File system testing with tmp_path in PyTest - Build an Automation Script

Create and verify a file using tmp_path fixture in pytest

Preconditions (2)

Step 1: Use the tmp_path fixture to get a temporary directory

Step 2: Create a new file named 'testfile.txt' inside the tmp_path directory

Step 3: Write the text 'Hello, pytest!' into the file

Step 4: Read the content of the file

Step 5: Verify that the content matches 'Hello, pytest!'

✅ Expected Result: The file 'testfile.txt' is created in the temporary directory with the exact content 'Hello, pytest!'

Automation Requirements - pytest

Assertions Needed:

Assert the file 'testfile.txt' exists in the tmp_path directory

Assert the file content equals 'Hello, pytest!'

Best Practices:

Use the tmp_path fixture for isolated temporary file system testing

Use context managers for file operations

Keep tests independent and clean up automatically

Automated Solution

PyTest

import pytest

def test_create_and_read_file(tmp_path):
    # Create a file path object for 'testfile.txt'
    file = tmp_path / "testfile.txt"
    # Write text to the file using context manager
    with file.open('w', encoding='utf-8') as f:
        f.write('Hello, pytest!')
    # Read the content back
    with file.open('r', encoding='utf-8') as f:
        content = f.read()
    # Assert the file exists
    assert file.exists(), f"File {file} should exist"
    # Assert the content is correct
    assert content == 'Hello, pytest!', f"File content should be 'Hello, pytest!' but was '{content}'"

This test uses the tmp_path fixture provided by pytest to get a temporary directory unique to this test run. It creates a file named testfile.txt inside this directory.

Using a context manager (with statement) ensures the file is properly opened and closed when writing and reading.

Assertions check that the file exists and that its content matches the expected string. This ensures the file system operations are working as intended in an isolated environment.

Using tmp_path keeps tests clean and avoids side effects on the real file system.

Common Mistakes - 3 Pitfalls

Not using tmp_path and writing files to a fixed directory

Not closing files properly after writing or reading

Not asserting file existence before reading content

Bonus Challenge

Now add data-driven testing to create and verify files with three different contents

Show Hint

Practice

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

easy

A. To mock file system calls without creating real files

B. To permanently store test files for later use

C. To speed up test execution by caching files

D. To create a temporary directory unique to each test for safe file operations

Solution

1. Step 1: Understand tmp_path functionality

   tmp_path provides a temporary directory unique to each test run.

2. Step 2: Identify its purpose in tests

   This temporary directory allows safe creation, reading, and deletion of files without affecting the real file system.

3. Final Answer:

   To create a temporary directory unique to each test for safe file operations -> Option D

4. Quick Check:

   tmp_path = safe temp folder [OK]

Hint: Remember tmp_path is a safe temp folder for each test [OK]

Common Mistakes:

• Thinking tmp_path stores files permanently
• Confusing tmp_path with mocking file calls
• Assuming tmp_path speeds up tests by caching

2. Which of the following is the correct way to create a new file named test.txt inside the tmp_path directory in pytest?

easy

A. (tmp_path / 'test.txt').write_text('Hello')

B. tmp_path / 'test.txt'.write_text('Hello')

C. tmp_path.write_text('test.txt', 'Hello')

D. tmp_path.write('test.txt', 'Hello')

Solution

1. Step 1: Understand tmp_path usage

   tmp_path is a pathlib.Path object, so to create a file, you combine paths with / operator.

2. Step 2: Correct syntax for writing text

   Use parentheses to combine path and then call write_text() on the file path: (tmp_path / 'test.txt').write_text('Hello').

3. Final Answer:

   (tmp_path / 'test.txt').write_text('Hello') -> Option A

4. Quick Check:

   Use (tmp_path / filename).write_text() [OK]

Hint: Use parentheses to combine path before write_text() [OK]

Common Mistakes:

• Calling write_text directly on tmp_path
• Missing parentheses around path combination
• Using incorrect method names like write()

3. Given the following pytest test code using tmp_path:

def test_write_and_read(tmp_path):
    file = tmp_path / 'data.txt'
    file.write_text('pytest rocks')
    content = file.read_text()
    assert content == 'pytest rocks'

What will be the result of running this test?

medium

A. Test passes because the file content matches the assertion

B. Test fails because the file is not created

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

D. Test raises a syntax error

Solution

1. Step 1: Analyze file creation and writing

   The code creates a file named 'data.txt' in tmp_path and writes the string 'pytest rocks' correctly.

2. Step 2: Analyze reading and assertion

   Reading the file with read_text() returns the string 'pytest rocks', which matches the assertion exactly.

3. Final Answer:

   Test passes because the file content matches the assertion -> Option A

4. Quick Check:

   write_text + read_text = matching string [OK]

Hint: write_text and read_text use strings, so assertion matches [OK]

Common Mistakes:

• Thinking read_text returns bytes instead of string
• Assuming file is not created automatically
• Confusing syntax errors with runtime behavior

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

def test_file_creation(tmp_path):
    file = tmp_path / 'log.txt'
    file.write('Log entry')
    assert file.read_text() == 'Log entry'

medium

A. Missing parentheses around tmp_path / 'log.txt'

B. The assertion compares bytes to string causing failure

C. Using write() instead of write_text() causes an AttributeError

D. The test will pass without errors

Solution

1. Step 1: Check method used to write file

   Path objects do not have a write() method; the correct method to write text is write_text().

2. Step 2: Understand error caused

   Calling write() will raise an AttributeError because it does not exist on the Path object.

3. Final Answer:

   Using write() instead of write_text() causes an AttributeError -> Option C

4. Quick Check:

   Use write_text() to write strings to files [OK]

Hint: Use write_text() for writing strings to files [OK]

Common Mistakes:

• Confusing write() with write_text()
• Ignoring method errors and expecting test pass
• Assuming parentheses are missing around path

5. You want to test a function that creates multiple files inside a directory using pytest's tmp_path. Which approach correctly verifies that exactly three files named a.txt, b.txt, and c.txt exist in the temporary directory after the function runs?

hard

A. Use assert tmp_path.read_text() == 'a.txtb.txtc.txt'

B. Use assert set(f.name for f in tmp_path.iterdir()) == {'a.txt', 'b.txt', 'c.txt'}

C. Use assert tmp_path.glob('*.txt') == ['a.txt', 'b.txt', 'c.txt']

D. Use assert tmp_path.exists('a.txt') and tmp_path.exists('b.txt') and tmp_path.exists('c.txt')

Solution

1. Step 1: Understand how to list files in tmp_path

   tmp_path.iterdir() returns an iterator of Path objects for all entries in the directory.

2. Step 2: Verify file names correctly

   Extract file names with f.name and compare as a set to ensure exactly the three expected files exist.

3. Final Answer:

   Use assert set(f.name for f in tmp_path.iterdir()) == {'a.txt', 'b.txt', 'c.txt'} -> Option B

4. Quick Check:

   Use iterdir() and set comparison for exact files [OK]

Hint: Use set of filenames from iterdir() to check exact files [OK]

Common Mistakes:

• Using glob() without converting to list or set
• Trying to read tmp_path as a file
• Using exists() incorrectly on tmp_path object