capsys for stdout/stderr in PyTest - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output captured by capsys?

Consider the following pytest test function using capsys to capture output. What will be the value of captured.out after the print statements?

PyTest

def test_capture_output(capsys):
    print("Hello")
    print("World", end="!")
    captured = capsys.readouterr()
    print("Again")
    return captured.out

A "Hello\nWorld!"

B "Hello\nWorld!\nAgain\n"

C "HelloWorld!\n"

D "Hello\nWorld!\n"

💡 Hint

capsys.readouterr() captures output up to the point it is called.

capsys.readouterr() captures all output printed before it is called. The first two print statements produce 'Hello\n' and 'World!' without a newline after 'World!'. So captured.out is 'Hello\nWorld!'. The last print("Again") happens after capture, so it is not included.

❓ assertion

intermediate

Which assertion correctly checks stderr output captured by capsys?

Given the following test code, which assertion will correctly verify that the error message 'Error occurred' was printed to stderr?

PyTest

def test_error_message(capsys):
    import sys
    print('Error occurred', file=sys.stderr)
    captured = capsys.readouterr()

A assert 'Error occurred' in captured.out

B assert captured.out == 'Error occurred\n'

C assert captured.err == ''

D assert 'Error occurred' in captured.err

💡 Hint

stderr output is captured in captured.err, not captured.out.

Output printed to sys.stderr is captured in captured.err. So to check for 'Error occurred' in stderr, the assertion must check captured.err.

🔧 Debug

advanced

Why does capsys.readouterr() not capture output after the test ends?

In the following test, the last print statement is not captured by capsys. Why does this happen?

PyTest

def test_late_print(capsys):
    print('Start')
    captured = capsys.readouterr()
    print('End')
    assert captured.out == 'Start\n'

A capsys only captures output printed before readouterr() is called

B capsys captures output only from stderr

C print statements after assertions are ignored by pytest

D capsys captures all output during the entire test function

💡 Hint

Think about when capsys.readouterr() captures output.

capsys.readouterr() captures output printed up to the moment it is called. Output printed after that is not included in the captured object.

❓ framework

advanced

How to capture output multiple times in one test using capsys?

You want to capture output twice in a single test function using capsys. Which code snippet correctly captures and asserts both outputs separately?

A

captured1 = capsys.readouterr()
print('First')
captured2 = capsys.readouterr()
print('Second')
assert captured1.out == ''
assert captured2.out == 'First\n'

B

print('First')
print('Second')
captured1 = capsys.readouterr()
captured2 = capsys.readouterr()
assert captured1.out == 'First\nSecond\n'
assert captured2.out == ''

C

print('First')
captured1 = capsys.readouterr()
print('Second')
captured2 = capsys.readouterr()
assert captured1.out == 'First\n'
assert captured2.out == 'Second\n'

D

print('First')
captured1 = capsys.readouterr()
print('Second')
captured2 = capsys.readouterr()
assert captured1.out == 'First'
assert captured2.out == 'Second'

💡 Hint

Call capsys.readouterr() after each print to capture output separately.

Calling capsys.readouterr() clears the captured output buffer. So capturing after each print separately allows checking outputs independently.

🧠 Conceptual

expert

What happens if you do not call capsys.readouterr() in a pytest test?

In a pytest test function using capsys, if you print output but never call capsys.readouterr(), what is the effect on the test output and captured output?

A The test raises an error because capsys.readouterr() was not called

B The printed output is captured silently and not shown unless the test fails or -s is used

C The printed output goes to the console normally and is not captured by pytest

D The printed output is lost and not shown anywhere

💡 Hint

Think about pytest's default behavior with output capture.

Pytest captures stdout and stderr by default during tests. If capsys.readouterr() is not called, output is captured silently and only shown on failure or with -s option.