capfd for file descriptors in PyTest - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the pytest fixture capfd capture?

The capfd fixture captures output sent to the system file descriptors stdout and stderr. This means it captures output from print statements and error messages during test execution.

beginner

How do you retrieve the captured output using capfd in a pytest test?

You call out, err = capfd.readouterr() inside your test. out contains the captured standard output, and err contains the captured standard error.

intermediate

Why is capfd useful compared to capturing output with capsys?

capfd captures output at the file descriptor level, so it can capture output from C extensions or subprocesses, while capsys captures output at the Python sys module level only.

intermediate

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

Each call to <code>readouterr()</code> returns the output captured since the last call and clears the buffer. So multiple calls let you check output in stages.

intermediate

Can capfd capture output from subprocesses spawned during a test?

Yes, because capfd captures output at the file descriptor level, it can capture output from subprocesses, unlike capsys.

What does capfd capture in pytest?

A Only logging messages

B Only Python print statements

C Output sent to system file descriptors stdout and stderr

D Only exceptions

capfd captures output sent to the system file descriptors stdout and stderr, including print statements and errors.

How do you get the captured output from capfd?

A Using <code>capfd.readouterr()</code>

B Using <code>capfd.get_output()</code>

C Using <code>capfd.capture()</code>

D Using <code>capfd.output()</code>

The method readouterr() returns a tuple with captured stdout and stderr.

Which is true about capfd compared to capsys?

A <code>capsys</code> captures output at the file descriptor level

B <code>capfd</code> only captures Python sys.stdout

C <code>capsys</code> captures subprocess output

D <code>capfd</code> captures output at the file descriptor level

capfd captures output at the OS file descriptor level, so it can capture subprocess output.

What happens when you call capfd.readouterr() multiple times?

A It returns output since last call and clears buffer

B It returns the same output every time

C It raises an error

D It disables output capture

Each call returns new output since last call and clears the captured output buffer.

Can capfd capture output from subprocesses?

A No, it only captures Python output

B Yes, because it captures at file descriptor level

C Only if subprocess uses Python print

D Only if subprocess is run with special flags

capfd captures output at the OS file descriptor level, so it captures subprocess output.

Explain how the pytest fixture capfd works and when you would use it.

Think about capturing output beyond just Python print statements.

Describe the difference between capfd and capsys in pytest.

Consider the level at which output is captured.