capturing_output.md

Capturing output

What is capturing? Some of your tasks may use print statements, have progress bars, require user input, or the libraries you are using show information during execution.

Since the output would pollute the terminal and the information shown by pytask, it captures all the output during execution and attaches it to the report of this task by default.

If the task fails, the output is shown along with the traceback to help you track down the error.

Default stdout/stderr/logging/stdin capturing behavior

Any output sent to stdout and stderr is captured during task execution. pytask displays it only if the task fails in addition to the traceback.

Log records emitted with Python's logging module are also captured during task execution and shown in their own report section for failing tasks.

In addition, stdin is set to a "null" object which will fail on attempts to read from it because it is rarely desired to wait for interactive input when running automated tasks.

By default, capturing is done by intercepting writes to low-level file descriptors. This allows capturing output from simple print statements as well as output from a subprocess started by a task.

!!! seealso

[Manage logging](../how_to_guides/logging.md) for a dedicated guide to captured logs,
live logs, log files, and logging configuration.

Setting capturing methods or disabling capturing

There are three ways in which pytask can perform capturing:

• fd (file descriptor) level capturing (default): All writes going to the operating system file descriptors 1 and 2 will be captured.
• sys level capturing: Only writes to Python files sys.stdout and sys.stderr will be captured. No capturing of writes to file descriptors is performed.
• tee-sys capturing: Python writes to sys.stdout and sys.stderr will be captured. However, the writes will also be passed through to the actual sys.stdout and sys.stderr.

You can influence output-capturing mechanisms from the command line with -s and --capture:

$ pytask -s                  # disable all capturing
$ pytask --capture=sys       # replace sys.stdout/stderr with in-mem files
$ pytask --capture=fd        # also point filedescriptors 1 and 2 to temp file
$ pytask --capture=tee-sys   # combines 'sys' and '-s', capturing sys.stdout/stderr
                             # and passing it along to the actual sys.stdout/stderr

Using print statements for debugging

One primary benefit of the default capturing of stdout/stderr output is that you can use print statements for debugging:

def task_func1():
    assert True


def task_func2():
    print("Debug statement")
    assert False

And running this module will show you precisely the output of the failing function and hide the other one:

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