How to use temporary directories and files in tests

The tmp_path fixture

You can use the tmp_path fixture which will provide a temporary directory unique to each test function.

tmp_path is a pathlib.Path object. Here is an example test usage:

# content of test_tmp_path.py
CONTENT = "content"


def test_create_file(tmp_path):
    d = tmp_path / "sub"
    d.mkdir()
    p = d / "hello.txt"
    p.write_text(CONTENT, encoding="utf-8")
    assert p.read_text(encoding="utf-8") == CONTENT
    assert len(list(tmp_path.iterdir())) == 1
    assert 0

Running this would result in a passed test except for the last assert 0 line which we use to look at values:

$ pytest test_tmp_path.py
=========================== test session starts ============================
platform linux -- Python 3.x.y, pytest-8.x.y, pluggy-1.x.y
rootdir: /home/sweet/project
collected 1 item

test_tmp_path.py F                                                   [100%]

================================= FAILURES =================================
_____________________________ test_create_file _____________________________

tmp_path = PosixPath('PYTEST_TMPDIR/test_create_file0')

    def test_create_file(tmp_path):
        d = tmp_path / "sub"
        d.mkdir()
        p = d / "hello.txt"
        p.write_text(CONTENT, encoding="utf-8")
        assert p.read_text(encoding="utf-8") == CONTENT
        assert len(list(tmp_path.iterdir())) == 1
>       assert 0
E       assert 0

test_tmp_path.py:11: AssertionError
========================= short test summary info ==========================
FAILED test_tmp_path.py::test_create_file - assert 0
============================ 1 failed in 0.12s =============================

By default, pytest retains the temporary directory for the last 3 pytest invocations. Concurrent invocations of the same test function are supported by configuring the base temporary directory to be unique for each concurrent run. See temporary directory location and retention for details.

The tmp_path_factory fixture

The tmp_path_factory is a session-scoped fixture which can be used to create arbitrary temporary directories from any other fixture or test.

For example, suppose your test suite needs a large image on disk, which is generated procedurally. Instead of computing the same image for each test that uses it into its own tmp_path, you can generate it once per-session to save time:

# contents of conftest.py
import pytest


@pytest.fixture(scope="session")
def image_file(tmp_path_factory):
    img = compute_expensive_image()
    fn = tmp_path_factory.mktemp("data") / "img.png"
    img.save(fn)
    return fn


# contents of test_image.py
def test_histogram(image_file):
    img = load_image(image_file)
    # compute and test histogram

See tmp_path_factory API for details.

The tmpdir and tmpdir_factory fixtures

The tmpdir and tmpdir_factory fixtures are similar to tmp_path and tmp_path_factory, but use/return legacy py.path.local objects rather than standard pathlib.Path objects.

Note

These days, it is preferred to use tmp_path and tmp_path_factory.

In order to help modernize old code bases, one can run pytest with the legacypath plugin disabled:

pytest -p no:legacypath

This will trigger errors on tests using the legacy paths. It can also be permanently set as part of the addopts parameter in the config file.

See tmpdir tmpdir_factory API for details.

Temporary directory location and retention

The temporary directories, as returned by the tmp_path and (now deprecated) tmpdir fixtures, are automatically created under a base temporary directory, in a structure that depends on the --basetemp option:

  • By default (when the --basetemp option is not set), the temporary directories will follow this template:

    {temproot}/pytest-of-{user}/pytest-{num}/{testname}/

    where:

    The auto-incrementing {num} placeholder provides a basic retention feature and avoids that existing results of previous test runs are blindly removed. By default, the last 3 temporary directories are kept, but this behavior can be configured with tmp_path_retention_count and tmp_path_retention_policy.

  • When the --basetemp option is used (e.g. pytest --basetemp=mydir), it will be used directly as base temporary directory:

    {basetemp}/{testname}/

    Note that there is no retention feature in this case: only the results of the most recent run will be kept.

    Warning

    The directory given to --basetemp will be cleared blindly before each test run, so make sure to use a directory for that purpose only.

When distributing tests on the local machine using pytest-xdist, care is taken to automatically configure a basetemp directory for the sub processes such that all temporary data lands below a single per-test run temporary directory.