Feature request: use caplog with custom formatter

[nicoulaj]

My application uses a quite heavily customized logging formatter.
Right now, the logging plugin only allows to specify the formats for the formatter, not completely replacing it.
It would be nice if there was a clean way to provide a custom formatter.

This is the workaround I use for now, patching the plugin class on the module:

from _pytest import logging as pytest_logging

def pytest_configure(config):
    config.pluginmanager.get_plugin('logging').LoggingPlugin = MyLoggingPlugin

class MyLoggingPlugin(pytest_logging.LoggingPlugin):
    def __init__(self, config):
        super().__init__(config)
        self.formatter = MyCustomFormatter()

[nicoddemus]

Perhaps this can be implemented as a hook:

@hookimpl(firstresult=True)
def pytest_logging_formatter(config):
    """Return a logging.Formatter instance that will be used by the internal logging capture"""

Then you can do this in your root conftest.py file:

def pytest_logging_formatter(config):
    return MyCustomFormatter()

[RonnyPfannschmidt]

as with other topics, i'd like to point to the fact that pytest hooks are a poor mechanism for selecting a "driver"

an ini option/section to select a formatter object and/or its parameters would be nice

[Zac-HD]

Closing this issue as the proposal has been inactive for over a year.

[twmr]

I think this should be reopened. It is needed for #4485.

[samvasko]

Expanding on @nicoulaj's solution, I recommend patching the _create_formatter method to replace the formatter for both caplog and live logging

class StructlogLoggingPlugin(pytest_logging.LoggingPlugin):
    """Replacement logging plugin that uses our formatter
    """
    def _create_formatter(self, log_format,
                          log_date_format) -> logging.Formatter:
        """Patch the logger method to always return out formatter
        Returns:
            logging.Formatter: Our structlog enhanced formatter
        """
        del log_format, log_date_format
        return <your formatter>

[brechin]

One option (definitely not optimal) would be to instantiate your custom formatter and set it explicitly on the caplog handler.

def test_my_function(caplog)
    formatter = MyCustomFormatter()
    caplog.handler.setFormatter(formatter)

    my_function()
    assert caplog.text == ....

[Emantor]

Simply replacing the formatter by retrieving the logging plugin also works.

logging = config.pluginmanager.getplugin('logging-plugin')
logging.log_cli_handler.setFormatter(YourCustomFormatter())
logging.log_file_handler.setFormatter(YourCustomFormatter())

There is only ever one formatter attached to handler, so you are effectively replacing the existing one.

[soxofaan]

One option (definitely not optimal) would be to instantiate your custom formatter and set it explicitly on the caplog handler.

def test_my_function(caplog)
   formatter = MyCustomFormatter()
   caplog.handler.setFormatter(formatter)

I've used this pattern, but just got bitten by an undesired side-effect: subsequent tests that also use caplog will also get the custom formatter because the original formatter is not automatically restored on tear down.

A slightly improved version that does take care of restoring the original formatter is as follows:

def test_my_function(caplog, monkeypatch)
    monkeypatch.setattr(caplog.handler, "formatter", MyCustomFormatter())

    my_function()
    assert caplog.text == ....

[jenstroeger]

Maybe somewhat related to issue #11610 where we use a context manager to (temporarily) add a filter to the fixture…