renumbering chapters >= 19

24-class-metaprog/metabunch/nutshell3e/bunch.py

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+import collections
+import warnings
+
+class MetaBunch(type):
+    """
+    Metaclass for new and improved "Bunch": implicitly defines
+    __slots__, __init__ and __repr__ from variables bound in
+    class scope.
+    A class statement for an instance of MetaBunch (i.e., for a
+    class whose metaclass is MetaBunch) must define only
+    class-scope data attributes (and possibly special methods, but
+    NOT __init__ and __repr__).  MetaBunch removes the data
+    attributes from class scope, snuggles them instead as items in
+    a class-scope dict named __dflts__, and puts in the class a
+    __slots__ with those attributes' names, an __init__ that takes
+    as optional named arguments each of them (using the values in
+    __dflts__ as defaults for missing ones), and a __repr__ that
+    shows the repr of each attribute that differs from its default
+    value (the output of __repr__ can be passed to __eval__ to make
+    an equal instance, as per usual convention in the matter, if
+    each non-default-valued attribute respects the convention too).
+
+    In v3, the order of data attributes remains the same as in the
+    class body; in v2, there is no such guarantee.
+    """
+    def __prepare__(name, *bases, **kwargs):
+        # precious in v3—harmless although useless in v2
+        return collections.OrderedDict()
+
+    def __new__(mcl, classname, bases, classdict):
+        """ Everything needs to be done in __new__, since
+            type.__new__ is where __slots__ are taken into account.
+        """
+        # define as local functions the __init__ and __repr__ that
+        # we'll use in the new class
+        def __init__(self, **kw):
+            """ Simplistic __init__: first set all attributes to
+                default values, then override those explicitly
+                passed in kw.
+            """
+            for k in self.__dflts__:
+                setattr(self, k, self.__dflts__[k])
+            for k in kw:
+                setattr(self, k, kw[k])
+        def __repr__(self):
+            """ Clever __repr__: show only attributes that differ
+                from default values, for compactness.
+            """
+            rep = ['{}={!r}'.format(k, getattr(self, k))
+                    for k in self.__dflts__
+                    if getattr(self, k) != self.__dflts__[k]
+                  ]
+            return '{}({})'.format(classname, ', '.join(rep))
+        # build the newdict that we'll use as class-dict for the
+        # new class
+        newdict = { '__slots__':[], 
+            '__dflts__':collections.OrderedDict(),
+            '__init__':__init__, '__repr__':__repr__, }
+        for k in classdict:
+            if k.startswith('__') and k.endswith('__'):
+                # dunder methods: copy to newdict, or warn
+                # about conflicts
+                if k in newdict:
+                    warnings.warn(
+                        "Can't set attr {!r} in bunch-class {!r}".
+                        format(k, classname))
+                else:
+                    newdict[k] = classdict[k]
+            else:
+                # class variables, store name in __slots__, and
+                # name and value as an item in __dflts__
+                newdict['__slots__'].append(k)
+                newdict['__dflts__'][k] = classdict[k]
+        # finally delegate the rest of the work to type.__new__
+        return super(MetaBunch, mcl).__new__(
+                     mcl, classname, bases, newdict)
+
+class Bunch(metaclass=MetaBunch):
+    """ For convenience: inheriting from Bunch can be used to get
+        the new metaclass (same as defining metaclass= yourself).
+
+        In v2, remove the (metaclass=MetaBunch) above and add
+        instead __metaclass__=MetaBunch as the class body.
+    """
+    pass

24-class-metaprog/metabunch/nutshell3e/bunch_test.py

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+import pytest
+
+from bunch import Bunch
+
+class Point(Bunch):
+    """ A point has x and y coordinates, defaulting to 0.0, 
+        and a color, defaulting to 'gray'—and nothing more, 
+        except what Python and the metaclass conspire to add, 
+        such as __init__ and __repr__
+    """
+    x = 0.0
+    y = 0.0
+    color = 'gray'
+
+
+def test_init_defaults():
+    p = Point()
+    assert repr(p) == 'Point()'
+
+
+def test_init():
+    p = Point(x=1.2, y=3.4, color='red')
+    assert repr(p) == "Point(x=1.2, y=3.4, color='red')"
+
+
+def test_init_wrong_argument():
+    with pytest.raises(AttributeError) as exc:
+        p = Point(x=1.2, y=3.4, flavor='coffee')
+    assert "no attribute 'flavor'" in str(exc.value)
+
+
+def test_slots():
+    p = Point()
+    with pytest.raises(AttributeError) as exc:
+        p.z = 5.6
+    assert "no attribute 'z'" in str(exc.value)
+
+