You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: lookup_key.rs
    path of file: ./repos/pydantic-core/src
    
    the code of the file until where you have to start completion: use core::slice::Iter;
use std::fmt;

use pyo3::exceptions::{PyAttributeError, PyTypeError};
use pyo3::prelude::*;
use pyo3::types::{PyDict, PyList, PyMapping, PyString};

use jiter::{JsonObject, JsonValue};

use crate::build_tools::py_schema_err;
use crate::errors::{py_err_string, ErrorType, ValError, ValLineError, ValResult};
use crate::input::{Input, StringMapping};
use crate::tools::{extract_i64, py_err};

/// Used for getting items from python dicts, python objects, or JSON objects, in different ways
#[derive(Debug, Clone)]
pub(crate) enum LookupKey {
    /// simply look up a key in a dict, equivalent to `d.get(key)`
    /// we save both the string and pystring to save creating th
    
    pub fn from_py(index: usize, obj: &PyAny) -> PyResult<Self> {
        if let Ok(py_str_key) = obj.downcast::<PyString>() {
            let str_key = py_str_key.to_str()?.to_string();
            Ok(Self::S(str_key, py_str_key.into()))
        } else if let Ok(usize_key) = obj.extract::<usize>() {
            if index == 0 {
                py_err!(PyTypeError; "The first item in an alias path should be a string")
            } else {
                Ok(Self::Pos(usize_key))
            }
        } else if let Some(int_key) = extract_i64(obj) {
            if index == 0 {
                py_err!(PyTypeError; "The first item in an alias path should be a string")
            } else {
                Ok(Self::Neg(int_key.unsigned_abs() as usize))
            }
        } else {
            py_err!(PyTypeError; "Item in an alias path should be a string or int")
        }
    }