# Temporary directories and files

## The ‘tmpdir’ fixture

You can use the `tmpdir` fixture which will provide a temporary directory unique to the test invocation, created in the base temporary directory.

`tmpdir` is a py.path.local object which offers `os.path` methods and more. Here is an example test usage:

    # content of test_tmpdir.py
    import os
    def test_create_file(tmpdir):
        p = tmpdir.mkdir("sub").join("hello.txt")
        p.write("content")
        assert p.read() == "content"
        assert len(tmpdir.listdir()) == 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:

    $ py.test test_tmpdir.py
    ======= test session starts ========
    platform linux2 -- Python 2.7.9, pytest-2.8.0.dev4, py-1.4.28, pluggy-0.3.0
    rootdir: $REGENDOC_TMPDIR, inifile:
    collected 1 items

    test_tmpdir.py F

    ======= FAILURES ========
    _______ test_create_file ________

    tmpdir = local('/tmp/pytest-NaN/test_create_file0')

        def test_create_file(tmpdir):
            p = tmpdir.mkdir("sub").join("hello.txt")
            p.write("content")
            assert p.read() == "content"
            assert len(tmpdir.listdir()) == 1
    >       assert 0
    E       assert 0

    test_tmpdir.py:7: AssertionError
    ======= 1 failed in 0.12 seconds ========

## The ‘tmpdir_factory’ fixture

New in version 2.8.

The `tmpdir_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 `tmpdir`, you can generate it once per-session to save time:

    # contents of conftest.py
    import pytest

    @pytest.fixture(scope='session')
    def image_file(tmpdir_factory):
        img = compute_expensive_image()
        fn = tmpdir_factory.mktemp('data').join('img.png')
        img.save(str(fn))
        return fn

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

`tmpdir_factory` instances have the following methods:

`TempdirFactory.``mktemp`(basename, numbered=True)

  Create a subdirectory of the base temporary directory and return it. If `numbered`, ensure the directory is unique by adding a number prefix greater than any existing one.

`TempdirFactory.``getbasetemp`()

  return base temporary directory.

## The default base temporary directory

Temporary directories are by default created as sub-directories of the system temporary directory. The base name will be `pytest-NUM` where `NUM` will be incremented with each test run. Moreover, entries older than 3 temporary directories will be removed.

You can override the default temporary directory setting like this:

    py.test --basetemp=mydir

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