capsys for stdout/stderr in PyTest - Build an Automation Script

Capture and verify printed output using capsys in pytest

Preconditions (2)

Step 1: Write a test function that calls the target function which prints to stdout and stderr

Step 2: Use the capsys fixture to capture the output

Step 3: After calling the function, use capsys.readouterr() to get the captured stdout and stderr

Step 4: Assert that the captured stdout contains the expected printed text

Step 5: Assert that the captured stderr contains the expected error text

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

Automation Requirements - pytest

Assertions Needed:

Assert captured stdout equals expected string

Assert captured stderr equals expected string

Best Practices:

Use the capsys fixture parameter in the test function

Call capsys.readouterr() only once after the function call

Avoid printing inside the test except for debugging

Keep assertions clear and specific

Automated Solution

PyTest

import sys

def greet():
    print("Hello, world!")
    print("Warning: something minor happened", file=sys.stderr)


def test_greet_output(capsys):
    greet()
    captured = capsys.readouterr()
    assert captured.out == "Hello, world!\n"
    assert captured.err == "Warning: something minor happened\n"

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

The test function test_greet_output uses the capsys fixture to capture these outputs.

After calling greet(), capsys.readouterr() returns an object with out and err strings.

Assertions check that the captured output matches exactly what greet printed, including newline characters.

This ensures the printed messages are correctly captured and verified.

Common Mistakes - 3 Pitfalls

Calling capsys.readouterr() multiple times after the function call

Not including newline characters in expected strings

Printing inside the test function for debugging without disabling

Bonus Challenge

Now add tests for a function that prints different messages based on input, capturing and asserting stdout and stderr for three different inputs.

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