Usage and Invocations(2)

  12 mins read  

Modifying Python traceback printing

Examples for modifying traceback printing:

pytest --showlocals # show local variables in tracebacks
pytest -l           # show local variables (shortcut)

pytest --tb=auto    # (default) 'long' tracebacks for the first and last
                     # entry, but 'short' style for the other entries
pytest --tb=long    # exhaustive, informative traceback formatting
pytest --tb=short   # shorter traceback format
pytest --tb=line    # only one line per failure
pytest --tb=native  # Python standard library formatting
pytest --tb=no      # no traceback at all

The –full-trace causes very long traces to be printed on error (longer than –tb=long). It also ensures that a stack trace is printed on KeyboardInterrupt (Ctrl+C). This is very useful if the tests are taking too long and you interrupt them with Ctrl+C to find out where the tests are hanging. By default no output will be shown (because KeyboardInterrupt is caught by pytest). By using this option you make sure a trace is shown.

Detailed summary report

The -r flag can be used to display a “short test summary info” at the end of the test session, making it easy in large test suites to get a clear picture of all failures, skips, xfails, etc.

Example:

# content of test_example.py
import pytest


@pytest.fixture
def error_fixture():
    assert 0


def test_ok():
    print("ok")


def test_fail():
    assert 0


def test_error(error_fixture):
    pass


def test_skip():
    pytest.skip("skipping this test")


def test_xfail():
    pytest.xfail("xfailing this test")


@pytest.mark.xfail(reason="always xfail")
def test_xpass():
    pass
$ pytest -ra
=========================== test session starts ============================
platform linux -- Python 3.x.y, pytest-4.x.y, py-1.x.y, pluggy-0.x.y
cachedir: $PYTHON_PREFIX/.pytest_cache
rootdir: $REGENDOC_TMPDIR
collected 6 items

test_example.py .FEsxX                                               [100%]

================================== ERRORS ==================================
_______________________ ERROR at setup of test_error _______________________

    @pytest.fixture
    def error_fixture():
>       assert 0
E       assert 0

test_example.py:6: AssertionError
================================= FAILURES =================================
________________________________ test_fail _________________________________

    def test_fail():
>       assert 0
E       assert 0

test_example.py:14: AssertionError
========================= short test summary info ==========================
SKIPPED [1] $REGENDOC_TMPDIR/test_example.py:23: skipping this test
XFAIL test_example.py::test_xfail
  reason: xfailing this test
XPASS test_example.py::test_xpass always xfail
ERROR test_example.py::test_error - assert 0
FAILED test_example.py::test_fail - assert 0
= 1 failed, 1 passed, 1 skipped, 1 xfailed, 1 xpassed, 1 error in 0.12 seconds =

The -r options accepts a number of characters after it, with a used above meaning “all except passes”.

Here is the full list of available characters that can be used:

  • f - failed
  • E - error
  • s - skipped
  • x - xfailed
  • X - xpassed
  • p - passed
  • P - passed with output
  • a - all except pP
  • A - all More than one character can be used, so for example to only see failed and skipped tests, you can execute:
$ pytest -rfs
=========================== test session starts ============================
platform linux -- Python 3.x.y, pytest-4.x.y, py-1.x.y, pluggy-0.x.y
cachedir: $PYTHON_PREFIX/.pytest_cache
rootdir: $REGENDOC_TMPDIR
collected 6 items

test_example.py .FEsxX                                               [100%]

================================== ERRORS ==================================
_______________________ ERROR at setup of test_error _______________________

    @pytest.fixture
    def error_fixture():
>       assert 0
E       assert 0

test_example.py:6: AssertionError
================================= FAILURES =================================
________________________________ test_fail _________________________________

    def test_fail():
>       assert 0
E       assert 0

test_example.py:14: AssertionError
========================= short test summary info ==========================
FAILED test_example.py::test_fail - assert 0
SKIPPED [1] $REGENDOC_TMPDIR/test_example.py:23: skipping this test
= 1 failed, 1 passed, 1 skipped, 1 xfailed, 1 xpassed, 1 error in 0.12 seconds =

Using p lists the passing tests, whilst P adds an extra section “PASSES” with those tests that passed but had captured output:

$ pytest -rpP
=========================== test session starts ============================
platform linux -- Python 3.x.y, pytest-4.x.y, py-1.x.y, pluggy-0.x.y
cachedir: $PYTHON_PREFIX/.pytest_cache
rootdir: $REGENDOC_TMPDIR
collected 6 items

test_example.py .FEsxX                                               [100%]

================================== ERRORS ==================================
_______________________ ERROR at setup of test_error _______________________

    @pytest.fixture
    def error_fixture():
>       assert 0
E       assert 0

test_example.py:6: AssertionError
================================= FAILURES =================================
________________________________ test_fail _________________________________

    def test_fail():
>       assert 0
E       assert 0

test_example.py:14: AssertionError
================================== PASSES ==================================
_________________________________ test_ok __________________________________
--------------------------- Captured stdout call ---------------------------
ok
========================= short test summary info ==========================
PASSED test_example.py::test_ok
= 1 failed, 1 passed, 1 skipped, 1 xfailed, 1 xpassed, 1 error in 0.12 seconds =

Dropping to PDB (Python Debugger) on failures

Python comes with a builtin Python debugger called PDB. pytest allows one to drop into the PDB prompt via a command line option.

pytest --pdb

This will invoke the Python debugger on every failure (or KeyboardInterrupt). Often you might only want to do this for the first failing test to understand a certain failure situation:

pytest -x --pdb   # drop to PDB on first failure, then end test session
pytest --pdb --maxfail=3  # drop to PDB for first three failures

Note that on any failure the exception information is stored on sys.last_value, sys.last_type and sys. last_traceback. In interactive use, this allows one to drop into postmortem debugging with any debug tool. One can also manually access the exception information, for example:

>>> import sys
>>> sys.last_traceback.tb_lineno
42
>>> sys.last_value
AssertionError('assert result == "ok"',)

Dropping to PDB (Python Debugger) at the start of a test

pytest allows one to drop into the PDB prompt immediately at the start of each test via a command line option:

pytest --trace

This will invoke the Python debugger at the start of every test.

Setting breakpoints

To set a breakpoint in your code use the native Python import pdb;pdb.set_trace() call in your code and pytest automatically disables its output capture for that test:

  • Output capture in other tests is not affected.
  • Any prior test output that has already been captured and will be processed as such.
  • Output capture gets resumed when ending the debugger session (via the continue command).

Using the builtin breakpoint function

Python 3.7 introduces a builtin breakpoint() function. Pytest supports the use of breakpoint() with the following behaviours:

  • When breakpoint() is called and PYTHONBREAKPOINT is set to the default value, pytest will use the custom internal PDB trace UI instead of the system default Pdb.
  • When tests are complete, the system will default back to the system Pdb trace UI.
  • With –pdb passed to pytest, the custom internal Pdb trace UI is used with both breakpoint() and failed tests/unhandled exceptions.
  • –pdbcls can be used to specify a custom debugger class.

Profiling test execution duration

To get a list of the slowest 10 test durations:

pytest --durations=10

By default, pytest will not show test durations that are too small (<0.01s) unless -vv is passed on the command-line.

Creating JUnitXML format files

To create result files which can be read by Jenkins or other Continuous integration servers, use this invocation:

pytest --junitxml=path

to create an XML file at path. To set the name of the root test suite xml item, you can configure the junit_suite_name option in your config file:

[pytest]
junit_suite_name = my_suite

New in version 4.0.

JUnit XML specification seems to indicate that “time” attribute should report total test execution times, including setup and teardown (1, 2). It is the default pytest behavior. To report just call durations instead, configure the junit_duration_report option like this:

[pytest]
junit_duration_report = call

record_property

If you want to log additional information for a test, you can use the record_property fixture:

def test_function(record_property):
    record_property("example_key", 1)
    assert True

This will add an extra property example_key=”1” to the generated testcase tag:

<testcase classname="test_function" file="test_function.py" line="0" name="test_function" time="0.0009">
  <properties>
    <property name="example_key" value="1" />
  </properties>
</testcase>

Alternatively, you can integrate this functionality with custom markers:

# content of conftest.py

def pytest_collection_modifyitems(session, config, items):
    for item in items:
        for marker in item.iter_markers(name="test_id"):
            test_id = marker.args[0]
            item.user_properties.append(("test_id", test_id))

And in your tests:

# content of test_function.py
import pytest


@pytest.mark.test_id(1501)
def test_function():
    assert True

Will result in:

<testcase classname="test_function" file="test_function.py" line="0" name="test_function" time="0.0009">
  <properties>
    <property name="test_id" value="1501" />
  </properties>
</testcase>

* Warning: * Please note that using this feature will break schema verifications for the latest JUnitXML schema.
This might be a problem when used with some CI servers.

record_xml_attribute

To add an additional xml attribute to a testcase element, you can use record_xml_attribute fixture. This can also be used to override existing values:

def test_function(record_xml_attribute):
  record_xml_attribute("assertions", "REQ-1234")
  record_xml_attribute("classname", "custom_classname")
  print("hello world")
  assert True

Unlike record_property, this will not add a new child element. Instead, this will add an attribute assertions=”REQ-1234” inside the generated testcase tag and override the default classname with “classname=custom_classname”:

<testcase classname="custom_classname" file="test_function.py" line="0" name="test_
˓→function" time="0.003" assertions="REQ-1234">
  <system-out>
    hello world
  </system-out>
</testcase>

* Warning: * record_xml_attribute is an experimental feature, and its interface might be replaced by something more powerful and general in future versions. The functionality per-se will be kept, however.
Using this over record_xml_property can help when using ci tools to parse the xml report. However, some
parsers are quite strict about the elements and attributes that are allowed. Many tools use an xsd schema (like the
example below) to validate incoming xml. Make sure you are using attribute names that are allowed by your parser.
Below is the Scheme used by Jenkins to validate the XML report:
<xs:element name="testcase">
  <xs:complexType>
    <xs:sequence>
      <xs:element ref="skipped" minOccurs="0" maxOccurs="1"/>
      <xs:element ref="error" minOccurs="0" maxOccurs="unbounded"/>
      <xs:element ref="failure" minOccurs="0" maxOccurs="unbounded"/>
      <xs:element ref="system-out" minOccurs="0" maxOccurs="unbounded"/>
      <xs:element ref="system-err" minOccurs="0" maxOccurs="unbounded"/>
    </xs:sequence>
    <xs:attribute name="name" type="xs:string" use="required"/>
    <xs:attribute name="assertions" type="xs:string" use="optional"/>
    <xs:attribute name="time" type="xs:string" use="optional"/>
    <xs:attribute name="classname" type="xs:string" use="optional"/>
    <xs:attribute name="status" type="xs:string" use="optional"/>
  </xs:complexType>
</xs:element>

* Warning: * Please note that using this feature will break schema verifications for the latest JUnitXML schema.
This might be a problem when used with some CI servers.

record_testsuite_property

New in version 4.5. If you want to add a properties node at the test-suite level, which may contains properties that are relevant to all tests, you can use the record_testsuite_property session-scoped fixture: The record_testsuite_property session-scoped fixture can be used to add properties relevant to all tests.

import pytest

@pytest.fixture(scope="session", autouse=True)
def log_global_env_facts(record_testsuite_property):
  record_testsuite_property("ARCH", "PPC")
  record_testsuite_property("STORAGE_TYPE", "CEPH")
  
class TestMe(object):
  def test_foo(self):
    assert True

The fixture is a callable which receives name and value of a tag added at the test-suite level of the generated xml:

<testsuite errors="0" failures="0" name="pytest" skipped="0" tests="1" time="0.006">
  <properties>
    <property name="ARCH" value="PPC"/>
    <property name="STORAGE_TYPE" value="CEPH"/>
  </properties>
  <testcase classname="test_me.TestMe" file="test_me.py" line="16" name="test_foo"
˓→time="0.000243663787842"/>
</testsuite>

name must be a string, value will be converted to a string and properly xml-escaped. The generated XML is compatible with the latest xunit standard, contrary to record_property and record_xml_attribute.

Creating resultlog format files

This option is rarely used and is scheduled for removal in 5.0. See the deprecation docs for more information. To create plain-text machine-readable result files you can issue:

pytest --resultlog=path

and look at the content at the path location. Such files are used e.g. by the PyPy-test web page to show test results over several revisions.

Sending test report to online pastebin service

Creating a URL for each test failure:

pytest --pastebin=failed

This will submit test run information to a remote Paste service and provide a URL for each failure. You may select tests as usual or add for example -x if you only want to send one particular failure.

Creating a URL for a whole test session log:

pytest --pastebin=all

Currently only pasting to the http://bpaste.net service is implemented.

Early loading plugins

You can early-load plugins (internal and external) explicitly in the command-line with the -p option:

pytest -p mypluginmodule

The option receives a name parameter, which can be:

  • A full module dotted name, for example myproject.plugins. This dotted name must be importable.
  • The entry-point name of a plugin. This is the name passed to setuptools when the plugin is registered. For example to early-load the pytest-cov plugin you can use:
    pytest -p pytest_cov

    Disabling plugins

    To disable loading specific plugins at invocation time, use the -p option together with the prefix no:. Example: to disable loading the plugin doctest, which is responsible for executing doctest tests from text files, invoke pytest like this:

    pytest -p no:doctest

    Calling pytest from Python code

    You can invoke pytest from Python code directly:

    pytest.main()

    this acts as if you would call “pytest” from the command line. It will not raise SystemExit but return the exitcode instead. You can pass in options and arguments:

    pytest.main(['-x', 'mytestdir'])

    You can specify additional plugins to pytest.main: ```

    content of myinvoke.py

    import pytest class MyPlugin(object): def pytest_sessionfinish(self): print(“*** test run reporting finishing”)

pytest.main([“-qq”], plugins=[MyPlugin()])

Running it will show that MyPlugin was added and its hook was invoked:

$ python myinvoke.py .FEsxX. [100%]*** test ˓→run reporting finishing ================================== ERRORS ================================== _________ ERROR at setup of test_error _________ @pytest.fixture def error_fixture():

assert 0 E assert 0

test_example.py:6: AssertionError ================================= FAILURES ================================= ____________ test_fail _____________ def test_fail():

assert 0 E assert 0 test_example.py:14: AssertionError

  • Note: * Calling pytest.main() will result in importing your tests and any modules that they import. Due to the caching mechanism of python’s import system, making subsequent calls to pytest.main() from the same process will not reflect changes to those files between the calls. For this reason, making multiple calls to pytest.main() from the same process (in order to re-run tests, for example) is not recommended. ```