[Python] How to unit test import error of a module that is actually available at testime?

How can I mock an import error of the module foo in the unit test?

# file: bla.py
try:
import foo

except ImportError:
print("error")
sys.exit(1)


Baseline for the unit test:

import bla
import unittest

class MyTest(unittest.TestCase):
# what to mock (or so) here in order to raise the 'ImportError' that then raises the `sys.exit(1)`
def test_1(self):
with self.assertRaises(SystemExit) as err:
importlib.reload(bla) # force reload, but fails as 'foo' is not importable
self.assertEqual(rt.exception.code,1)

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