How to Create a Dummy Context Manager for Python/Pytest Unit Tests

The aim of this page📝 is to explain how to use the nullcontext context manager in Python to fix a common error that occurs when testing code that uses context managers.

• The error message TypeError: 'dict' object does not support the context manager protocol indicates that a dictionary object is being used as a context manager, but dictionaries do not support the context manager protocol.
• This error can occur when testing code that uses context managers if the context manager is replaced with a dummy object that does not support the context manager protocol.

from contextlib import nullcontext

def test_patch_schema_usual(monkeypatch, iglu):
    monkeypatch.setattr(schema_patcher, "get_env", mock_get_env)
    monkeypatch.setattr(schema_patcher, "get_schema_endpoint",
                        mock_get_schema_endpoint)
    monkeypatch.setattr(bdp_console_handlers, "sync_UI",
                        mock_sync_schema_in_env)
    monkeypatch.setattr(schema_patcher, "handle_patching", lambda _: _) # HERE

    with push_and_pop_path(MODULE_PATH):
        schema_patcher.patch_schema(iglu)

• To fix this error, you can use the nullcontext context manager from the contextlib module as a stand-in for the context manager being tested.
• The nullcontext context manager does nothing but returns the value passed to its enter_result parameter from its __enter__ method.

Here’s an example of how you can use the nullcontext context manager to fix this error:

from contextlib import nullcontext

def test_patch_schema_usual(monkeypatch, iglu):
    monkeypatch.setattr(schema_patcher, "get_env", mock_get_env)
    monkeypatch.setattr(schema_patcher, "get_schema_endpoint",
                        mock_get_schema_endpoint)
    monkeypatch.setattr(bdp_console_handlers, "sync_UI",
                        mock_sync_schema_in_env)
    monkeypatch.setattr(schema_patcher, "handle_patching", lambda _: nullcontext()) # HERE

    with push_and_pop_path(MODULE_PATH):
        schema_patcher.patch_schema(iglu

In this example, we’re testing the patch_schema function, which uses the handle_patching context manager. To prevent the error from occurring, we replace the handle_patching function with a lambda function that returns a nullcontext object.

I hope this helps you understand how to use the nullcontext context manager to fix this common error when testing code that uses context managers.

LINKS

• https://docs.python.org/3/library/contextlib.html#contextlib.nullcontext
• https://stackoverflow.com/questions/45187286/how-do-i-use-contextlib-nullcontext