Weekly backup.

20220519082710-la_methode_new.org

@@ -0,0 +1,62 @@
+:PROPERTIES:
+:ID:       aaf77222-82d2-492c-bf5f-2dd47659b1be
+:mtime:    20220519210820
+:ctime:    20220519082710
+:END:
+#+title: La méthode __new__
+
+* Synthaxe
+#+BEGIN_SRC python
+object.__new__(cls, *args, **kwargs)
+#+END_SRC
+La méthode ~object.__new__~ :
+ * Crée les instances d'une classe (constructeur),
+ * Est appelée avant ~object.__init__~,
+ * Retourne une nouvelle instance ou une référence à une précedente (classes /singleton/).
+
+* Exemples
+** Utilisation classique
+#+BEGIN_SRC python :results output
+class B:
+
+    def __new__(cls, *args, **kwargs):
+        print("__new__ running", cls, args, kwargs)
+        return super().__new__(cls)
+
+    def __init__(self, *args, **kwargs):
+        print("__init__ running", self, args, kwargs)
+        return super().__init__(*args, **kwargs)
+
+for i in range(2):
+    B()
+#+END_SRC
+#+RESULTS:
+: __new__ running <class '__main__.B'> () {}
+: __init__ running <__main__.B object at 0x7f78f614c040> () {}
+: __new__ running <class '__main__.B'> () {}
+: __init__ running <__main__.B object at 0x7f78f614c040> () {}
+
+** Singleton /design pattern/
+#+BEGIN_SRC python :results output
+class SingletonClass:     
+
+    _instance = None
+    
+    def __new__(cls, *args, **kwargs):   
+        print("__new__ running", cls, args, kwargs)    
+        if cls._instance is None: # Checking if an instance of this class exists            
+            cls._instance = super().__new__(cls, *args, **kwargs) # Creating a new instace           
+        return cls._instance # Returing the instance 
+
+for i in range(2):
+    print(f'{id(SingletonClass())=}')
+
+ #+END_SRC
+#+RESULTS:
+: __new__ running <class '__main__.SingletonClass'> () {}
+: id(SingletonClass())=140699167274272
+: __new__ running <class '__main__.SingletonClass'> () {}
+: id(SingletonClass())=140699167274272
+
+* Références
+ * [[https://python.plainenglish.io/advanced-python-classes-objects-and-mro-423bb01521fb][Advanced Python classes objects and MRO - Medium]]