tmp_path and tmp_path_factory in PyTest

Introduction

We use tmp_path and tmp_path_factory to create temporary folders during tests. This helps keep tests clean and avoids messing with real files.

When you need a fresh folder to save files during a test.

When you want to test file creation or modification without affecting real data.

When multiple tests need separate temporary folders to avoid conflicts.

When you want to clean up files automatically after tests finish.

Syntax

PyTest

def test_example(tmp_path):
    # tmp_path is a pathlib.Path object for a temp directory
    file = tmp_path / "test.txt"
    file.write_text("hello")


def test_factory(tmp_path_factory):
    # tmp_path_factory creates multiple temp directories
    dir1 = tmp_path_factory.mktemp("data1")
    dir2 = tmp_path_factory.mktemp("data2")

tmp_path gives a unique temporary directory for each test function.

tmp_path_factory lets you create multiple temporary directories if needed.

Examples

This test writes and reads a file inside a temporary folder.

PyTest

def test_write_file(tmp_path):
    file = tmp_path / "file.txt"
    file.write_text("hello world")
    assert file.read_text() == "hello world"

This test creates two separate temp folders and writes files in each.

PyTest

def test_multiple_dirs(tmp_path_factory):
    dir_a = tmp_path_factory.mktemp("a")
    dir_b = tmp_path_factory.mktemp("b")
    (dir_a / "a.txt").write_text("A")
    (dir_b / "b.txt").write_text("B")
    assert (dir_a / "a.txt").read_text() == "A"
    assert (dir_b / "b.txt").read_text() == "B"

Sample Program

This script has two tests. The first uses tmp_path to create and check a file. The second uses tmp_path_factory to create two folders and files inside them, then checks their contents.

PyTest

import pytest

def test_tmp_path_creates_file(tmp_path):
    file_path = tmp_path / "sample.txt"
    file_path.write_text("pytest temp file")
    content = file_path.read_text()
    assert content == "pytest temp file"

def test_tmp_path_factory_creates_dirs(tmp_path_factory):
    dir1 = tmp_path_factory.mktemp("dir1")
    dir2 = tmp_path_factory.mktemp("dir2")
    (dir1 / "file1.txt").write_text("data1")
    (dir2 / "file2.txt").write_text("data2")
    assert (dir1 / "file1.txt").read_text() == "data1"
    assert (dir2 / "file2.txt").read_text() == "data2"

Important Notes

Temporary folders are deleted automatically after tests finish.

Use tmp_path for simple cases needing one temp folder per test.

Use tmp_path_factory when you need multiple temp folders in one test.

Summary

tmp_path gives a fresh temp folder for each test.

tmp_path_factory lets you create many temp folders as needed.

Both help keep tests clean and avoid changing real files.

Practice

1. What is the main purpose of the tmp_path fixture in pytest?

easy

A. To provide a unique temporary directory for each test function

B. To create multiple temporary directories during a single test

C. To permanently store test files after tests finish

D. To mock file system calls without creating real files

Solution

1. Step 1: Understand the role of tmp_path

   The tmp_path fixture gives each test a fresh temporary directory to use.

2. Step 2: Compare with other options

   It does not create multiple directories per test (that's tmp_path_factory), nor does it store files permanently or mock file calls.

3. Final Answer:

   To provide a unique temporary directory for each test function -> Option A

4. Quick Check:

   tmp_path = unique temp folder per test [OK]

Hint: Remember: tmp_path = one temp folder per test [OK]

Common Mistakes:

• Confusing tmp_path with tmp_path_factory
• Thinking tmp_path stores files permanently
• Assuming tmp_path mocks file system calls

2. Which of the following is the correct way to use tmp_path_factory to create a new temporary directory named 'data' inside a test?

easy

A. new_dir = tmp_path_factory.make_directory('data')

B. new_dir = tmp_path_factory.create_temp('data')

C. new_dir = tmp_path_factory.tempdir('data')

D. new_dir = tmp_path_factory.mktemp('data')

Solution

1. Step 1: Recall the correct method name

   The method to create a new temporary directory with tmp_path_factory is mktemp().

2. Step 2: Verify method usage

   Calling tmp_path_factory.mktemp('data') creates a temp directory named 'data'. Other options are not valid pytest methods.

3. Final Answer:

   new_dir = tmp_path_factory.mktemp('data') -> Option D

4. Quick Check:

   mktemp() creates temp dirs [OK]

Hint: Use mktemp() method to create temp dirs with tmp_path_factory [OK]

Common Mistakes:

• Using incorrect method names like create_temp or tempdir
• Trying to call make_directory which doesn't exist
• Confusing tmp_path and tmp_path_factory usage

3. Given the following pytest test code, what will be the output of the assertion?

def test_create_file(tmp_path):
    file = tmp_path / 'test.txt'
    file.write_text('hello')
    content = file.read_text()
    assert content == 'hello'

medium

A. Test fails because read_text() returns bytes, not string

B. Test fails because the file is not created

C. Test passes because the file content matches 'hello'

D. Test raises an exception due to invalid file path

Solution

1. Step 1: Analyze file creation and writing

   The code creates a file 'test.txt' in the temporary directory and writes the string 'hello' to it.

2. Step 2: Check reading and assertion

   Reading the file with read_text() returns the string 'hello', which matches the assertion.

3. Final Answer:

   Test passes because the file content matches 'hello' -> Option C

4. Quick Check:

   write_text/read_text with tmp_path works [OK]

Hint: write_text and read_text handle strings correctly [OK]

Common Mistakes:

• Assuming read_text returns bytes instead of string
• Thinking file is not created automatically
• Expecting exceptions from valid path operations

4. Identify the error in this pytest test using tmp_path_factory:

def test_dir_creation(tmp_path_factory):
    dir1 = tmp_path_factory.mktemp('dir')
    dir2 = tmp_path_factory.mktemp('dir')
    assert dir1 != dir2

medium

A. Error because mktemp cannot create directories with the same name

B. No error; test passes because mktemp creates unique dirs

C. Error because tmp_path_factory is not a valid fixture

D. Error because assert compares directory contents, not paths

Solution

1. Step 1: Understand mktemp behavior

   Each call to mktemp('dir') creates a unique temporary directory, even if the name is the same.

2. Step 2: Check assertion validity

   The assertion compares the two directory paths and expects them to be different, which is true.

3. Final Answer:

   No error; test passes because mktemp creates unique dirs -> Option B

4. Quick Check:

   mktemp creates unique dirs even with same name [OK]

Hint: mktemp always creates unique dirs, names can repeat [OK]

Common Mistakes:

• Thinking mktemp fails on duplicate names
• Confusing fixture names or usage
• Assuming assert compares contents, not paths

5. You want to write a pytest test that creates two temporary directories: one shared across multiple tests and one unique per test. Which combination of fixtures should you use?

hard

A. Use tmp_path_factory to create the shared directory and tmp_path for the unique per-test directory

B. Use tmp_path for both shared and unique directories

C. Use tmp_path_factory for both shared and unique directories

D. Use tmp_path to create the shared directory and tmp_path_factory for unique directories

Solution

1. Step 1: Identify fixture roles

   tmp_path provides a fresh temp directory unique to each test. tmp_path_factory can create directories shared or reused across tests.

2. Step 2: Match requirements to fixtures

   For a shared directory across tests, use tmp_path_factory. For unique per-test directories, use tmp_path.

3. Final Answer:

   Use tmp_path_factory to create the shared directory and tmp_path for the unique per-test directory -> Option A

4. Quick Check:

   tmp_path unique per test, tmp_path_factory for shared [OK]

Hint: tmp_path unique per test; tmp_path_factory for shared dirs [OK]

Common Mistakes:

• Using tmp_path for shared directories
• Using tmp_path_factory only for unique per-test dirs
• Mixing up fixture purposes