serde_json::from_str(&contents)  // Returns Result
        .map_err(|e| ConfigError::ParseError(e.to_string()))?;

    if data.get("version").is_none() {
        return Err(ConfigError::MissingField("version".to_string()));
    }

    Ok(data)
}

#[derive(Debug)]
enum ConfigError {
    FileError(String),
    ParseError(String),
    MissingField(String),
}

Python:                                 Rust:
─────────                               ─────
- Errors are exceptions (thrown)        - Errors are values (returned)
- Hidden control flow (stack unwinding) - Explicit control flow (? operator)
- Can't tell what errors from signature- MUST see errors in return type
- Uncaught exceptions crash at runtime - Unhandled Results produce compile warnings (always handle them)
- try/except is optional               - Handling Result is required
- Broad except catches everything      - match arms are exhaustive

// Result<T, E> has exactly two variants:
enum Result<T, E> {
    Ok(T),    // Success — contains the value (like Python's return value)
    Err(E),   // Failure — contains the error (like Python's raised exception)
}

// Using Result:
fn divide(a: f64, b: f64) -> Result<f64, String> {
    if b == 0.0 {
        Err("Division by zero".to_string())  // Like: raise ValueError("...")
    } else {
        Ok(a / b)                             // Like: return a / b
    }
}

// Handling Result — like try/except but explicit
match divide(10.0, 0.0) {
    Ok(result) => println!("Result: {result}"),
    Err(msg) => println!("Error: {msg}"),
}

# Python — exceptions propagate silently up the call stack
def read_username() -> str:
    with open("config.txt") as f:      # FileNotFoundError propagates
        return f.readline().strip()    # IOError propagates

def greet():
    name = read_username()             # If this throws, greet() also throws
    print(f"Hello, {name}!")           # This is skipped on error

# The error propagation is INVISIBLE — you have to read the implementation
# to know what exceptions might escape.

// Rust — ? propagates errors, but it's visible in the code AND the signature
use std::fs;
use std::io;

fn read_username() -> Result<String, io::Error> {
    let contents = fs::read_to_string("config.txt")?;  // ? = propagate on Err
    Ok(contents.lines().next().unwrap_or("").to_string())
}

fn greet() -> Result<(), io::Error> {
    let name = read_username()?;       // ? = if Err, return Err immediately
    println!("Hello, {name}!");        // Only reached on Ok
    Ok(())
}

// The ? says: "if this is Err, return it from THIS function immediately."
// It's like Python's exception propagation, but:
// 1. It's visible (you see the ?)
// 2. It's in the return type (Result<..., io::Error>)
// 3. The compiler ensures you handle it somewhere

# Python — multiple operations that might fail
def process_file(path: str) -> dict:
    with open(path) as f:                    # Might fail
        text = f.read()                       # Might fail
    data = json.loads(text)                   # Might fail
    validate(data)                            # Might fail
    return transform(data)                    # Might fail
    # Any of these can throw — and the exception type varies!

// Rust — same chain, but explicit
fn process_file(path: &str) -> Result<Data, AppError> {
    let text = fs::read_to_string(path)?;     // ? propagates io::Error
    let data: Value = serde_json::from_str(&text)?;  // ? propagates serde error
    let validated = validate(&data)?;          // ? propagates validation error
    let result = transform(&validated)?;       // ? propagates transform error
    Ok(result)
}
// Every ? is a potential early return — and they're all visible!

# Python — custom exception classes
class AppError(Exception):
    pass

class NotFoundError(AppError):
    def __init__(self, entity: str, id: int):
        self.entity = entity
        self.id = id
        super().__init__(f"{entity} with id {id} not found")

class ValidationError(AppError):
    def __init__(self, field: str, message: str):
        self.field = field
        super().__init__(f"Validation error on {field}: {message}")

# Usage:
def find_user(user_id: int) -> dict:
    if user_id not in users:
        raise NotFoundError("User", user_id)
    return users[user_id]

// Rust — error enums with thiserror (most popular approach)
// Cargo.toml: thiserror = "2"

use thiserror::Error;

#[derive(Debug, Error)]
enum AppError {
    #[error("{entity} with id {id} not found")]
    NotFound { entity: String, id: i64 },

    #[error("Validation error on {field}: {message}")]
    Validation { field: String, message: String },

    #[error("IO error: {0}")]
    Io(#[from] std::io::Error),        // Auto-convert from io::Error

    #[error("JSON error: {0}")]
    Json(#[from] serde_json::Error),   // Auto-convert from serde error
}

// Usage:
fn find_user(user_id: i64) -> Result<User, AppError> {
    users.get(&user_id)
        .cloned()
        .ok_or(AppError::NotFound {
            entity: "User".to_string(),
            id: user_id,
        })
}

// The #[from] attribute means ? auto-converts io::Error → AppError::Io
fn load_users(path: &str) -> Result<Vec<User>, AppError> {
    let data = fs::read_to_string(path)?;  // io::Error → AppError::Io automatically
    let users: Vec<User> = serde_json::from_str(&data)?;  // → AppError::Json
    Ok(users)
}