updade from Atlas repo

25-class-metaprog/metabunch/from3.6/bunch.py

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+"""
+The `MetaBunch` metaclass is a simplified version of the
+last example in the _How a Metaclass Creates a Class_ section
+of _Chapter 4: Object Oriented Python_ from
+[_Python in a Nutshell, 3rd edition_](https://learning.oreilly.com/library/view/python-in-a/9781491913833)
+by Alex Martelli, Anna Ravenscroft, and Steve Holden.
+
+Here are a few tests. ``bunch_test.py`` has a few more.
+
+# tag::BUNCH_POINT_DEMO_1[]
+    >>> class Point(Bunch):
+    ...     x = 0.0
+    ...     y = 0.0
+    ...     color = 'gray'
+    ...
+    >>> Point(x=1.2, y=3, color='green')
+    Point(x=1.2, y=3, color='green')
+    >>> p = Point()
+    >>> p.x, p.y, p.color
+    (0.0, 0.0, 'gray')
+    >>> p
+    Point()
+
+# end::BUNCH_POINT_DEMO_1[]
+
+# tag::BUNCH_POINT_DEMO_2[]
+
+    >>> Point(x=1, y=2, z=3)
+    Traceback (most recent call last):
+      ...
+    AttributeError: 'Point' object has no attribute 'z'
+    >>> p = Point(x=21)
+    >>> p.y = 42
+    >>> p
+    Point(x=21, y=42)
+    >>> p.flavor = 'banana'
+    Traceback (most recent call last):
+      ...
+    AttributeError: 'Point' object has no attribute 'flavor'
+
+# end::BUNCH_POINT_DEMO_2[]
+"""
+
+# tag::METABUNCH[]
+class MetaBunch(type):  # <1>
+    def __new__(meta_cls, cls_name, bases, cls_dict):  # <2>
+
+        defaults = {}  # <3>
+
+        def __init__(self, **kwargs):  # <4>
+            for name, default in defaults.items():  # <5>
+                setattr(self, name, kwargs.pop(name, default))
+            if kwargs:  # <6>
+                setattr(self, *kwargs.popitem())
+
+        def __repr__(self):  # <7>
+            rep = ', '.join(f'{name}={value!r}'
+                            for name, default in defaults.items()
+                            if (value := getattr(self, name)) != default)
+            return f'{cls_name}({rep})'
+
+        new_dict = dict(__slots__=[], __init__=__init__, __repr__=__repr__)  # <8>
+
+        for name, value in cls_dict.items():  # <9>
+            if name.startswith('__') and name.endswith('__'):  # <10>
+                if name in new_dict:
+                    raise AttributeError(f"Can't set {name!r} in {cls_name!r}")
+                new_dict[name] = value
+            else:  # <11>
+                new_dict['__slots__'].append(name)
+                defaults[name] = value
+        return super().__new__(meta_cls, cls_name, bases, new_dict)  # <12>
+
+
+class Bunch(metaclass=MetaBunch):  # <13>
+    pass
+# end::METABUNCH[]