capfd for file descriptors in PyTest - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output captured by capfd?

Consider this pytest test function using capfd to capture output from print statements. What will be the value of captured output after running this test?

PyTest

def test_capture_output(capfd):
    print('Hello')
    print('World')
    out, err = capfd.readouterr()
    assert out == 'Hello\nWorld\n'
    assert err == ''

A ''

B 'Hello World\n'

C 'HelloWorld'

D 'Hello\nWorld\n'

💡 Hint

Remember that print adds a newline after each call.

The capfd fixture captures output sent to stdout and stderr. Each print adds a newline, so the captured stdout is 'Hello\nWorld\n'.

❓ assertion

intermediate

Which assertion correctly checks stderr captured by capfd?

Given a function that writes an error message to stderr, which assertion correctly verifies the captured error output using capfd?

PyTest

def test_error_message(capfd):
    import sys
    print('Error occurred', file=sys.stderr)
    out, err = capfd.readouterr()

A assert err == 'Error occurred'

B assert err == 'Error occurred\n'

C assert out == 'Error occurred\n'

D assert out == ''

💡 Hint

Error messages printed to stderr are captured in err.

The error message is printed to stderr, so capfd captures it in err with a newline added by print.

🔧 Debug

advanced

Why does capfd.readouterr() return empty strings after first call?

In this test, capfd.readouterr() is called twice. Why does the second call return empty strings?

PyTest

def test_double_read(capfd):
    print('Test')
    out1, err1 = capfd.readouterr()
    out2, err2 = capfd.readouterr()
    assert out1 == 'Test\n'
    assert out2 == ''

A Because readouterr() clears the captured output after reading

B Because print only outputs once and capfd caches it

C Because capfd only captures stderr, not stdout

D Because the second print statement is missing

💡 Hint

Think about what happens to the buffer after reading.

capfd.readouterr() returns the current captured output and then clears the buffer. So the second call returns empty strings.

🧠 Conceptual

advanced

What is the main difference between capfd and capsys in pytest?

Choose the correct statement about the difference between capfd and capsys fixtures in pytest.

A capfd captures output only from Python print, capsys captures all system output

B capfd captures only stdout, capsys captures both stdout and stderr

C capfd captures output at the file descriptor level, capsys captures at the Python sys module level

D capfd is slower than capsys because it uses subprocesses

💡 Hint

Think about how low-level the capture happens.

capfd captures output by redirecting the OS file descriptors (low-level), while capsys captures output by replacing sys.stdout and sys.stderr (Python-level).

❓ framework

expert

How to test a function that writes to both stdout and stderr using capfd?

You have a function that prints 'Info' to stdout and 'Warning' to stderr. Which test code correctly asserts both outputs using capfd?

PyTest

def log_messages():
    import sys
    print('Info')
    print('Warning', file=sys.stderr)

A

def test_log(capfd):
    log_messages()
    out, err = capfd.readouterr()
    assert out == 'Info\n'
    assert err == 'Warning\n'

B

def test_log(capfd):
    log_messages()
    out, err = capfd.readouterr()
    assert out == 'Warning\n'
    assert err == 'Info\n'

C

def test_log(capfd):
    log_messages()
    out, err = capfd.readouterr()
    assert out == 'InfoWarning\n'
    assert err == ''

D

def test_log(capfd):
    log_messages()
    out, err = capfd.readouterr()
    assert out == ''
    assert err == 'Info\nWarning\n'

💡 Hint

Remember which stream each print writes to.

The function prints 'Info' to stdout and 'Warning' to stderr. capfd captures both separately in out and err.