1. Introduction¶
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.
1.1. 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
1.2. Installation¶
1.2.1. Supported environments¶
Yagot is supported in these environments:
- Operating Systems: Linux, Windows (native, and with UNIX-like environments), OS-X
- Python: 2.7, 3.4, and higher
1.2.2. Installing¶
Prerequisites:
- The Python environment into which you want to install must be the current
Python environment, and must have at least the following Python packages
installed:
- setuptools
- wheel
- pip
- The Python environment into which you want to install must be the current
Python environment, and must have at least the following Python packages
installed:
Install the yagot package and its prerequisite Python packages into the active Python environment:
$ pip install yagot
1.2.3. Installing a different version¶
The examples in the previous sections install the latest version of Yagot that is released on PyPI. This section describes how different versions of Yagot can be installed.
To install an older released version of Yagot, Pip supports specifying a version requirement. The following example installs Yagot version 0.1.0 from PyPI:
$ pip install yagot==0.1.0
If you need to get a certain new functionality or a new fix that is not yet part of a version released to PyPI, Pip supports installation from a Git repository. The following example installs yagot from the current code level in the master branch of the python-yagot repository:
$ pip install git+https://github.com/andy-maier/python-yagot.git@master#egg=yagot
1.2.4. Verifying the installation¶
You can verify that yagot is installed correctly by importing the package into Python (using the Python environment you installed it to):
$ python -c "import yagot; print('ok')"
ok
In addition, you can verify that pytest picks up the yagot plugin, by looking at the pytest help. If it shows the yagot options, the plugin has been properly picked up:
$ pytest --help | grep yagot
--yagot Enables checking for collected and uncollectable
--yagot-leaks-only Limits the checking to only uncollectable (=leak)
--yagot-ignore-types=TYPE[,TYPE[...]]
In case of trouble with the installation, see the Troubleshooting section.