capfd for file descriptors in PyTest - Build an Automation Script

Capture and verify output printed to stdout and stderr using capfd

Preconditions (2)

Step 1: Write a test function that uses the capfd fixture

Step 2: Call the function that prints to stdout and stderr inside the test

Step 3: Use capfd.readouterr() to capture the output

Step 4: Verify that the captured stdout contains the expected message

Step 5: Verify that the captured stderr contains the expected error message

✅ Expected Result: The test passes if the captured stdout and stderr match the expected messages

Automation Requirements - pytest

Assertions Needed:

Assert that captured stdout contains the expected string

Assert that captured stderr contains the expected string

Best Practices:

Use capfd fixture parameter in the test function

Call capfd.readouterr() only once after the output is generated

Avoid printing inside assertions

Keep test functions simple and focused

Automated Solution

PyTest

import sys
import pytest

def print_messages():
    print("Hello stdout")
    print("Error message", file=sys.stderr)


def test_capture_output(capfd):
    print_messages()
    out, err = capfd.readouterr()
    assert "Hello stdout" in out
    assert "Error message" in err

The print_messages function prints a message to standard output and another to standard error.

The test function test_capture_output uses the capfd fixture to capture these outputs.

After calling print_messages(), it calls capfd.readouterr() once to get both stdout and stderr.

Assertions check that the expected messages appear in the captured output streams.

This approach ensures the test verifies exactly what the function prints, without showing output during test runs.

Common Mistakes - 3 Pitfalls

Calling capfd.readouterr() multiple times in the same test

Not using the capfd fixture parameter in the test function

Printing inside the test assertions

Bonus Challenge

Now add data-driven testing with 3 different functions that print different messages to stdout and stderr, and verify each output using capfd.

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