In Why your mock doesn’t work I explained this rule of mocking:

Mock where the object is used, not where it’s defined.

That blog post explained why that rule was important: often a mock doesn’t work at all if you do it wrong. But in some cases, the mock will work even if you don’t follow this rule, and then it can break much later. Why?

Let’s say you have code like this:

# user.py
def get_user_settings() -> str:
    with open(Path("~/settings.json").expanduser()) as f:
        return json.load(f)
def add_two_settings() -> int:
    settings = get_user_settings()
    return settings["opt1"] + settings["opt2"]

You write a simple test:

def test_add_two_settings():
    # NOTE: need to create ~/settings.json for this to work:
    #   {"opt1": 10, "opt2": 7}
    assert add_two_settings() == 17

As the comment in the test points out, the test will only pass if you create the correct settings.json file in your home directory. This is bad: you don’t want to require finicky environments for your tests to pass.

The thing we want to avoid is opening a real file, so it’s a natural impulse to mock out open():

# test_user.py
from io import StringIO
from unittest.mock import patch
@patch("builtins.open")
def test_add_two_settings(mock_open):
    mock_open.return_value = StringIO('{"opt1": 10, "opt2": 7}')
    assert add_two_settings() == 17

Nice, the test works without needing to create a file in our home directory!

One day your test suite fails with an error like:

...
  File ".../site-packages/coverage/python.py", line 55, in get_python_source
    source_bytes = read_python_source(try_filename)
  File ".../site-packages/coverage/python.py", line 39, in read_python_source
    return source.replace(b"\r\n", b"\n").replace(b"\r", b"\n")
           ~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^
TypeError: replace() argument 1 must be str, not bytes

What happened!? Coverage.py code runs during your tests, invoked by the Python interpreter. The mock in the test changed the builtin open, so any use of it anywhere during the test is affected. In some cases, coverage.py needs to read your source code to record the execution properly. When that happens, coverage.py unknowingly uses the mocked open, and bad things happen.

When you use a mock, patch it where it’s used, not where it’s defined. In this case, the patch would be:

@patch("myproduct.user.open")
def test_add_two_settings(mock_open):
    ... etc ...

With a mock like this, the coverage.py code would be unaffected.

Keep in mind: it’s not just coverage.py that could trip over this mock. There could be other libraries used by your code, or you might use open yourself in another part of your product. Mocking the definition means anything using the object will be affected. Your intent is to only mock in one place, so target that place.

I decided to add some code to coverage.py to defend against this kind of over-mocking. There is a lot of over-mocking out there, and this problem only shows up in coverage.py with Python 3.14. It’s not happening to many people yet, but it will happen more and more as people start testing with 3.14. I didn’t want to have to answer this question many times, and I didn’t want to force people to fix their mocks.

From a certain perspective, I shouldn’t have to do this. They are in the wrong, not me. But this will reduce the overall friction in the universe. And the fix was really simple:

open = open

This is a top-level statement in my module, so it runs when the module is imported, long before any tests are run. The assignment to open will create a global in my module, using the current value of open, the one found in the builtins. This saves the original open for use in my module later, isolated from how builtins might be changed later.

This is an ad-hoc fix: it only defends one builtin. Mocking other builtins could still break coverage.py. But open is a common one, and this will keep things working smoothly for those cases. And there’s precedent: I’ve already been using a more involved technique to defend against mocking of the os module for ten years.

No blog post about mocking is complete without encouraging a number of other best practices, some of which could get you out of the mocking mess: