FM / library

/usr/share/doc/python3-docs/html/library/

/usr/share/doc/python3-docs/html/library

Name	Size	Mode	Actions
2to3.html	61709	0644	edit dl rm
abc.html	41394	0644	edit dl rm
aifc.html	28114	0644	edit dl rm
allos.html	32236	0644	edit dl rm
archiving.html	11992	0644	edit dl rm
argparse.html	274984	0644	edit dl rm
array.html	33547	0644	edit dl rm
ast.html	39595	0644	edit dl rm
asynchat.html	32546	0644	edit dl rm
asyncio-dev.html	51869	0644	edit dl rm
asyncio-eventloop.html	111215	0644	edit dl rm
asyncio-eventloops.html	33026	0644	edit dl rm
asyncio-protocol.html	87431	0644	edit dl rm
asyncio-queue.html	23957	0644	edit dl rm
asyncio-stream.html	61005	0644	edit dl rm
asyncio-subprocess.html	58015	0644	edit dl rm
asyncio-sync.html	35678	0644	edit dl rm
asyncio-task.html	83968	0644	edit dl rm
asyncio.html	28026	0644	edit dl rm
asyncore.html	42706	0644	edit dl rm
atexit.html	18433	0644	edit dl rm
audioop.html	35442	0644	edit dl rm
base64.html	36454	0644	edit dl rm
bdb.html	43423	0644	edit dl rm
binary.html	12723	0644	edit dl rm
binascii.html	24745	0644	edit dl rm
binhex.html	11669	0644	edit dl rm
bisect.html	25602	0644	edit dl rm
builtins.html	11604	0644	edit dl rm
bz2.html	33358	0644	edit dl rm
calendar.html	39539	0644	edit dl rm
cgi.html	55881	0644	edit dl rm
cgitb.html	12765	0644	edit dl rm
chunk.html	16872	0644	edit dl rm
cmath.html	34171	0644	edit dl rm
cmd.html	43811	0644	edit dl rm
code.html	27886	0644	edit dl rm
codecs.html	134071	0644	edit dl rm
codeop.html	14304	0644	edit dl rm
collections.abc.html	60912	0644	edit dl rm
collections.html	154946	0644	edit dl rm
colorsys.html	12627	0644	edit dl rm
compileall.html	28263	0644	edit dl rm
concurrency.html	21203	0644	edit dl rm
concurrent.futures.html	60660	0644	edit dl rm
concurrent.html	7959	0644	edit dl rm
configparser.html	141052	0644	edit dl rm
constants.html	17500	0644	edit dl rm
contextlib.html	88801	0644	edit dl rm
copy.html	13841	0644	edit dl rm
copyreg.html	14725	0644	edit dl rm
crypt.html	20331	0644	edit dl rm
crypto.html	10914	0644	edit dl rm
csv.html	69043	0644	edit dl rm
ctypes.html	264529	0644	edit dl rm
curses.ascii.html	25197	0644	edit dl rm
curses.html	175551	0644	edit dl rm
curses.panel.html	16706	0644	edit dl rm
custominterp.html	8734	0644	edit dl rm
datatypes.html	23950	0644	edit dl rm
datetime.html	306435	0644	edit dl rm
dbm.html	43119	0644	edit dl rm
debug.html	13983	0644	edit dl rm
decimal.html	233486	0644	edit dl rm
development.html	25123	0644	edit dl rm
difflib.html	96156	0644	edit dl rm
dis.html	106191	0644	edit dl rm
distribution.html	10506	0644	edit dl rm
distutils.html	10884	0644	edit dl rm
doctest.html	179752	0644	edit dl rm
dummy_threading.html	9316	0644	edit dl rm
email.charset.html	26431	0644	edit dl rm
email.compat32-message.html	86150	0644	edit dl rm
email.contentmanager.html	30028	0644	edit dl rm
email.encoders.html	14250	0644	edit dl rm
email.errors.html	19704	0644	edit dl rm
email.examples.html	57872	0644	edit dl rm
email.generator.html	41798	0644	edit dl rm
email.header.html	31219	0644	edit dl rm
email.headerregistry.html	57670	0644	edit dl rm
email.html	27741	0644	edit dl rm
email.iterators.html	13532	0644	edit dl rm
email.message.html	87210	0644	edit dl rm
email.mime.html	36081	0644	edit dl rm
email.parser.html	47430	0644	edit dl rm
email.policy.html	75241	0644	edit dl rm
email.util.html	33853	0644	edit dl rm
ensurepip.html	18700	0644	edit dl rm
enum.html	133949	0644	edit dl rm
errno.html	41331	0644	edit dl rm
exceptions.html	85151	0644	edit dl rm
faulthandler.html	23396	0644	edit dl rm
fcntl.html	27321	0644	edit dl rm
filecmp.html	25036	0644	edit dl rm
fileformats.html	11129	0644	edit dl rm
fileinput.html	30030	0644	edit dl rm
filesys.html	13418	0644	edit dl rm
fnmatch.html	16987	0644	edit dl rm
formatter.html	37666	0644	edit dl rm
fpectl.html	17538	0644	edit dl rm
fractions.html	29918	0644	edit dl rm
frameworks.html	13471	0644	edit dl rm
ftplib.html	54429	0644	edit dl rm
functional.html	9002	0644	edit dl rm
functions.html	201486	0644	edit dl rm
functools.html	64951	0644	edit dl rm
gc.html	29524	0644	edit dl rm
getopt.html	25353	0644	edit dl rm
getpass.html	12061	0644	edit dl rm
gettext.html	76641	0644	edit dl rm
glob.html	18432	0644	edit dl rm
grp.html	12383	0644	edit dl rm
gzip.html	32669	0644	edit dl rm
hashlib.html	79554	0644	edit dl rm
heapq.html	35529	0644	edit dl rm
hmac.html	17827	0644	edit dl rm
html.entities.html	11361	0644	edit dl rm
html.html	11108	0644	edit dl rm
html.parser.html	41842	0644	edit dl rm
http.client.html	65250	0644	edit dl rm
http.cookiejar.html	96195	0644	edit dl rm
http.cookies.html	39329	0644	edit dl rm
http.html	37602	0644	edit dl rm
http.server.html	58191	0644	edit dl rm
i18n.html	10464	0644	edit dl rm
idle.html	48412	0644	edit dl rm
imaplib.html	67339	0644	edit dl rm
imghdr.html	13456	0644	edit dl rm
imp.html	48153	0644	edit dl rm
importlib.html	173109	0644	edit dl rm
index.html	67768	0644	edit dl rm
inspect.html	130355	0644	edit dl rm
internet.html	31087	0644	edit dl rm
intro.html	9904	0644	edit dl rm
io.html	122685	0644	edit dl rm
ipaddress.html	105873	0644	edit dl rm
ipc.html	33743	0644	edit dl rm
itertools.html	132824	0644	edit dl rm
json.html	81078	0644	edit dl rm
keyword.html	8918	0644	edit dl rm
language.html	13648	0644	edit dl rm
linecache.html	13492	0644	edit dl rm
locale.html	61540	0644	edit dl rm
logging.config.html	85489	0644	edit dl rm
logging.handlers.html	112397	0644	edit dl rm
logging.html	134850	0644	edit dl rm
lzma.html	56484	0644	edit dl rm
macpath.html	9257	0644	edit dl rm
mailbox.html	157315	0644	edit dl rm
mailcap.html	14676	0644	edit dl rm
markup.html	19356	0644	edit dl rm
marshal.html	18317	0644	edit dl rm
math.html	53661	0644	edit dl rm
mimetypes.html	31410	0644	edit dl rm
misc.html	8123	0644	edit dl rm
mm.html	10022	0644	edit dl rm
mmap.html	34294	0644	edit dl rm
modulefinder.html	18559	0644	edit dl rm
modules.html	10914	0644	edit dl rm
msilib.html	57205	0644	edit dl rm
msvcrt.html	21989	0644	edit dl rm
multiprocessing.html	365487	0644	edit dl rm
netdata.html	17470	0644	edit dl rm
netrc.html	14568	0644	edit dl rm
nis.html	11659	0644	edit dl rm
nntplib.html	69601	0644	edit dl rm
numbers.html	41432	0644	edit dl rm
numeric.html	15535	0644	edit dl rm
operator.html	81460	0644	edit dl rm
optparse.html	249942	0644	edit dl rm
os.html	398039	0644	edit dl rm
os.path.html	53818	0644	edit dl rm
ossaudiodev.html	47947	0644	edit dl rm
othergui.html	10256	0644	edit dl rm
parser.html	42408	0644	edit dl rm
pathlib.html	116726	0644	edit dl rm
pdb.html	52661	0644	edit dl rm
persistence.html	15281	0644	edit dl rm
pickle.html	115052	0644	edit dl rm
pickletools.html	17094	0644	edit dl rm
pipes.html	15991	0644	edit dl rm
pkgutil.html	31878	0644	edit dl rm
platform.html	33728	0644	edit dl rm
plistlib.html	29290	0644	edit dl rm
poplib.html	30940	0644	edit dl rm
posix.html	17608	0644	edit dl rm
pprint.html	41969	0644	edit dl rm
profile.html	71848	0644	edit dl rm
pty.html	17594	0644	edit dl rm
pwd.html	13098	0644	edit dl rm
pyclbr.html	16520	0644	edit dl rm
pydoc.html	15991	0644	edit dl rm
pyexpat.html	88938	0644	edit dl rm
python.html	17133	0644	edit dl rm
py_compile.html	15508	0644	edit dl rm
queue.html	28199	0644	edit dl rm
quopri.html	13858	0644	edit dl rm
random.html	60646	0644	edit dl rm
re.html	191176	0644	edit dl rm
readline.html	42133	0644	edit dl rm
reprlib.html	24964	0644	edit dl rm
resource.html	36552	0644	edit dl rm
rlcompleter.html	14392	0644	edit dl rm
runpy.html	26607	0644	edit dl rm
sched.html	20034	0644	edit dl rm
secrets.html	24734	0644	edit dl rm
select.html	62819	0644	edit dl rm
selectors.html	35845	0644	edit dl rm
shelve.html	28777	0644	edit dl rm
shlex.html	48979	0644	edit dl rm
shutil.html	74207	0644	edit dl rm
signal.html	55064	0644	edit dl rm
site.html	32242	0644	edit dl rm
smtpd.html	35892	0644	edit dl rm
smtplib.html	68433	0644	edit dl rm
sndhdr.html	12439	0644	edit dl rm
socket.html	196077	0644	edit dl rm
socketserver.html	81918	0644	edit dl rm
spwd.html	12328	0644	edit dl rm
sqlite3.html	161872	0644	edit dl rm
ssl.html	266327	0644	edit dl rm
stat.html	44722	0644	edit dl rm
statistics.html	50050	0644	edit dl rm
stdtypes.html	465658	0644	edit dl rm
string.html	95262	0644	edit dl rm
stringprep.html	18654	0644	edit dl rm
struct.html	51100	0644	edit dl rm
subprocess.html	141926	0644	edit dl rm
sunau.html	31757	0644	edit dl rm
superseded.html	13781	0644	edit dl rm
symbol.html	8848	0644	edit dl rm
symtable.html	26112	0644	edit dl rm
sys.html	141670	0644	edit dl rm
sysconfig.html	30259	0644	edit dl rm
syslog.html	21026	0644	edit dl rm
tabnanny.html	12269	0644	edit dl rm
tarfile.html	96758	0644	edit dl rm
telnetlib.html	30409	0644	edit dl rm
tempfile.html	41394	0644	edit dl rm
termios.html	18181	0644	edit dl rm
test.html	78555	0644	edit dl rm
text.html	13786	0644	edit dl rm
textwrap.html	37998	0644	edit dl rm
threading.html	103464	0644	edit dl rm
time.html	79629	0644	edit dl rm
timeit.html	45249	0644	edit dl rm
tk.html	23299	0644	edit dl rm
tkinter.html	82932	0644	edit dl rm
tkinter.scrolledtext.html	9957	0644	edit dl rm
tkinter.tix.html	51858	0644	edit dl rm
tkinter.ttk.html	110673	0644	edit dl rm
token.html	22660	0644	edit dl rm
tokenize.html	32149	0644	edit dl rm
trace.html	29145	0644	edit dl rm
traceback.html	57588	0644	edit dl rm
tracemalloc.html	101144	0644	edit dl rm
tty.html	10540	0644	edit dl rm
turtle.html	250367	0644	edit dl rm
types.html	37060	0644	edit dl rm
typing.html	139175	0644	edit dl rm
undoc.html	9181	0644	edit dl rm
unicodedata.html	21108	0644	edit dl rm
unittest.html	250689	0644	edit dl rm
unittest.mock-examples.html	168144	0644	edit dl rm
unittest.mock.html	300089	0644	edit dl rm
unix.html	11780	0644	edit dl rm
urllib.error.html	14474	0644	edit dl rm
urllib.html	9566	0644	edit dl rm
urllib.parse.html	76334	0644	edit dl rm
urllib.request.html	177199	0644	edit dl rm
urllib.robotparser.html	17156	0644	edit dl rm
uu.html	12209	0644	edit dl rm
uuid.html	31957	0644	edit dl rm
venv.html	73909	0644	edit dl rm
warnings.html	50320	0644	edit dl rm
wave.html	29868	0644	edit dl rm
weakref.html	65292	0644	edit dl rm
webbrowser.html	26901	0644	edit dl rm
windows.html	10626	0644	edit dl rm
winreg.html	66730	0644	edit dl rm
winsound.html	21268	0644	edit dl rm
wsgiref.html	94492	0644	edit dl rm
xdrlib.html	33751	0644	edit dl rm
xml.dom.html	97570	0644	edit dl rm
xml.dom.minidom.html	41481	0644	edit dl rm
xml.dom.pulldom.html	24214	0644	edit dl rm
xml.etree.elementtree.html	135491	0644	edit dl rm
xml.html	19261	0644	edit dl rm
xml.sax.handler.html	42659	0644	edit dl rm
xml.sax.html	25515	0644	edit dl rm
xml.sax.reader.html	45869	0644	edit dl rm
xml.sax.utils.html	16928	0644	edit dl rm
xmlrpc.client.html	71947	0644	edit dl rm
xmlrpc.html	8535	0644	edit dl rm
xmlrpc.server.html	49195	0644	edit dl rm
zipapp.html	45105	0644	edit dl rm
zipfile.html	80039	0644	edit dl rm
zipimport.html	23052	0644	edit dl rm
zlib.html	34557	0644	edit dl rm
_dummy_thread.html	9193	0644	edit dl rm
_thread.html	22588	0644	edit dl rm
__future__.html	15792	0644	edit dl rm
__main__.html	9019	0644	edit dl rm

Edit: /usr/share/doc/python3-docs/html/library/weakref.html (65292B)

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
 "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">

<html xmlns="http://www.w3.org/1999/xhtml">
 <head>
 <meta http-equiv="X-UA-Compatible" content="IE=Edge" />
 <meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
 <title>8.8. weakref — Weak references &#8212; Python 3.6.7 documentation</title>
 <link rel="stylesheet" href="../_static/pydoctheme.css" type="text/css" />
 <link rel="stylesheet" href="../_static/pygments.css" type="text/css" />
 <script type="text/javascript" id="documentation_options" data-url_root="../" src="../_static/documentation_options.js"></script>
 <script type="text/javascript" src="../_static/jquery.js"></script>
 <script type="text/javascript" src="../_static/underscore.js"></script>
 <script type="text/javascript" src="../_static/doctools.js"></script>
 <script type="text/javascript" src="../_static/sidebar.js"></script>
 <link rel="search" type="application/opensearchdescription+xml"
 title="Search within Python 3.6.7 documentation"
 href="../_static/opensearch.xml"/>
 <link rel="author" title="About these documents" href="../about.html" />
 <link rel="index" title="Index" href="../genindex.html" />
 <link rel="search" title="Search" href="../search.html" />
 <link rel="copyright" title="Copyright" href="../copyright.html" />
 <link rel="next" title="8.9. types — Dynamic type creation and names for built-in types" href="types.html" />
 <link rel="prev" title="8.7. array — Efficient arrays of numeric values" href="array.html" />
 <link rel="shortcut icon" type="image/png" href="../_static/py.png" />
 <link rel="canonical" href="https://docs.python.org/3/library/weakref.html" />
 
 <script type="text/javascript" src="../_static/copybutton.js"></script>

</head><body> 
 <div class="related" role="navigation" aria-label="related navigation">
 <h3>Navigation</h3>
 <ul>
 <li class="right" style="margin-right: 10px">
 <a href="../genindex.html" title="General Index"
 accesskey="I">index</a></li>
 <li class="right" >
 <a href="../py-modindex.html" title="Python Module Index"
 >modules</a> |</li>
 <li class="right" >
 <a href="types.html" title="8.9. types — Dynamic type creation and names for built-in types"
 accesskey="N">next</a> |</li>
 <li class="right" >
 <a href="array.html" title="8.7. array — Efficient arrays of numeric values"
 accesskey="P">previous</a> |</li>
 <li><img src="../_static/py.png" alt=""
 style="vertical-align: middle; margin-top: -1px"/></li>
 <li><a href="https://www.python.org/">Python</a> &#187;</li>
 <li>
 <a href="../index.html">3.6.7 Documentation</a> &#187;
 </li>

<li class="nav-item nav-item-1"><a href="index.html" >The Python Standard Library</a> &#187;</li>
 <li class="nav-item nav-item-2"><a href="datatypes.html" accesskey="U">8. Data Types</a> &#187;</li>
 <li class="right">

<div class="inline-search" style="display: none" role="search">
 <form class="inline-search" action="../search.html" method="get">
 <input placeholder="Quick search" type="text" name="q" />
 <input type="submit" value="Go" />
 <input type="hidden" name="check_keywords" value="yes" />
 <input type="hidden" name="area" value="default" />
 </form>
 </div>
 <script type="text/javascript">$('.inline-search').show(0);</script>
 |
 </li>

</ul>
 </div>

<div class="document">
 <div class="documentwrapper">
 <div class="bodywrapper">
 <div class="body" role="main">
 
 <div class="section" id="module-weakref">
<h1>8.8. <a class="reference internal" href="#module-weakref" title="weakref: Support for weak references and weak dictionaries."><code class="xref py py-mod docutils literal notranslate">weakref</code></a> — Weak references<a class="headerlink" href="#module-weakref" title="Permalink to this headline">¶</a></h1>
Source code: <a class="reference external" href="https://github.com/python/cpython/tree/3.6/Lib/weakref.py">Lib/weakref.py</a>
<hr class="docutils" />
The <a class="reference internal" href="#module-weakref" title="weakref: Support for weak references and weak dictionaries."><code class="xref py py-mod docutils literal notranslate">weakref</code></a> module allows the Python programmer to create weak
references to objects.
In the following, the term referent means the object which is referred to
by a weak reference.
A weak reference to an object is not enough to keep the object alive: when the
only remaining references to a referent are weak references,
<a class="reference internal" href="../glossary.html#term-garbage-collection">garbage collection</a> is free to destroy the referent and reuse its memory
for something else. However, until the object is actually destroyed the weak
reference may return the object even if there are no strong references to it.
A primary use for weak references is to implement caches or
mappings holding large objects, where it’s desired that a large object not be
kept alive solely because it appears in a cache or mapping.
For example, if you have a number of large binary image objects, you may wish to
associate a name with each. If you used a Python dictionary to map names to
images, or images to names, the image objects would remain alive just because
they appeared as values or keys in the dictionaries. The
<a class="reference internal" href="#weakref.WeakKeyDictionary" title="weakref.WeakKeyDictionary"><code class="xref py py-class docutils literal notranslate">WeakKeyDictionary</code></a> and <a class="reference internal" href="#weakref.WeakValueDictionary" title="weakref.WeakValueDictionary"><code class="xref py py-class docutils literal notranslate">WeakValueDictionary</code></a> classes supplied by
the <a class="reference internal" href="#module-weakref" title="weakref: Support for weak references and weak dictionaries."><code class="xref py py-mod docutils literal notranslate">weakref</code></a> module are an alternative, using weak references to construct
mappings that don’t keep objects alive solely because they appear in the mapping
objects. If, for example, an image object is a value in a
<a class="reference internal" href="#weakref.WeakValueDictionary" title="weakref.WeakValueDictionary"><code class="xref py py-class docutils literal notranslate">WeakValueDictionary</code></a>, then when the last remaining references to that
image object are the weak references held by weak mappings, garbage collection
can reclaim the object, and its corresponding entries in weak mappings are
simply deleted.
<a class="reference internal" href="#weakref.WeakKeyDictionary" title="weakref.WeakKeyDictionary"><code class="xref py py-class docutils literal notranslate">WeakKeyDictionary</code></a> and <a class="reference internal" href="#weakref.WeakValueDictionary" title="weakref.WeakValueDictionary"><code class="xref py py-class docutils literal notranslate">WeakValueDictionary</code></a> use weak references
in their implementation, setting up callback functions on the weak references
that notify the weak dictionaries when a key or value has been reclaimed by
garbage collection. <a class="reference internal" href="#weakref.WeakSet" title="weakref.WeakSet"><code class="xref py py-class docutils literal notranslate">WeakSet</code></a> implements the <a class="reference internal" href="stdtypes.html#set" title="set"><code class="xref py py-class docutils literal notranslate">set</code></a> interface,
but keeps weak references to its elements, just like a
<a class="reference internal" href="#weakref.WeakKeyDictionary" title="weakref.WeakKeyDictionary"><code class="xref py py-class docutils literal notranslate">WeakKeyDictionary</code></a> does.
<a class="reference internal" href="#weakref.finalize" title="weakref.finalize"><code class="xref py py-class docutils literal notranslate">finalize</code></a> provides a straight forward way to register a
cleanup function to be called when an object is garbage collected.
This is simpler to use than setting up a callback function on a raw
weak reference, since the module automatically ensures that the finalizer
remains alive until the object is collected.
Most programs should find that using one of these weak container types
or <a class="reference internal" href="#weakref.finalize" title="weakref.finalize"><code class="xref py py-class docutils literal notranslate">finalize</code></a> is all they need – it’s not usually necessary to
create your own weak references directly. The low-level machinery is
exposed by the <a class="reference internal" href="#module-weakref" title="weakref: Support for weak references and weak dictionaries."><code class="xref py py-mod docutils literal notranslate">weakref</code></a> module for the benefit of advanced uses.
Not all objects can be weakly referenced; those objects which can include class
instances, functions written in Python (but not in C), instance methods, sets,
frozensets, some <a class="reference internal" href="../glossary.html#term-file-object">file objects</a>, <a class="reference internal" href="../glossary.html#term-generator">generator</a>s, type
objects, sockets, arrays, deques, regular expression pattern objects, and code
objects.
<div class="versionchanged">
Changed in version 3.2: Added support for thread.lock, threading.Lock, and code objects.
</div>
Several built-in types such as <a class="reference internal" href="stdtypes.html#list" title="list"><code class="xref py py-class docutils literal notranslate">list</code></a> and <a class="reference internal" href="stdtypes.html#dict" title="dict"><code class="xref py py-class docutils literal notranslate">dict</code></a> do not directly
support weak references but can add support through subclassing:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>class Dict(dict):
 pass

obj = Dict(red=1, green=2, blue=3) # this object is weak referenceable
</pre></div>
</div>
Other built-in types such as <a class="reference internal" href="stdtypes.html#tuple" title="tuple"><code class="xref py py-class docutils literal notranslate">tuple</code></a> and <a class="reference internal" href="functions.html#int" title="int"><code class="xref py py-class docutils literal notranslate">int</code></a> do not support weak
references even when subclassed (This is an implementation detail and may be
different across various Python implementations.).
Extension types can easily be made to support weak references; see
<a class="reference internal" href="../extending/newtypes.html#weakref-support">Weak Reference Support</a>.
<dl class="class">
<dt id="weakref.ref">
class <code class="descclassname">weakref.</code><code class="descname">ref</code>(object[, callback])<a class="headerlink" href="#weakref.ref" title="Permalink to this definition">¶</a></dt>
<dd>Return a weak reference to object. The original object can be retrieved by
calling the reference object if the referent is still alive; if the referent is
no longer alive, calling the reference object will cause <a class="reference internal" href="constants.html#None" title="None"><code class="xref py py-const docutils literal notranslate">None</code></a> to be
returned. If callback is provided and not <a class="reference internal" href="constants.html#None" title="None"><code class="xref py py-const docutils literal notranslate">None</code></a>, and the returned
weakref object is still alive, the callback will be called when the object is
about to be finalized; the weak reference object will be passed as the only
parameter to the callback; the referent will no longer be available.
It is allowable for many weak references to be constructed for the same object.
Callbacks registered for each weak reference will be called from the most
recently registered callback to the oldest registered callback.
Exceptions raised by the callback will be noted on the standard error output,
but cannot be propagated; they are handled in exactly the same way as exceptions
raised from an object’s <a class="reference internal" href="../reference/datamodel.html#object.__del__" title="object.__del__"><code class="xref py py-meth docutils literal notranslate">__del__()</code></a> method.
Weak references are <a class="reference internal" href="../glossary.html#term-hashable">hashable</a> if the object is hashable. They will
maintain their hash value even after the object was deleted. If
<a class="reference internal" href="functions.html#hash" title="hash"><code class="xref py py-func docutils literal notranslate">hash()</code></a> is called the first time only after the object was deleted,
the call will raise <a class="reference internal" href="exceptions.html#TypeError" title="TypeError"><code class="xref py py-exc docutils literal notranslate">TypeError</code></a>.
Weak references support tests for equality, but not ordering. If the referents
are still alive, two references have the same equality relationship as their
referents (regardless of the callback). If either referent has been deleted,
the references are equal only if the reference objects are the same object.
This is a subclassable type rather than a factory function.
<dl class="attribute">
<dt id="weakref.ref.__callback__">
<code class="descname">__callback__</code><a class="headerlink" href="#weakref.ref.__callback__" title="Permalink to this definition">¶</a></dt>
<dd>This read-only attribute returns the callback currently associated to the
weakref. If there is no callback or if the referent of the weakref is
no longer alive then this attribute will have value <code class="docutils literal notranslate">None</code>.
</dd></dl>

<div class="versionchanged">
Changed in version 3.4: Added the <a class="reference internal" href="#weakref.ref.__callback__" title="weakref.ref.__callback__"><code class="xref py py-attr docutils literal notranslate">__callback__</code></a> attribute.
</div>
</dd></dl>

<dl class="function">
<dt id="weakref.proxy">
<code class="descclassname">weakref.</code><code class="descname">proxy</code>(object[, callback])<a class="headerlink" href="#weakref.proxy" title="Permalink to this definition">¶</a></dt>
<dd>Return a proxy to object which uses a weak reference. This supports use of
the proxy in most contexts instead of requiring the explicit dereferencing used
with weak reference objects. The returned object will have a type of either
<code class="docutils literal notranslate">ProxyType</code> or <code class="docutils literal notranslate">CallableProxyType</code>, depending on whether object is
callable. Proxy objects are not <a class="reference internal" href="../glossary.html#term-hashable">hashable</a> regardless of the referent; this
avoids a number of problems related to their fundamentally mutable nature, and
prevent their use as dictionary keys. callback is the same as the parameter
of the same name to the <a class="reference internal" href="#weakref.ref" title="weakref.ref"><code class="xref py py-func docutils literal notranslate">ref()</code></a> function.
</dd></dl>

<dl class="function">
<dt id="weakref.getweakrefcount">
<code class="descclassname">weakref.</code><code class="descname">getweakrefcount</code>(object)<a class="headerlink" href="#weakref.getweakrefcount" title="Permalink to this definition">¶</a></dt>
<dd>Return the number of weak references and proxies which refer to object.
</dd></dl>

<dl class="function">
<dt id="weakref.getweakrefs">
<code class="descclassname">weakref.</code><code class="descname">getweakrefs</code>(object)<a class="headerlink" href="#weakref.getweakrefs" title="Permalink to this definition">¶</a></dt>
<dd>Return a list of all weak reference and proxy objects which refer to object.
</dd></dl>

<dl class="class">
<dt id="weakref.WeakKeyDictionary">
class <code class="descclassname">weakref.</code><code class="descname">WeakKeyDictionary</code>([dict])<a class="headerlink" href="#weakref.WeakKeyDictionary" title="Permalink to this definition">¶</a></dt>
<dd>Mapping class that references keys weakly. Entries in the dictionary will be
discarded when there is no longer a strong reference to the key. This can be
used to associate additional data with an object owned by other parts of an
application without adding attributes to those objects. This can be especially
useful with objects that override attribute accesses.
<div class="admonition note">
Note
Caution: Because a <a class="reference internal" href="#weakref.WeakKeyDictionary" title="weakref.WeakKeyDictionary"><code class="xref py py-class docutils literal notranslate">WeakKeyDictionary</code></a> is built on top of a Python
dictionary, it must not change size when iterating over it. This can be
difficult to ensure for a <a class="reference internal" href="#weakref.WeakKeyDictionary" title="weakref.WeakKeyDictionary"><code class="xref py py-class docutils literal notranslate">WeakKeyDictionary</code></a> because actions
performed by the program during iteration may cause items in the
dictionary to vanish “by magic” (as a side effect of garbage collection).
</div>
</dd></dl>

<a class="reference internal" href="#weakref.WeakKeyDictionary" title="weakref.WeakKeyDictionary"><code class="xref py py-class docutils literal notranslate">WeakKeyDictionary</code></a> objects have an additional method that
exposes the internal references directly. The references are not guaranteed to
be “live” at the time they are used, so the result of calling the references
needs to be checked before being used. This can be used to avoid creating
references that will cause the garbage collector to keep the keys around longer
than needed.
<dl class="method">
<dt id="weakref.WeakKeyDictionary.keyrefs">
<code class="descclassname">WeakKeyDictionary.</code><code class="descname">keyrefs</code>()<a class="headerlink" href="#weakref.WeakKeyDictionary.keyrefs" title="Permalink to this definition">¶</a></dt>
<dd>Return an iterable of the weak references to the keys.
</dd></dl>

<dl class="class">
<dt id="weakref.WeakValueDictionary">
class <code class="descclassname">weakref.</code><code class="descname">WeakValueDictionary</code>([dict])<a class="headerlink" href="#weakref.WeakValueDictionary" title="Permalink to this definition">¶</a></dt>
<dd>Mapping class that references values weakly. Entries in the dictionary will be
discarded when no strong reference to the value exists any more.
<div class="admonition note">
Note
Caution: Because a <a class="reference internal" href="#weakref.WeakValueDictionary" title="weakref.WeakValueDictionary"><code class="xref py py-class docutils literal notranslate">WeakValueDictionary</code></a> is built on top of a Python
dictionary, it must not change size when iterating over it. This can be
difficult to ensure for a <a class="reference internal" href="#weakref.WeakValueDictionary" title="weakref.WeakValueDictionary"><code class="xref py py-class docutils literal notranslate">WeakValueDictionary</code></a> because actions performed
by the program during iteration may cause items in the dictionary to vanish “by
magic” (as a side effect of garbage collection).
</div>
</dd></dl>

<a class="reference internal" href="#weakref.WeakValueDictionary" title="weakref.WeakValueDictionary"><code class="xref py py-class docutils literal notranslate">WeakValueDictionary</code></a> objects have an additional method that has the
same issues as the <code class="xref py py-meth docutils literal notranslate">keyrefs()</code> method of <a class="reference internal" href="#weakref.WeakKeyDictionary" title="weakref.WeakKeyDictionary"><code class="xref py py-class docutils literal notranslate">WeakKeyDictionary</code></a>
objects.
<dl class="method">
<dt id="weakref.WeakValueDictionary.valuerefs">
<code class="descclassname">WeakValueDictionary.</code><code class="descname">valuerefs</code>()<a class="headerlink" href="#weakref.WeakValueDictionary.valuerefs" title="Permalink to this definition">¶</a></dt>
<dd>Return an iterable of the weak references to the values.
</dd></dl>

<dl class="class">
<dt id="weakref.WeakSet">
class <code class="descclassname">weakref.</code><code class="descname">WeakSet</code>([elements])<a class="headerlink" href="#weakref.WeakSet" title="Permalink to this definition">¶</a></dt>
<dd>Set class that keeps weak references to its elements. An element will be
discarded when no strong reference to it exists any more.
</dd></dl>

<dl class="class">
<dt id="weakref.WeakMethod">
class <code class="descclassname">weakref.</code><code class="descname">WeakMethod</code>(method)<a class="headerlink" href="#weakref.WeakMethod" title="Permalink to this definition">¶</a></dt>
<dd>A custom <a class="reference internal" href="#weakref.ref" title="weakref.ref"><code class="xref py py-class docutils literal notranslate">ref</code></a> subclass which simulates a weak reference to a bound
method (i.e., a method defined on a class and looked up on an instance).
Since a bound method is ephemeral, a standard weak reference cannot keep
hold of it. <a class="reference internal" href="#weakref.WeakMethod" title="weakref.WeakMethod"><code class="xref py py-class docutils literal notranslate">WeakMethod</code></a> has special code to recreate the bound
method until either the object or the original function dies:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; class C:
... def method(self):
... print(&quot;method called!&quot;)
...
&gt;&gt;&gt; c = C()
&gt;&gt;&gt; r = weakref.ref(c.method)
&gt;&gt;&gt; r()
&gt;&gt;&gt; r = weakref.WeakMethod(c.method)
&gt;&gt;&gt; r()
&lt;bound method C.method of &lt;__main__.C object at 0x7fc859830220&gt;&gt;
&gt;&gt;&gt; r()()
method called!
&gt;&gt;&gt; del c
&gt;&gt;&gt; gc.collect()
0
&gt;&gt;&gt; r()
&gt;&gt;&gt;
</pre></div>
</div>
<div class="versionadded">
New in version 3.4.
</div>
</dd></dl>

<dl class="class">
<dt id="weakref.finalize">
class <code class="descclassname">weakref.</code><code class="descname">finalize</code>(obj, func, *args, **kwargs)<a class="headerlink" href="#weakref.finalize" title="Permalink to this definition">¶</a></dt>
<dd>Return a callable finalizer object which will be called when obj
is garbage collected. Unlike an ordinary weak reference, a finalizer
will always survive until the reference object is collected, greatly
simplifying lifecycle management.
A finalizer is considered alive until it is called (either explicitly
or at garbage collection), and after that it is dead. Calling a live
finalizer returns the result of evaluating <code class="docutils literal notranslate">func(*arg, **kwargs)</code>,
whereas calling a dead finalizer returns <a class="reference internal" href="constants.html#None" title="None"><code class="xref py py-const docutils literal notranslate">None</code></a>.
Exceptions raised by finalizer callbacks during garbage collection
will be shown on the standard error output, but cannot be
propagated. They are handled in the same way as exceptions raised
from an object’s <a class="reference internal" href="../reference/datamodel.html#object.__del__" title="object.__del__"><code class="xref py py-meth docutils literal notranslate">__del__()</code></a> method or a weak reference’s
callback.
When the program exits, each remaining live finalizer is called
unless its <a class="reference internal" href="atexit.html#module-atexit" title="atexit: Register and execute cleanup functions."><code class="xref py py-attr docutils literal notranslate">atexit</code></a> attribute has been set to false. They
are called in reverse order of creation.
A finalizer will never invoke its callback during the later part of
the <a class="reference internal" href="../glossary.html#term-interpreter-shutdown">interpreter shutdown</a> when module globals are liable to have
been replaced by <a class="reference internal" href="constants.html#None" title="None"><code class="xref py py-const docutils literal notranslate">None</code></a>.
<dl class="method">
<dt id="weakref.finalize.__call__">
<code class="descname">__call__</code>()<a class="headerlink" href="#weakref.finalize.__call__" title="Permalink to this definition">¶</a></dt>
<dd>If self is alive then mark it as dead and return the result of
calling <code class="docutils literal notranslate">func(*args, **kwargs)</code>. If self is dead then return
<a class="reference internal" href="constants.html#None" title="None"><code class="xref py py-const docutils literal notranslate">None</code></a>.
</dd></dl>

<dl class="method">
<dt id="weakref.finalize.detach">
<code class="descname">detach</code>()<a class="headerlink" href="#weakref.finalize.detach" title="Permalink to this definition">¶</a></dt>
<dd>If self is alive then mark it as dead and return the tuple
<code class="docutils literal notranslate">(obj, func, args, kwargs)</code>. If self is dead then return
<a class="reference internal" href="constants.html#None" title="None"><code class="xref py py-const docutils literal notranslate">None</code></a>.
</dd></dl>

<dl class="method">
<dt id="weakref.finalize.peek">
<code class="descname">peek</code>()<a class="headerlink" href="#weakref.finalize.peek" title="Permalink to this definition">¶</a></dt>
<dd>If self is alive then return the tuple <code class="docutils literal notranslate">(obj, func, args,
kwargs)</code>. If self is dead then return <a class="reference internal" href="constants.html#None" title="None"><code class="xref py py-const docutils literal notranslate">None</code></a>.
</dd></dl>

<dl class="attribute">
<dt id="weakref.finalize.alive">
<code class="descname">alive</code><a class="headerlink" href="#weakref.finalize.alive" title="Permalink to this definition">¶</a></dt>
<dd>Property which is true if the finalizer is alive, false otherwise.
</dd></dl>

<dl class="attribute">
<dt id="weakref.finalize.atexit">
<code class="descname">atexit</code><a class="headerlink" href="#weakref.finalize.atexit" title="Permalink to this definition">¶</a></dt>
<dd>A writable boolean property which by default is true. When the
program exits, it calls all remaining live finalizers for which
<a class="reference internal" href="#weakref.finalize.atexit" title="weakref.finalize.atexit"><code class="xref py py-attr docutils literal notranslate">atexit</code></a> is true. They are called in reverse order of
creation.
</dd></dl>

<div class="admonition note">
Note
It is important to ensure that func, args and kwargs do
not own any references to obj, either directly or indirectly,
since otherwise obj will never be garbage collected. In
particular, func should not be a bound method of obj.
</div>
<div class="versionadded">
New in version 3.4.
</div>
</dd></dl>

<dl class="data">
<dt id="weakref.ReferenceType">
<code class="descclassname">weakref.</code><code class="descname">ReferenceType</code><a class="headerlink" href="#weakref.ReferenceType" title="Permalink to this definition">¶</a></dt>
<dd>The type object for weak references objects.
</dd></dl>

<dl class="data">
<dt id="weakref.ProxyType">
<code class="descclassname">weakref.</code><code class="descname">ProxyType</code><a class="headerlink" href="#weakref.ProxyType" title="Permalink to this definition">¶</a></dt>
<dd>The type object for proxies of objects which are not callable.
</dd></dl>

<dl class="data">
<dt id="weakref.CallableProxyType">
<code class="descclassname">weakref.</code><code class="descname">CallableProxyType</code><a class="headerlink" href="#weakref.CallableProxyType" title="Permalink to this definition">¶</a></dt>
<dd>The type object for proxies of callable objects.
</dd></dl>

<dl class="data">
<dt id="weakref.ProxyTypes">
<code class="descclassname">weakref.</code><code class="descname">ProxyTypes</code><a class="headerlink" href="#weakref.ProxyTypes" title="Permalink to this definition">¶</a></dt>
<dd>Sequence containing all the type objects for proxies. This can make it simpler
to test if an object is a proxy without being dependent on naming both proxy
types.
</dd></dl>

<dl class="exception">
<dt id="weakref.ReferenceError">
exception <code class="descclassname">weakref.</code><code class="descname">ReferenceError</code><a class="headerlink" href="#weakref.ReferenceError" title="Permalink to this definition">¶</a></dt>
<dd>Exception raised when a proxy object is used but the underlying object has been
collected. This is the same as the standard <a class="reference internal" href="exceptions.html#ReferenceError" title="ReferenceError"><code class="xref py py-exc docutils literal notranslate">ReferenceError</code></a> exception.
</dd></dl>

<div class="admonition seealso">
See also
<dl class="last docutils">
<dt><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0205">PEP 205</a> - Weak References</dt>
<dd>The proposal and rationale for this feature, including links to earlier
implementations and information about similar features in other languages.</dd>
</dl>
</div>
<div class="section" id="weak-reference-objects">
<h2>8.8.1. Weak Reference Objects<a class="headerlink" href="#weak-reference-objects" title="Permalink to this headline">¶</a></h2>
Weak reference objects have no methods and no attributes besides
<a class="reference internal" href="#weakref.ref.__callback__" title="weakref.ref.__callback__"><code class="xref py py-attr docutils literal notranslate">ref.__callback__</code></a>. A weak reference object allows the referent to be
obtained, if it still exists, by calling it:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; import weakref
&gt;&gt;&gt; class Object:
... pass
...
&gt;&gt;&gt; o = Object()
&gt;&gt;&gt; r = weakref.ref(o)
&gt;&gt;&gt; o2 = r()
&gt;&gt;&gt; o is o2
True
</pre></div>
</div>
If the referent no longer exists, calling the reference object returns
<a class="reference internal" href="constants.html#None" title="None"><code class="xref py py-const docutils literal notranslate">None</code></a>:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; del o, o2
&gt;&gt;&gt; print(r())
None
</pre></div>
</div>
Testing that a weak reference object is still live should be done using the
expression <code class="docutils literal notranslate">ref() is not None</code>. Normally, application code that needs to use
a reference object should follow this pattern:
<div class="highlight-python3 notranslate"><div class="highlight"><pre># r is a weak reference object
o = r()
if o is None:
 # referent has been garbage collected
 print(&quot;Object has been deallocated; can&#39;t frobnicate.&quot;)
else:
 print(&quot;Object is still live!&quot;)
 o.do_something_useful()
</pre></div>
</div>
Using a separate test for “liveness” creates race conditions in threaded
applications; another thread can cause a weak reference to become invalidated
before the weak reference is called; the idiom shown above is safe in threaded
applications as well as single-threaded applications.
Specialized versions of <a class="reference internal" href="#weakref.ref" title="weakref.ref"><code class="xref py py-class docutils literal notranslate">ref</code></a> objects can be created through subclassing.
This is used in the implementation of the <a class="reference internal" href="#weakref.WeakValueDictionary" title="weakref.WeakValueDictionary"><code class="xref py py-class docutils literal notranslate">WeakValueDictionary</code></a> to reduce
the memory overhead for each entry in the mapping. This may be most useful to
associate additional information with a reference, but could also be used to
insert additional processing on calls to retrieve the referent.
This example shows how a subclass of <a class="reference internal" href="#weakref.ref" title="weakref.ref"><code class="xref py py-class docutils literal notranslate">ref</code></a> can be used to store
additional information about an object and affect the value that’s returned when
the referent is accessed:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>import weakref

class ExtendedRef(weakref.ref):
 def __init__(self, ob, callback=None, **annotations):
 super(ExtendedRef, self).__init__(ob, callback)
 self.__counter = 0
 for k, v in annotations.items():
 setattr(self, k, v)

def __call__(self):
 &quot;&quot;&quot;Return a pair containing the referent and the number of
 times the reference has been called.
 &quot;&quot;&quot;
 ob = super(ExtendedRef, self).__call__()
 if ob is not None:
 self.__counter += 1
 ob = (ob, self.__counter)
 return ob
</pre></div>
</div>
</div>
<div class="section" id="example">
<h2>8.8.2. Example<a class="headerlink" href="#example" title="Permalink to this headline">¶</a></h2>
This simple example shows how an application can use object IDs to retrieve
objects that it has seen before. The IDs of the objects can then be used in
other data structures without forcing the objects to remain alive, but the
objects can still be retrieved by ID if they do.
<div class="highlight-python3 notranslate"><div class="highlight"><pre>import weakref

_id2obj_dict = weakref.WeakValueDictionary()

def remember(obj):
 oid = id(obj)
 _id2obj_dict[oid] = obj
 return oid

def id2obj(oid):
 return _id2obj_dict[oid]
</pre></div>
</div>
</div>
<div class="section" id="finalizer-objects">
<h2>8.8.3. Finalizer Objects<a class="headerlink" href="#finalizer-objects" title="Permalink to this headline">¶</a></h2>
The main benefit of using <a class="reference internal" href="#weakref.finalize" title="weakref.finalize"><code class="xref py py-class docutils literal notranslate">finalize</code></a> is that it makes it simple
to register a callback without needing to preserve the returned finalizer
object. For instance
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; import weakref
&gt;&gt;&gt; class Object:
... pass
...
&gt;&gt;&gt; kenny = Object()
&gt;&gt;&gt; weakref.finalize(kenny, print, &quot;You killed Kenny!&quot;) 
&lt;finalize object at ...; for &#39;Object&#39; at ...&gt;
&gt;&gt;&gt; del kenny
You killed Kenny!
</pre></div>
</div>
The finalizer can be called directly as well. However the finalizer
will invoke the callback at most once.
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; def callback(x, y, z):
... print(&quot;CALLBACK&quot;)
... return x + y + z
...
&gt;&gt;&gt; obj = Object()
&gt;&gt;&gt; f = weakref.finalize(obj, callback, 1, 2, z=3)
&gt;&gt;&gt; assert f.alive
&gt;&gt;&gt; assert f() == 6
CALLBACK
&gt;&gt;&gt; assert not f.alive
&gt;&gt;&gt; f() # callback not called because finalizer dead
&gt;&gt;&gt; del obj # callback not called because finalizer dead
</pre></div>
</div>
You can unregister a finalizer using its <a class="reference internal" href="#weakref.finalize.detach" title="weakref.finalize.detach"><code class="xref py py-meth docutils literal notranslate">detach()</code></a>
method. This kills the finalizer and returns the arguments passed to
the constructor when it was created.
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; obj = Object()
&gt;&gt;&gt; f = weakref.finalize(obj, callback, 1, 2, z=3)
&gt;&gt;&gt; f.detach() 
(&lt;__main__.Object object ...&gt;, &lt;function callback ...&gt;, (1, 2), {&#39;z&#39;: 3})
&gt;&gt;&gt; newobj, func, args, kwargs = _
&gt;&gt;&gt; assert not f.alive
&gt;&gt;&gt; assert newobj is obj
&gt;&gt;&gt; assert func(*args, **kwargs) == 6
CALLBACK
</pre></div>
</div>
Unless you set the <a class="reference internal" href="#weakref.finalize.atexit" title="weakref.finalize.atexit"><code class="xref py py-attr docutils literal notranslate">atexit</code></a> attribute to
<a class="reference internal" href="constants.html#False" title="False"><code class="xref py py-const docutils literal notranslate">False</code></a>, a finalizer will be called when the program exits if it
is still alive. For instance
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; obj = Object()
&gt;&gt;&gt; weakref.finalize(obj, print, &quot;obj dead or exiting&quot;) 
&lt;finalize object at ...; for &#39;Object&#39; at ...&gt;
&gt;&gt;&gt; exit() 
obj dead or exiting
</pre></div>
</div>
</div>
<div class="section" id="comparing-finalizers-with-del-methods">
<h2>8.8.4. Comparing finalizers with <a class="reference internal" href="../reference/datamodel.html#object.__del__" title="object.__del__"><code class="xref py py-meth docutils literal notranslate">__del__()</code></a> methods<a class="headerlink" href="#comparing-finalizers-with-del-methods" title="Permalink to this headline">¶</a></h2>
Suppose we want to create a class whose instances represent temporary
directories. The directories should be deleted with their contents
when the first of the following events occurs:
<ul class="simple">
<li>the object is garbage collected,</li>
<li>the object’s <code class="xref py py-meth docutils literal notranslate">remove()</code> method is called, or</li>
<li>the program exits.</li>
</ul>
We might try to implement the class using a <a class="reference internal" href="../reference/datamodel.html#object.__del__" title="object.__del__"><code class="xref py py-meth docutils literal notranslate">__del__()</code></a> method as
follows:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>class TempDir:
 def __init__(self):
 self.name = tempfile.mkdtemp()

def remove(self):
 if self.name is not None:
 shutil.rmtree(self.name)
 self.name = None

def __del__(self):
 self.remove()
</pre></div>
</div>
Starting with Python 3.4, <a class="reference internal" href="../reference/datamodel.html#object.__del__" title="object.__del__"><code class="xref py py-meth docutils literal notranslate">__del__()</code></a> methods no longer prevent
reference cycles from being garbage collected, and module globals are
no longer forced to <a class="reference internal" href="constants.html#None" title="None"><code class="xref py py-const docutils literal notranslate">None</code></a> during <a class="reference internal" href="../glossary.html#term-interpreter-shutdown">interpreter shutdown</a>.
So this code should work without any issues on CPython.
However, handling of <a class="reference internal" href="../reference/datamodel.html#object.__del__" title="object.__del__"><code class="xref py py-meth docutils literal notranslate">__del__()</code></a> methods is notoriously implementation
specific, since it depends on internal details of the interpreter’s garbage
collector implementation.
A more robust alternative can be to define a finalizer which only references
the specific functions and objects that it needs, rather than having access
to the full state of the object:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>class TempDir:
 def __init__(self):
 self.name = tempfile.mkdtemp()
 self._finalizer = weakref.finalize(self, shutil.rmtree, self.name)

def remove(self):
 self._finalizer()

@property
 def removed(self):
 return not self._finalizer.alive
</pre></div>
</div>
Defined like this, our finalizer only receives a reference to the details
it needs to clean up the directory appropriately. If the object never gets
garbage collected the finalizer will still be called at exit.
The other advantage of weakref based finalizers is that they can be used to
register finalizers for classes where the definition is controlled by a
third party, such as running code when a module is unloaded:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>import weakref, sys
def unloading_module():
 # implicit reference to the module globals from the function body
weakref.finalize(sys.modules[__name__], unloading_module)
</pre></div>
</div>
<div class="admonition note">
Note
If you create a finalizer object in a daemonic thread just as the program
exits then there is the possibility that the finalizer
does not get called at exit. However, in a daemonic thread
<a class="reference internal" href="atexit.html#atexit.register" title="atexit.register"><code class="xref py py-func docutils literal notranslate">atexit.register()</code></a>, <code class="docutils literal notranslate">try: ... finally: ...</code> and <code class="docutils literal notranslate">with: ...</code>
do not guarantee that cleanup occurs either.
</div>
</div>
</div>

</div>
 </div>
 </div>
 <div class="sphinxsidebar" role="navigation" aria-label="main navigation">
 <div class="sphinxsidebarwrapper">
 <h3><a href="../contents.html">Table Of Contents</a></h3>
 <ul>
<li><a class="reference internal" href="#">8.8. <code class="docutils literal notranslate">weakref</code> — Weak references</a><ul>
<li><a class="reference internal" href="#weak-reference-objects">8.8.1. Weak Reference Objects</a></li>
<li><a class="reference internal" href="#example">8.8.2. Example</a></li>
<li><a class="reference internal" href="#finalizer-objects">8.8.3. Finalizer Objects</a></li>
<li><a class="reference internal" href="#comparing-finalizers-with-del-methods">8.8.4. Comparing finalizers with <code class="docutils literal notranslate">__del__()</code> methods</a></li>
</ul>
</li>
</ul>

<h4>Previous topic</h4>
 <a href="array.html"
 title="previous chapter">8.7. <code class="docutils literal notranslate">array</code> — Efficient arrays of numeric values</a>
 <h4>Next topic</h4>
 <a href="types.html"
 title="next chapter">8.9. <code class="docutils literal notranslate">types</code> — Dynamic type creation and names for built-in types</a>
 <div role="note" aria-label="source link">
 <h3>This Page</h3>
 <ul class="this-page-menu">
 <li><a href="../bugs.html">Report a Bug</a></li>
 <li>
 <a href="https://github.com/python/cpython/blob/3.6/Doc/library/weakref.rst"
 rel="nofollow">Show Source
 </a>
 </li>
 </ul>
 </div>
 </div>
 </div>
 <div class="clearer"></div>
 </div> 
 <div class="related" role="navigation" aria-label="related navigation">
 <h3>Navigation</h3>
 <ul>
 <li class="right" style="margin-right: 10px">
 <a href="../genindex.html" title="General Index"
 >index</a></li>
 <li class="right" >
 <a href="../py-modindex.html" title="Python Module Index"
 >modules</a> |</li>
 <li class="right" >
 <a href="types.html" title="8.9. types — Dynamic type creation and names for built-in types"
 >next</a> |</li>
 <li class="right" >
 <a href="array.html" title="8.7. array — Efficient arrays of numeric values"
 >previous</a> |</li>
 <li><img src="../_static/py.png" alt=""
 style="vertical-align: middle; margin-top: -1px"/></li>
 <li><a href="https://www.python.org/">Python</a> &#187;</li>
 <li>
 <a href="../index.html">3.6.7 Documentation</a> &#187;
 </li>

<li class="nav-item nav-item-1"><a href="index.html" >The Python Standard Library</a> &#187;</li>
 <li class="nav-item nav-item-2"><a href="datatypes.html" >8. Data Types</a> &#187;</li>
 <li class="right">

</ul>
 </div> 
 <div class="footer">
 &copy; <a href="../copyright.html">Copyright</a> 2001-2023, Python Software Foundation.
 
 The Python Software Foundation is a non-profit corporation.
 <a href="https://www.python.org/psf/donations/">Please donate.</a>
 
 Last updated on Dec 18, 2023.
 <a href="../bugs.html">Found a bug</a>?
 
 Created using <a href="http://sphinx.pocoo.org/">Sphinx</a> 1.7.6.
 </div>

</body>
</html>

cmd: