Logging within pytest tests

Question

I would like to put some logging statements within test function to examine some state variables.

I have the following code snippet:

import pytest,os
i         


        

Answer 1

Works for me, here's the output I get: [snip -> example was incorrect]

Edit: It seems that you have to pass the -s option to py.test so it won't capture stdout. Here (py.test not installed), it was enough to use python pytest.py -s pyt.py.

For your code, all you need is to pass -s in args to main:

 pytest.main(args=['-s', os.path.abspath(__file__)])

See the py.test documentation on capturing output.

Answer 2

Since version 3.3, pytest supports live logging, meaning that all the log records emitted in tests will be printed to the terminal immediately. The feature is documented under Live Logs section. Live logging is disabled by default; to enable it, set log_cli = 1 in the pytest.ini config¹. Live logging supports emitting to terminal and file; the relevant options allow records customizing:

terminal:

• log_cli_level
• log_cli_format
• log_cli_date_format

file:

• log_file
• log_file_level
• log_file_format
• log_file_date_format

Note: log_cli flag can't be passed from command line and must be set in pytest.ini. All the other options can be both passed from command line or set in the config file. As pointed out by Kévin Barré in this comment, overriding ini options from command line can be done via the -o/--override option. So instead of declaring log_cli in pytest.ini, you can simply call:

$ pytest -o log_cli=true ...

Examples

Simple test file used for demonstrating:

# test_spam.py

import logging

LOGGER = logging.getLogger(__name__)


def test_eggs():
    LOGGER.info('eggs info')
    LOGGER.warning('eggs warning')
    LOGGER.error('eggs error')
    LOGGER.critical('eggs critical')
    assert True

As you can see, no extra configuration needed; pytest will setup the logger automatically, based on options specified in pytest.ini or passed from command line.

Live logging to terminal, INFO level, fancy output

Configuration in pytest.ini:

[pytest]
log_cli = 1
log_cli_level = INFO
log_cli_format = %(asctime)s [%(levelname)8s] %(message)s (%(filename)s:%(lineno)s)
log_cli_date_format=%Y-%m-%d %H:%M:%S

Running the test:

$ pytest test_spam.py
=============================== test session starts ================================
platform darwin -- Python 3.6.4, pytest-3.7.0, py-1.5.3, pluggy-0.7.1 -- /Users/hoefling/.virtualenvs/stackoverflow/bin/python3.6
cachedir: .pytest_cache
rootdir: /Users/hoefling/projects/private/stackoverflow/so-4673373, inifile: pytest.ini
collected 1 item

test_spam.py::test_eggs
---------------------------------- live log call -----------------------------------
2018-08-01 14:33:20 [    INFO] eggs info (test_spam.py:7)
2018-08-01 14:33:20 [ WARNING] eggs warning (test_spam.py:8)
2018-08-01 14:33:20 [   ERROR] eggs error (test_spam.py:9)
2018-08-01 14:33:20 [CRITICAL] eggs critical (test_spam.py:10)
PASSED                                                                        [100%]

============================= 1 passed in 0.01 seconds =============================

Live logging to terminal and file, only message & CRITICAL level in terminal, fancy output in pytest.log file

Configuration in pytest.ini:

[pytest]
log_cli = 1
log_cli_level = CRITICAL
log_cli_format = %(message)s

log_file = pytest.log
log_file_level = DEBUG
log_file_format = %(asctime)s [%(levelname)8s] %(message)s (%(filename)s:%(lineno)s)
log_file_date_format=%Y-%m-%d %H:%M:%S

Test run:

$ pytest test_spam.py
=============================== test session starts ================================
platform darwin -- Python 3.6.4, pytest-3.7.0, py-1.5.3, pluggy-0.7.1 -- /Users/hoefling/.virtualenvs/stackoverflow/bin/python3.6
cachedir: .pytest_cache
rootdir: /Users/hoefling/projects/private/stackoverflow/so-4673373, inifile: pytest.ini
collected 1 item

test_spam.py::test_eggs
---------------------------------- live log call -----------------------------------
eggs critical
PASSED                                                                        [100%]

============================= 1 passed in 0.01 seconds =============================

$ cat pytest.log
2018-08-01 14:38:09 [    INFO] eggs info (test_spam.py:7)
2018-08-01 14:38:09 [ WARNING] eggs warning (test_spam.py:8)
2018-08-01 14:38:09 [   ERROR] eggs error (test_spam.py:9)
2018-08-01 14:38:09 [CRITICAL] eggs critical (test_spam.py:10)

---

¹ Although you can configure pytest in setup.cfg under the [tool:pytest] section, don't be tempted to do that when you want to provide custom live logging format. Other tools reading setup.cfg might treat stuff like %(message)s as string interpolation and fail. Use pytest.ini to avoid errors.