capsys for capturing output in PyTest - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output captured by capsys?

Consider this pytest test function using capsys to capture printed output. What will be the value of captured.out after the test runs?

PyTest

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

A 'Hello, world!\n'

B 'Hello, world!'

C ''

D 'hello, world!\n'

💡 Hint

Remember that print adds a newline character at the end.

The print function adds a newline character '\n' after the string. capsys.readouterr().out captures exactly what was printed, including the newline.

❓ assertion

intermediate

Which assertion correctly checks captured stderr output?

Given this test code snippet, which assertion correctly verifies that the error message 'Error occurred' was printed to standard error?

PyTest

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

A assert captured.err == 'Error occurred\n'

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

C assert 'Error occurred' in captured.out

D assert captured.err == 'error occurred\n'

💡 Hint

Standard error output is captured in captured.err, not captured.out.

The error message is printed to sys.stderr, so capsys.readouterr().err contains the output. The exact string includes a newline.

🔧 Debug

advanced

Why does this test fail to capture output?

This test tries to capture printed output but fails. What is the reason?

PyTest

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

A capsys.readouterr() clears output after reading, so print is lost

B print does not output anything to capture

C captured is read before print, so it captures nothing

D assertion syntax is incorrect

💡 Hint

Think about when capsys.readouterr() is called relative to print.

capsys.readouterr() captures output up to the point it is called. Since it is called before print, it captures nothing. The print output is not captured in captured.

❓ framework

advanced

How to capture output from a function called multiple times?

You want to test a function that prints output multiple times. How do you capture all output using capsys?

PyTest

def greet():
    print('Hello')
    print('World')

def test_greet(capsys):
    greet()
    captured = capsys.readouterr()

A Call capsys.readouterr() before greet() to capture output

B Use capsys.readouterr() only once before test starts

C Call capsys.readouterr() after each print inside greet()

D Call capsys.readouterr() after all prints to capture combined output

💡 Hint

capsys accumulates output until readouterr() is called.

capsys captures all output printed until readouterr() is called. Calling it after greet() captures all prints combined.

🧠 Conceptual

expert

What happens if you call capsys.readouterr() multiple times?

In a pytest test, what is the effect of calling capsys.readouterr() multiple times during the test?

A Only the first call returns output; others return empty strings

B Each call returns output since last call and clears the buffer

C Calling multiple times causes an error

D Each call returns all output from test start without clearing

💡 Hint

Think of capsys.readouterr() like reading and emptying a mailbox.

Each call to capsys.readouterr() returns the output printed since the last call and clears the internal buffer, so subsequent calls only get new output.