dynamic attributes, descriptors and first concurrency examples

descriptors/descriptorkinds_dump.py

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+
+
+"""
+Overriding descriptor (a.k.a. data descriptor or enforced descriptor):
+
+    >>> obj = Model()
+    >>> obj.over  # doctest: +ELLIPSIS
+    Overriding.__get__() invoked with args:
+        self     = <descriptorkinds.Overriding object at 0x...>
+        instance = <descriptorkinds.Model object at 0x...>
+        owner    = <class 'descriptorkinds.Model'>
+    >>> Model.over  # doctest: +ELLIPSIS
+    Overriding.__get__() invoked with args:
+        self     = <descriptorkinds.Overriding object at 0x...>
+        instance = None
+        owner    = <class 'descriptorkinds.Model'>
+
+
+An overriding descriptor cannot be shadowed by assigning to an instance:
+
+    >>> obj = Model()
+    >>> obj.over = 7  # doctest: +ELLIPSIS
+    Overriding.__set__() invoked with args:
+        self     = <descriptorkinds.Overriding object at 0x...>
+        instance = <descriptorkinds.Model object at 0x...>
+        value    = 7
+    >>> obj.over  # doctest: +ELLIPSIS
+    Overriding.__get__() invoked with args:
+        self     = <descriptorkinds.Overriding object at 0x...>
+        instance = <descriptorkinds.Model object at 0x...>
+        owner    = <class 'descriptorkinds.Model'>
+
+
+Not even by poking the attribute into the instance ``__dict__``:
+
+    >>> obj.__dict__['over'] = 8
+    >>> obj.over  # doctest: +ELLIPSIS
+    Overriding.__get__() invoked with args:
+        self     = <descriptorkinds.Overriding object at 0x...>
+        instance = <descriptorkinds.Model object at 0x...>
+        owner    = <class 'descriptorkinds.Model'>
+    >>> vars(obj)
+    {'over': 8}
+
+Overriding descriptor without ``__get__``:
+
+    >>> obj.over_no_get  # doctest: +ELLIPSIS
+    <descriptorkinds.OverridingNoGet object at 0x...>
+    >>> Model.over_no_get   # doctest: +ELLIPSIS
+    <descriptorkinds.OverridingNoGet object at 0x...>
+    >>> obj.over_no_get = 7  # doctest: +ELLIPSIS
+    OverridingNoGet.__set__() invoked with args:
+        self     = <descriptorkinds.OverridingNoGet object at 0x...>
+        instance = <descriptorkinds.Model object at 0x...>
+        value    = 7
+    >>> obj.over_no_get  # doctest: +ELLIPSIS
+    <descriptorkinds.OverridingNoGet object at 0x...>
+
+
+Poking the attribute into the instance ``__dict__`` means you can read the new
+value for the attribute, but setting it still triggers ``__set__``:
+
+    >>> obj.__dict__['over_no_get'] = 9
+    >>> obj.over_no_get
+    9
+    >>> obj.over_no_get = 7  # doctest: +ELLIPSIS
+    OverridingNoGet.__set__() invoked with args:
+        self     = <descriptorkinds.OverridingNoGet object at 0x...>
+        instance = <descriptorkinds.Model object at 0x...>
+        value    = 7
+    >>> obj.over_no_get
+    9
+
+
+Non-overriding descriptor (a.k.a. non-data descriptor or shadowable descriptor):
+
+    >>> obj = Model()
+    >>> obj.non_over  # doctest: +ELLIPSIS
+    NonOverriding.__get__() invoked with args:
+        self     = <descriptorkinds.NonOverriding object at 0x...>
+        instance = <descriptorkinds.Model object at 0x...>
+        owner    = <class 'descriptorkinds.Model'>
+    >>> Model.non_over  # doctest: +ELLIPSIS
+    NonOverriding.__get__() invoked with args:
+        self     = <descriptorkinds.NonOverriding object at 0x...>
+        instance = None
+        owner    = <class 'descriptorkinds.Model'>
+
+
+A non-overriding descriptor can be shadowed by assigning to an instance:
+
+    >>> obj.non_over = 7
+    >>> obj.non_over
+    7
+
+
+Methods are non-over descriptors:
+
+    >>> obj.spam  # doctest: +ELLIPSIS
+    <bound method Model.spam of <descriptorkinds.Model object at 0x...>>
+    >>> Model.spam  # doctest: +ELLIPSIS
+    <function Model.spam at 0x...>
+    >>> obj.spam()  # doctest: +ELLIPSIS
+    Model.spam() invoked with arg:
+        self = <descriptorkinds.Model object at 0x...>
+    >>> obj.spam = 7
+    >>> obj.spam
+    7
+
+
+No descriptor type survives being overwritten on the class itself:
+
+    >>> Model.over = 1
+    >>> obj.over
+    1
+    >>> Model.over_no_get = 2
+    >>> obj.over_no_get
+    2
+    >>> Model.non_over = 3
+    >>> obj.non_over
+    7
+
+"""
+
+# BEGIN DESCRIPTORKINDS
+def print_args(name, *args):  # <1>
+    cls_name = args[0].__class__.__name__
+    arg_names = ['self', 'instance', 'owner']
+    if name == 'set':
+        arg_names[-1] = 'value'
+    print('{}.__{}__() invoked with args:'.format(cls_name, name))
+    for arg_name, value in zip(arg_names, args):
+        print('    {:8} = {}'.format(arg_name, value))
+
+
+class Overriding:  # <2>
+    """a.k.a. data descriptor or enforced descriptor"""
+
+    def __get__(self, instance, owner):
+        print_args('get', self, instance, owner)  # <3>
+
+    def __set__(self, instance, value):
+        print_args('set', self, instance, value)
+
+
+class OverridingNoGet:  # <4>
+    """an overriding descriptor without ``__get__``"""
+
+    def __set__(self, instance, value):
+        print_args('set', self, instance, value)
+
+
+class NonOverriding:  # <5>
+    """a.k.a. non-data or shadowable descriptor"""
+
+    def __get__(self, instance, owner):
+        print_args('get', self, instance, owner)
+
+
+class Model:  # <6>
+    over = Overriding()
+    over_no_get = OverridingNoGet()
+    non_over = NonOverriding()
+
+    def spam(self):  # <7>
+        print('Model.spam() invoked with arg:')
+        print('    self =', self)
+
+#END DESCRIPTORKINDS