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POHow to adapt the Singleton pattern? (Deprecation warning)
 

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  1. POHow to adapt the Singleton pattern? (Deprecation warning)
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    <p>Few years ago I found an implementation of the Singleton pattern in Python by <a href="http://web.archive.org/web/20090619190842/http://www.suttoncourtenay.org.uk/duncan/accu/pythonpatterns.html" rel="nofollow noreferrer">Duncan Booth</a>:</p> <pre><code>class Singleton(object): """ Singleton class by Duncan Booth. Multiple object variables refers to the same object. http://web.archive.org/web/20090619190842/http://www.suttoncourtenay.org.uk/duncan/accu/pythonpatterns.html#singleton-and-the-borg """ _instance = None def __new__(cls, *args, **kwargs): if not cls._instance: cls._instance = super(Singleton, cls).__new__( cls, *args, **kwargs) return cls._instance </code></pre> <p>The same approach is also described in question "<a href="https://stackoverflow.com/questions/31875/is-there-a-simple-elegant-way-to-define-singletons-in-python/1810391#1810391">Is there a simple, elegant way to define Singletons in Python?</a>"</p> <p>I use the Singleton via sub-classing:<br> <code>class Settings(Singleton)</code><br> <code>class Debug(Singleton)</code> </p> <p>Recently I made some changes to the program and got this warning:</p> <pre><code>/media/KINGSTON/Sumid/src/miscutil.py:39: DeprecationWarning: object.__new__() takes no parameters cls._instance = super(Singleton, cls).__new__(cls, *args, **kwargs) </code></pre> <p>I found <a href="http://mail.python.org/pipermail/python-dev/2008-February/076854.html" rel="nofollow noreferrer">explanation</a> (by Guido) about <code>__new__</code> deprecation which says that the parameters are not used at all. Passing an argument which is not needed can be symptom of a bug.</p> <p>So I decided to clear the parameters:</p> <pre><code>class Singleton(object): _instance = None def __new__(cls): if not cls._instance: cls._instance = super(Singleton, cls).__new__() return cls._instance </code></pre> <p>Which resulted in following exception:</p> <pre><code>Traceback (most recent call last): File "sumid.py", line 1168, in &lt;module&gt; settings = Settings() File "/media/KINGSTON/Sumid/src/miscutil.py", line 45, in __new__ cls._instance = super(Singleton, cls).__new__() TypeError: object.__new__(): not enough arguments </code></pre> <p>When I modify the line to <code>cls._instance = super(Singleton, cls).__new__(cls)</code>, I'll get:</p> <pre><code>Traceback (most recent call last): File "sumid.py", line 1174, in &lt;module&gt; debug = Debug(settings) TypeError: __new__() takes exactly 1 argument (2 given) </code></pre> <p>Gimmel suggest another <a href="https://stackoverflow.com/questions/1590477/python-deprecation-warnings-with-monostate-new-can-someone-explain-why/1590586#1590586">solution</a>: <code>_instance = type.__new__(cls)</code>. For me it breaks the inheritance:</p> <pre><code>Traceback (most recent call last): File "sumid.py", line 1168, in &lt;module&gt; settings = Settings() File "/media/KINGSTON/Sumid/src/miscutil.py", line 40, in __new__ _instance = type.__new__(cls) TypeError: type.__new__(Settings): Settings is not a subtype of type </code></pre> <p>The same <a href="http://freshfoo.com/blog/object__init__takes_no_parameters" rel="nofollow noreferrer">problem</a> has also Menno Smits. But I don't understand the <a href="http://www.artima.com/weblogs/viewpost.jsp?thread=281127" rel="nofollow noreferrer">solution</a> suggested. Moreover I have no multiple inheritance in relevant code.</p> <p>I didn't try the another <a href="https://stackoverflow.com/questions/31875/is-there-a-simple-elegant-way-to-define-singletons-in-python/33201#33201">example</a> in "Is there a simple, elegant way to define Singletons in Python?", but at a glance it probably will have the same problem.</p> <p>I use the Singleton pattern in a program and I don't want to rewrite it completely just because one warning. Thus following answers wont help me: </p> <ul> <li>Singleton is wrong. Don't use the Singleton at all. </li> <li>Use Borg instead, it's more pythonic. </li> <li>Use module instead of a class. </li> </ul> <p>To conclude I'll repeat the question:<br> How to <strong>adapt the Singleton</strong> pattern with consideration of the <strong>deprecation warning</strong> with the <strong>minimal impact</strong> to existing code?</p> <p><strong>Edit:</strong> Line <code>cls._instance = object.__new__(cls)</code> raises the TypeError when the Child's init takes an argument:</p> <pre><code>class Child(Singleton): def __init__(self,param=None): print(param) print("Doing another stuff.") ch = Child("Some stuff") </code></pre>
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    1. COYou have probably already looked at Alex Martelli's http://www.aleax.it/gdd_pydp.pdf (slides 20-..), but anyway. (I just remember him talking about the Singleton's cons in that talk (one can watch it at http://www.youtube.com/watch?v=0vJJlVBVTFg))
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      1. POHow to adapt the Singleton pattern? (Deprecation warning)
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    2. CO@mlvljr interesting. Thanks. But the Singleton pattern on P20 has the same problem - deprecation warning. They solve it with Borg - which will raise the same warning.
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    3. COA few links discussing this issue: 1. [The bug report that caused the issue](http://bugs.python.org/issue1683368) 2. [A discussion of a similar problem](http://freshfoo.com/blog/object__init__takes_no_parameters).
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