yagot 0.5.0

Yet Another Garbage Object Tracker for Python

Overview

Yagot (Yet Another Garbage Object Tracker) is a tool for Python developers to help find issues with garbage collection and memory leaks:

• It can determine the set of collected objects caused by a function or method.

  Collected objects are objects Python could not immediately release when they became unreachable and that were eventually released by the Python garbage collector. Frequently this is caused by the presence of circular references into which the object to be released is involved. The garbage collector is designed to handle circular references when releasing objects.

  Collected objects are not a problem per se, but they can contribute to large memory use and can often be eliminated.

• It can determine the set of uncollectable objects caused by a function or method.

  Uncollectable objects are objects Python was unable to release during garbage collection, even when running a full collection (i.e. on all generations of the Python generational garbage collector).

  Uncollectable objects remain allocated in the last generation of the garbage collector. On each run on its last generation, the garbage collector attempts to release these objects. It seems to be rare that these continued attempts eventually succeed, so these objects can basically be considered memory leaks.

See section Background for more detailed explanations about object release in Python.

Yagot is simple to use in either of the following ways:

• It provides a pytest plugin named yagot that detects collected and uncollectable objects caused by the test cases. This detection is enabled by specifying command line options or environment variables and does not require modifying the test cases.

• It provides a Python decorator named garbage_checked that detects collected and uncollectable objects caused by the decorated function or method. This allows using Yagot independent of any test framework or with other test frameworks such as nose or unittest.

Yagot works with a normal (non-debug) build of Python.

Installation

To install the latest released version of the yagot package into your active Python environment:

$ pip install yagot

This will also install any prerequisite Python packages.

For more details and alternative ways to install, see Installation.

Usage

Here is an example of how to use Yagot to detect collected objects caused by pytest test cases using the command line options provided by the yagot pytest plugin:

$ cat examples/test_1.py
def test_selfref_dict():
    d1 = dict()
    d1['self'] = d1

$ pytest examples --yagot -k test_1.py
===================================== test session starts ======================================
platform darwin -- Python 3.7.5, pytest-5.3.5, py-1.8.1, pluggy-0.13.1
rootdir: /Users/maiera/PycharmProjects/yagot/python-yagot
plugins: cov-2.8.1, yagot-0.1.0.dev1
yagot: Checking for collected and uncollectable objects, ignoring types: (none)
collected 2 items / 1 deselected / 1 selected

examples/test_1.py .E                                                                    [100%]

============================================ ERRORS ============================================
____________________________ ERROR at teardown of test_selfref_dict ____________________________

item = <Function test_selfref_dict>

    def pytest_runtest_teardown(item):
        """
        py.test hook that is called when tearing down a test item.

        We use this hook to stop tracking and check the track result.
        """
        config = item.config
        enabled = config.getvalue('yagot')
        if enabled:
            import yagot
            tracker = yagot.GarbageTracker.get_tracker()
            tracker.stop()
            location = "{file}::{func}". \
                format(file=item.location[0], func=item.name)
>           assert not tracker.garbage, tracker.assert_message(location)
E           AssertionError:
E             There were 1 collected or uncollectable object(s) caused by function examples/test_1.py::test_selfref_dict:
E
E             1: <class 'dict'> object at 0x10df6ceb0:
E             {'self': <Recursive reference to dict object at 0x10df6ceb0>}
E
E           assert not [{'self': {'self': {'self': {'self': {'self': {...}}}}}}]
E            +  where [{'self': {'self': {'self': {'self': {'self': {...}}}}}}] = <yagot._garbagetracker.GarbageTracker object at 0x10df15f10>.garbage

yagot_pytest/plugin.py:148: AssertionError
=========================== 1 passed, 1 deselected, 1 error in 0.07s ===========================

Here is an example of how to use Yagot to detect collected objects caused by a function using the garbage_checked decorator on the function. The yagot pytest plugin is loaded in this example and it presence is reported by pytest, but it is not used:

$ cat examples/test_2.py
import yagot

@yagot.garbage_checked()
def test_selfref_dict():
    d1 = dict()
    d1['self'] = d1

$ pytest examples -k test_2.py
===================================== test session starts ======================================
platform darwin -- Python 3.7.5, pytest-5.3.5, py-1.8.1, pluggy-0.13.1
rootdir: /Users/maiera/PycharmProjects/yagot/python-yagot
plugins: cov-2.8.1, yagot-0.1.0.dev1
collected 2 items / 1 deselected / 1 selected

examples/test_2.py F                                                                     [100%]

=========================================== FAILURES ===========================================
______________________________________ test_selfref_dict _______________________________________

args = (), kwargs = {}, tracker = <yagot._garbagetracker.GarbageTracker object at 0x1078853d0>
ret = None, location = 'test_2::test_selfref_dict'
@py_assert1 = [{'self': {'self': {'self': {'self': {'self': {...}}}}}}], @py_assert3 = False
@py_format4 = "\n~There were 1 collected or uncollectable object(s) caused by function test_2::test_selfref_dict:\n~\n~1: <class 'di...elf': {'self': {'self': {'self': {...}}}}}}] = <yagot._garbagetracker.GarbageTracker object at 0x1078853d0>.garbage\n}"

    @functools.wraps(func)
    def wrapper_garbage_checked(*args, **kwargs):
        "Wrapper function for the garbage_checked decorator"
        tracker = GarbageTracker.get_tracker()
        tracker.enable(leaks_only=leaks_only)
        tracker.start()
        tracker.ignore_types(type_list=ignore_types)
        ret = func(*args, **kwargs)  # The decorated function
        tracker.stop()
        location = "{module}::{function}".format(
            module=func.__module__, function=func.__name__)
>       assert not tracker.garbage, tracker.assert_message(location)
E       AssertionError:
E         There were 1 collected or uncollectable object(s) caused by function test_2::test_selfref_dict:
E
E         1: <class 'dict'> object at 0x1078843c0:
E         {'self': <Recursive reference to dict object at 0x1078843c0>}
E
E       assert not [{'self': {'self': {'self': {'self': {'self': {...}}}}}}]
E        +  where [{'self': {'self': {'self': {'self': {'self': {...}}}}}}] = <yagot._garbagetracker.GarbageTracker object at 0x1078853d0>.garbage

yagot/_decorators.py:67: AssertionError
=============================== 1 failed, 1 deselected in 0.07s ================================

In both usages, Yagot reports that there was one collected or uncollectable object caused by the test function. The assertion message provides some details about that object. In this case, we can see that the object is a dict object, and that its ‘self’ item references back to the same dict object, so there was a circular reference that caused the object to become a collectable object.

That circular reference is simple enough for the Python garbage collector to break it up, so this object does not become uncollectable.

The failure location and source code shown by pytest is the wrapper function of the garbage_checked decorator and the pytest_runtest_teardown function since this is where it is detected. The decorated function or pytest test case that caused the objects to be created is reported in the assertion message using a “module::function” notation.

Knowing the test function test_selfref_dict() that caused the object to become a collectable object is a good start for identifying the problem code, and in our example case it is easy to do because the test function is simple enough. If the test function is too complex to identify the culprit, it can be split into multiple simpler test functions, or new test functions can be added to check out specific types of objects that were used.

As an exercise, test the standard dict class and the collections.OrderedDict class by creating empty dictionaries. You will find that on CPython 2.7, collections.OrderedDict causes collected objects (see issue9825).

The garbage_checked decorator can be combined with any other decorators in any order. Note that it always tracks the next inner function, so unless you want to track what garbage other decorators create, you want to have it directly on the test function, as the innermost decorator, like in the following example:

import pytest
import yagot

@pytest.mark.parametrize('parm2', [ ... ])
@pytest.mark.parametrize('parm1', [ ... ])
@yagot.garbage_checked()
def test_something(parm1, parm2):
    pass  # some test code

Documentation

• Documentation

Change History

• Change history

Contributing

For information on how to contribute to the Yagot project, see Contributing.

License

The Yagot project is provided under the Apache Software License 2.0.