How to test if import raises ImportError if package is missing?

I am writing a module which can have pandas as an optional package. The import statement at the top of the file my_submodule.py looks like this.

try:
    import pandas as pd
except (ImportError, ModuleNotFoundError):
    pd = None

Now I want to test that pandas is not installed and either ImportError or ModuleNotFoundError is raised.

How to do this?

At the moment my test file looks like this:

from unittest import TestCase
from unittest.mock import patch

def test_no_pandas_import():

    with patch('sys.path', []):
        from my_module import my_submodule
        assert my_submodule.pd is None

but the assertion is not True, pandas is imported and the errors are not checked.

In my system (where pandas is not installed) I have created exactly your file my_submodule.py, inside the package my_module.

I have introduced your test function test_no_pandas_import() into the followed test class (in the file test_pandas_exception.py):

class MyTestCase(TestCase)

So the test code becomes as below:

from unittest import TestCase, main
from unittest.mock import patch

class MyTestCase(TestCase):

    def test_no_pandas_import(self):
        with patch('sys.path', []):
            # ---------> Note the followed instruction <---------------
            with self.assertRaises(ModuleNotFoundError):
                from my_module import my_submodule
                assert my_submodule.pd is None


if __name__ == '__main__':
    main()

The main difference between my test code and your test code is that I have added the instruction:

with self.assertRaises(ModuleNotFoundError):

If I execute the test method test_no_pandas_import() in my system, it pass, but if I remove your instruction:

with patch('sys.path', []):

the test fails with the following output:

F
======================================================================
FAIL: test_no_pandas_import (__main__.MyTestCase)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/path/to/stack_overflow/test_pandas_exception.py", line 10, in test_no_pandas_import
    assert my_submodule.pd is None
AssertionError: ModuleNotFoundError not raised

----------------------------------------------------------------------
Ran 1 test in 0.002s

FAILED (failures=1)

In this case the test fails because the Exception ModuleNotFoundError is not raised. The exception is not raised because it has caught inside your file my_submodule.py.

Removing the exception catch

If I change the code in your file my_submodule.py so that it doesn't catch the exceptions I obtain the followed code:

#try:
import pandas as pd
#except (ImportError, ModuleNotFoundError):
#    pd = None

where remains only the import.

With the previous code I can use the followed test code (without the instruction with patch('sys.path', []):):

from unittest import TestCase, main

class MyTestCase(TestCase):

    def test_no_pandas_import(self):
        with self.assertRaises(ModuleNotFoundError):
            from my_module import my_submodule
            assert my_submodule.pd is None

if __name__ == '__main__':
    main()

If I execute the previous test code in my system (where pandas is not installed) the test pass and this proof that the ModuleNotFoundError is raised.