capsys for stdout/stderr in PyTest - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is capsys in pytest?

capsys is a pytest fixture that captures output sent to stdout and stderr during test execution. It allows you to check what your code prints or logs.

beginner

How do you access captured stdout and stderr using capsys?

Use capsys.readouterr() which returns a namedtuple with out (stdout) and err (stderr) strings.

beginner

Why is capturing stdout and stderr useful in tests?

It helps verify that your program prints the correct messages or error logs, similar to checking a receipt after a purchase to confirm details.

beginner

Show a simple pytest test function using capsys to check printed output.

def test_print_message(capsys):
    print('Hello, world!')
    captured = capsys.readouterr()
    assert captured.out == 'Hello, world!\n'

intermediate

What happens if you call capsys.readouterr() multiple times in the same test?

Each call returns the output captured since the last call, then clears the buffer. This lets you check output in parts.

What does capsys.readouterr() return?

A A namedtuple with <code>stdout</code> and <code>stderr</code> output

B Only the <code>stdout</code> output

C Only the <code>stderr</code> output

D A boolean indicating if output was captured

capsys.readouterr() returns a namedtuple with two strings: out for stdout and err for stderr.

Why use capsys in pytest tests?

A To mock database calls

B To capture and check printed output

C To speed up test execution

D To generate test data

capsys is used to capture printed output so tests can verify it.

If your code prints 'Error!' to stderr, how do you check it with capsys?

A Use <code>assert False</code>

B Check <code>captured.out</code> after <code>capsys.readouterr()</code>

C Use <code>print()</code> in the test

D Check <code>captured.err</code> after <code>capsys.readouterr()</code>

stderr output is stored in captured.err.

What happens if you forget to call capsys.readouterr() in your test?

A Output is printed to the console

B The test will fail automatically

C You cannot access the captured output

D The test runs faster

You must call capsys.readouterr() to get the captured output strings.

Can capsys capture output from functions called inside your test?

A Yes, it captures all output during the test

B No, only output from the test function itself

C Only output from imported modules

D Only output from print statements in the test

capsys captures all output sent to stdout and stderr during the test, including from called functions.

Explain how to use capsys in a pytest test to verify printed output.

Think about how you capture and check what your code prints.

Describe the difference between stdout and stderr and how capsys helps test both.

Consider normal messages vs error messages.

Practice

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

easy

A. To capture and check printed output sent to stdout and stderr

B. To speed up test execution by skipping print statements

C. To automatically fix syntax errors in print statements

D. To generate random test data for print output

Solution

1. Step 1: Understand what capsys does

   capsys is a pytest feature that captures output sent to the console, including both stdout and stderr.

2. Step 2: Identify its main use in tests

   It allows tests to check what was printed, so you can verify messages or errors your code produces.

3. Final Answer:

   To capture and check printed output sent to stdout and stderr -> Option A

4. Quick Check:

   capsys captures output = C [OK]

Hint: capsys catches print output for verification [OK]

Common Mistakes:

• Thinking capsys speeds up tests
• Believing capsys fixes code errors
• Confusing capsys with test data generation

2. Which of the following is the correct way to use capsys to capture printed output in a pytest test function?

easy

A. def test_func(capsys): captured = capsys.capture(); print('Hello')

B. def test_func(): capsys = print('Hello'); captured = capsys.readouterr()

C. def test_func(capsys): print('Hello'); captured = capsys.readouterr()

D. def test_func(): captured = capsys.readouterr(); print('Hello')

Solution

1. Step 1: Check how capsys is passed and used

   In pytest, capsys is passed as a function argument and used after printing to capture output.

2. Step 2: Verify correct method to read output

   The method to get output is capsys.readouterr(), which returns captured stdout and stderr.

3. Final Answer:

   def test_func(capsys): print('Hello'); captured = capsys.readouterr() -> Option C

4. Quick Check:

   capsys as argument + readouterr() = A [OK]

Hint: Pass capsys as argument and call readouterr() after print [OK]

Common Mistakes:

• Not passing capsys as a test function argument
• Calling non-existent capture() method
• Calling readouterr() before printing

3. What will be the output of the following pytest test code snippet?

def test_output(capsys):
    print('Test message')
    captured = capsys.readouterr()
    assert captured.out == 'Test message\n'

medium

A. Test passes because printed output matches assertion

B. Test fails because captured.out does not include newline

C. Test fails due to syntax error in print statement

D. Test fails because capsys.readouterr() is called before print

Solution

1. Step 1: Understand output capture timing

   The print statement runs first, sending 'Test message' plus a newline to stdout.

2. Step 2: Check captured output and assertion

   capsys.readouterr() captures the output including the newline. The assertion checks for 'Test message\n', which matches exactly.

3. Final Answer:

   Test passes because printed output matches assertion -> Option A

4. Quick Check:

   Printed output includes newline = B [OK]

Hint: Remember print adds newline; assert with '\n' [OK]

Common Mistakes:

• Forgetting newline in assertion string
• Calling readouterr() before print
• Assuming captured.out excludes newlines

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

def test_error(capsys):
    captured = capsys.readouterr()
    print('Hello')
    assert captured.out == 'Hello\n'

medium

A. Missing parentheses in print statement

B. Calling capsys.readouterr() before printing captures nothing

C. Assertion string missing newline character

D. capsys must be imported before use

Solution

1. Step 1: Check order of output capture and print

   capsys.readouterr() is called before the print statement, so it captures output up to that point, which is empty.

2. Step 2: Understand assertion failure reason

   The assertion expects 'Hello\n' but captured.out is empty, causing test failure.

3. Final Answer:

   Calling capsys.readouterr() before printing captures nothing -> Option B

4. Quick Check:

   Read output after print, not before = A [OK]

Hint: Always call readouterr() after print to capture output [OK]

Common Mistakes:

• Calling readouterr() before print
• Forgetting newline in assertion
• Assuming capsys needs import

5. You want to test a function that prints error messages to stderr using pytest and capsys. Which code snippet correctly captures and asserts the stderr output?

hard

A. def test_error_message(capsys):
print('Error occurred', file=sys.stdout)
captured = capsys.readouterr()
assert captured.err == 'Error occurred\n'

B. def test_error_message(capsys):
print('Error occurred')
captured = capsys.readouterr()
assert captured.err == 'Error occurred\n'

C. def test_error_message():
print('Error occurred', file=sys.stderr)
captured = capsys.readouterr()
assert captured.err == 'Error occurred\n'

D. def test_error_message(capsys):
print('Error occurred', file=sys.stderr)
captured = capsys.readouterr()
assert captured.err == 'Error occurred\n'

Solution

1. Step 1: Identify how to print to stderr

   Use print(..., file=sys.stderr) to send output to stderr stream.

2. Step 2: Check correct use of capsys and assertion

   capsys.readouterr() captures both stdout and stderr. To check stderr, assert captured.err matches expected string including newline.

3. Final Answer:

   def test_error_message(capsys): print('Error occurred', file=sys.stderr) captured = capsys.readouterr() assert captured.err == 'Error occurred\n' -> Option D

4. Quick Check:

   Print to stderr + assert captured.err = D [OK]

Hint: Print with file=sys.stderr and assert captured.err [OK]

Common Mistakes:

• Not passing capsys as argument
• Printing to stdout instead of stderr
• Asserting captured.out instead of captured.err