tmp_path and tmp_path_factory in PyTest - Test Execution Trace

Test Overview

This test demonstrates how to use tmp_path and tmp_path_factory in pytest to create temporary directories and files. It verifies that files created in these temporary paths exist and contain the expected content.

Test Code - pytest

PyTest

import pytest

# Test using tmp_path fixture

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

# Test using tmp_path_factory fixture
@pytest.fixture

def custom_tmp_dir(tmp_path_factory):
    return tmp_path_factory.mktemp("custom")


def test_tmp_path_factory_creates_file(custom_tmp_dir):
    file = custom_tmp_dir / "data.txt"
    file.write_text("pytest tmp_path_factory")
    assert file.exists()
    assert file.read_text() == "pytest tmp_path_factory"

Execution Trace - 9 Steps

Step	Action	System State	Assertion	Result
1	Test starts	pytest test runner initialized	—	PASS
2	pytest injects tmp_path fixture into test_tmp_path_creates_file	Temporary directory created for this test	—	PASS
3	Create file 'testfile.txt' in tmp_path and write 'hello world'	File 'testfile.txt' exists in temporary directory with content 'hello world'	Check file exists	PASS
4	Read file content and assert it equals 'hello world'	File content is 'hello world'	Assert file.read_text() == 'hello world'	PASS
5	pytest injects tmp_path_factory fixture into custom_tmp_dir fixture	Temporary directory 'custom' created by tmp_path_factory	—	PASS
6	custom_tmp_dir fixture returns the 'custom' temporary directory	Test receives custom temporary directory path	—	PASS
7	Create file 'data.txt' in custom_tmp_dir and write 'pytest tmp_path_factory'	File 'data.txt' exists in 'custom' directory with content 'pytest tmp_path_factory'	Check file exists	PASS
8	Read file content and assert it equals 'pytest tmp_path_factory'	File content is 'pytest tmp_path_factory'	Assert file.read_text() == 'pytest tmp_path_factory'	PASS
9	Test ends	All assertions passed, temporary files cleaned up after test	—	PASS

Failure Scenario

Failing Condition: File is not created or content does not match expected string

Execution Trace Quiz - 3 Questions

What does the tmp_path fixture provide in pytest?

A A permanent directory shared across tests

B A temporary directory unique to the test function

C A temporary file only

D A database connection

Key Result

Using tmp_path and tmp_path_factory fixtures in pytest helps create isolated temporary directories for tests, ensuring no leftover files affect other tests and making tests reliable and clean.

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