Attempting to `dill` a function defined in a `doctest` run with `pytest` causes a `TypeError: cannot pickle 'EncodedFile' object`.

• a detailed description of the bug or problem you are having
• output of pip list from the virtual environment you are using
• pytest and operating system versions
• minimal example if possible

Description of the Problem

When I attempt to dill a function defined inside of a doctest, pytest raises an error, complaining that TypeError: cannot pickle 'EncodedFile' object. Best I can tell, this is the EncodedFile object in pytest.

This error only occurs when I run those doctests via pytest. They run just fine via vanilla doctest. The same code also works just fine in a script and as a regular pytest test function.

I fear that this is related to https://github.com/pytest-dev/pytest/issues/10845, which doesn't seem to have a solution.

Minimal Reproducible Example

class MyClass:
    """Running a file containing this class with `python -m pytest this_file.py --doctest-modules` will fail with `TypeError: cannot pickle 'EncodedFile' object`.

    Examples:
        >>> def template_function():
        ...     return "Hello, World!"

        >>> import dill
        >>> dill.dumps(template_function)
    """

Using pytest to run those doctests will cause the TypeError: cannot pickle 'EncodedFile' object.

Using vanilla doctest works fine. (Ignore the fact that the test failed. The expected output did not match. Note that it did successfully dump the function using dill.)

Failed example:
dill.dumps(template_function)
Expected nothing
Got:
b'\x80\x04\x95L\x03\x00\x00\x00\x00\x00\x00\x8c\ndill.dill\x94\x8c\x10_create_function\x94\x93\x94(h\x00\x8c\x0c_create_code\x94\x93\x94(C\x02\x02\x01\x94K\x00K\x00K\x00K\x00K\x01K\x03C\x06\x97\x00d\x01S\x00\x94N\x8c\rHello, World!\x94\x86\x94))\x8c <doctest test_client.MyClass[0]>\x94\x8c\x11template_function\x94h\nK\x01C\x07\x80\x00\xd8\x0b\x1a\x88?\x94C\x00\x94))t\x94R\x94}\x94\x8c\x08__name_\x94\x8c\x0btest_client\x94sh\nNNt\x94R\x94}\x94}\x94\x8c\x0f__annotations__\x94}\x94s\x86\x94bh\x0f(h\x10h\x11\x8c\x07__doc__\x94N\x8c\x0b__package__\x94\x8c\x00\x94\x8c\n__loader__\x94\x8c\x1a_frozen_importlib_external\x94\x8c\x10SourceFileLoader\x94\x93\x94)\x81\x94}\x94(\x8c\x04name\x94h\x11\x8c\x04path\x94\x8c@/Users/username/test_client.py\x94ub\x8c\x08__spec__\x94\x8c\x11_frozen_importlib\x94\x8c\nModuleSpec\x94\x93\x94)\x81\x94}\x94(h"h\x11\x8c\x06loader\x94h \x8c\x06origin\x94h$\x8c\x0cloader_state\x94N\x8c\x1asubmodule_search_locations\x94N\x8c\x19_uninitialized_submodules\x94]\x94\x8c\r_set_fileattr\x94\x88\x8c\x07_cached\x94\x8cY/Users/username/pycache/test_client.cpython-311.pyc\x94\x8c\r_initializing\x94\x89ub\x8c\x08__file__\x94h$\x8c\n__cached__\x94h3\x8c\x0c__builtins__\x94cbuiltins\n__dict__\n\x8c\x07MyClass\x94h\x11h8\x93\x94h\nh\x13\x8c\x04dill\x94h\x00\x8c\x0e_import_module\x94\x93\x94h:\x85\x94R\x94u0.'

---

1 items had failures:
1 of 3 in test_client.MyClass

Using a regular pytest function to perform the test also works fine. This is only when attempting to dill a function defined in a doctest run with pytest.

Pytest and Operating System versions

This was run on Pytest 8.2.2, Python 3.11 on an M1 Macbook Air (macOS Sonoma 14.4.1). The same errors were also seen on Python 3.9 on the same Macbook and on Python 3.11 on an amd64 Ubuntu Machine (Ubuntu 22.04 LTS).

Pip list

Package    Version
---------- -------
coverage   7.5.3
dill       0.3.8
iniconfig  2.0.0
packaging  24.1
pip        24.0
pluggy     1.5.0
pytest     8.2.2
pytest-cov 5.0.0
setuptools 70.0.0
uv         0.2.10
wheel      0.43.0

Start with the minimal doctest example and run it through python -m pytest this_file.py --doctest-modules, then compare vanilla doctest and a regular pytest test. Inspect src/_pytest/capture.py around line 166 and trace how the doctest environment reaches the dill.dumps call. Done means the doctest case no longer raises TypeError: cannot pickle 'EncodedFile' object while the other cases continue to work.