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20220519090118-la_methode_prepare.org

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+:PROPERTIES:
+:ID:       0d9a7351-babd-4394-9d95-1d40cb46f615
+:mtime:    20220520230717
+:ctime:    20220519090118
+:END:
+#+title: La méthode __prepare__
+
+* Synthaxe
+#+BEGIN_SRC python
+class.__prepare__()
+#+END_SRC
+La méthode ~class.__prepare__~ :
+ * Cf. [[https://peps.python.org/pep-3115/][PEP-3115 - Metaclasses in Python 3000]],
+ * Est utilisé comme /namespace/ local pour tout le code du corps de la
+   classe (prépare le contenu du dictionnaire /__dict__/ de l'instance).
+ * Est appelée avant ~class.__new__~,
+ * Doit retourner un objet /dictionary-like/,
+ 
+* Exemples
+
+#+BEGIN_SRC python :results output
+class DummyMeta(type):
+
+    @classmethod
+    def __prepare__(mcs, name, bases, **kwargs):
+        print('DummyMeta.__prepare__')
+        return {'special': 1}
+            
+class Dummy(metaclass=DummyMeta):
+    
+    def __new__(cls, name):
+        print(f'Dummy.__new__({name})')
+        return super().__new__(cls)
+    
+    def __init__(self, name):
+        super().__init__()
+        self.name = name
+        print(f'Dummy.__init__({self.name})')
+
+dummy1, dummy2 = (Dummy(f'dummy{i + 1}') for i in range(2))
+print(f'{dummy1.special=}')
+dummy1.special += 1
+print(f'{dummy1.special=}')
+print(f'{dummy2.special=}')
+#+END_SRC
+
+#+RESULTS:
+: DummyMeta.__prepare__
+: Dummy.__new__(dummy1)
+: Dummy.__init__(dummy1)
+: Dummy.__new__(dummy2)
+: Dummy.__init__(dummy2)
+: dummy1.special=1
+: dummy1.special=2
+: dummy2.special=1
+
+* Références
+ * [[https://blog.ionelmc.ro/2015/02/09/understanding-python-metaclasses/][Understanding Python metaclasses - Ionel codelog]]
+ * [[https://zestedesavoir.com/tutoriels/954/notions-de-python-avancees/4-classes/2-metaclasses/][Metaclasses - Zeste de savoir]]