pytest parametrised fixture with different cases

I have several test cases and test functions, and the list of test cases is different for the different functions. This can be easily done with pytest.mark.parametrize. The extra need I have is to load a resource (a file in my case) and I'd like to have this file only loaded once per test session and cached.

Below an example illustrating what I want. It's working, but I would like to find a way to use pytest fixtures or some other caching mechanism so that I don't have to do the caching myself and have the pars=load_file(pars) line in each test function.

Can someone please explain how to do this with pytest?

import pytest

case_1 = dict(label='case_1', spam=1)
case_2 = dict(label='case_2', spam=2)
case_3 = dict(label='case_3', spam=3)

_cache = {}


def load_file(pars):
    if pars['label'] in _cache:
        print('load_file from cache', pars)
        return _cache[pars['label']]
    else:
        print('load_file loading', pars)
        pars['file'] = pars['label'] + ' spam!'
        _cache[pars['label']] = pars
        return pars


@pytest.mark.parametrize('pars', [case_1, case_2])
def test_a(pars):
    pars = load_file(pars)
    print('test_a', pars)


@pytest.mark.parametrize('pars', [case_2, case_3])
def test_b(pars):
    pars = load_file(pars)
    print('test_b', pars)


@pytest.mark.parametrize('pars', [case1, case_2, case_3])
def test_c(pars):
    pars = load_file(pars)
    print('test_c', pars)

### more tests here for various combinations of test cases