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Properties with item-style access

Project description

Author: Rob Gaddi, Highland Technology
Date: 19-Sep-2017
Version: 0.1.4


This package supports indexed properties on class instances. Whereas the builtin property function allows you create functions for single-valued properties, the indexedproperty package allows you to create properties that accept an index argument like the __getitem__/__setitem__/__delitem__ set:

>>> from indexedproperty import indexedproperty
>>> class PropertyTest:
...     def __init__(self):
...         self._singleprop = None
...         self._dict = {}
...     # Standard builtin property
...     @property
...     def sprop(self):
...         return self._singleprop
...     @sprop.setter
...     def sprop(self, value):
...         print('Setting sprop', value)
...         self._singleprop = value
...     # Fancy new indexed property
...     @indexedproperty
...     def iprop(self, key):
...         return self._dict[key]
...     @iprop.setter
...     def iprop(self, key, value):
...         print('Setting iprop {0}={1}'.format(key, value))
...         self._dict[key] = value
>>> x = PropertyTest()
>>> x.sprop = 5
Setting sprop 5
>>> x.sprop
>>> x.iprop['Gilliam'] = 'Terry'
Setting iprop Gilliam=Terry
>>> x.iprop['Gilliam']

Decorator Functions

The most convenient use for this library is through the decorator functions.

Marks a function as the getter (__getitem__) for an IndexedProperty. The .setter (__setitem__) and .deleter(__delitem__) methods may be called on the returned IndexedProperty to add additional functionality for the property

Marks a function as the getter (__getitem__) for a ContainerProperty. A ContainerProperty is indexed on the container provided by base and raises a KeyError if the users’s key is not “in” the base.

If the base supports iter() and len(), the ContainerProperty does as well, and adds an items() iterator similar to the one provided by dict. Broadcasting is supported on list and tuple keys.

rangeproperty([start=0], stop)

Marks a function as the getter (__getitem__) for an RangeProperty. This property is indexed on the half-open numeric range defined by range(start, stop), and behaves like a Sequence, though with a potentially non-zero lower bound.

RangeProperty elements support iteration, negative index wraparound (if start >= 0), and broadcasting on list, tuple, range, and slice keys.

Decorating Methods

Akin to property, the basic operations are to define the setter, getter, and deleter for an IndexedProperty. The functional form of the decorator assumes the getter, and further assignments are made by the .setter and .deleter methods of the property.

Unlike a property, however, decorators can be created to provide any function including “magic” methods like __iter__ and __contains__, which will work as expected. In all cases, the first argument to the function (typically self) will point to an instance of the class that the property is defined in.

Or to say it another way, when you have:

class C:
    def x(self, key):

    def fancyproperty(self):

    def unfancymethod(self):

All of those self references will be to an instance of a C. __getitem__, __setitem__, and __delitem__ cannot be patched, as it would break the magic, these are assigned with .getter, .setter, and .deleter respectively.

On the base IndexedProperty, you can assign to .iterable_indices with a tuple of classes that should be given special treatment. If the key in the brackets is an instance of one of then, then rather than pass the key on to the getter/setter/deleter functions directly, it will be iterated into successive calls. So if x.iterable_indices = (list, tuple)

Doing Does
return x.getter(self, 5)
return [
    x.getter(self, 5),
    x.getter(self, 10),
    x.getter(self, 15)
x[5] = 'Larry'
x.setter(self, 5, 'Larry')
x[5,10,15] = 'Larry'
x.setter(self, 5, 'Larry')
x.setter(self, 10, 'Larry')
x.setter(self, 15, 'Larry')
x[5,10,15] = [
    "Larry", "can't",
x.setter(self, 5, 'Larry')
x.setter(self, 10, "can't")
x.setter(self, 15, 'spel')

The setter broadcasting concept is taken from numpy; you can assign either a single value or an iterable of values that is the same length as the list of keys. Strings are treated as single values, and non-string iterables of a different length than the key list raise ValueError.



ContainerProperty is an IndexedProperty linked to a, which is anything with a __contains__ method (i.e. that supports the x in y construction). ContainerProperty automatically checks to ensure that the key provided on accesses is in the container and raises KeyError otherwise. This saves having to check explicitly in the accessor code.

ContainerProperty also supports broadcasting over list and tuple keys.

In almost every case, the container will actually be a, meaning that it also supports len() and iter(). Expecting this, the ContainerProperty also provides:

  • len(prop) : Returns the number of keys in the collection.
  • iter(prop) : Iterates over the keys in the collection.
  • prop.items() : Iterates over (key, value) pairs in the collection.

In this way, a ContainerProperty implements much of the functionality of a dict.

>>> import indexedproperty as ix
>>> class FoodRestrictions:
...     _foodlist = ['apples', 'bananas', 'pears']
...     def __init__(self):
...         self.fooddict = { k : [] for k in self._foodlist }
...     @ix.containerproperty(_foodlist)
...     def lunch(self, idx):
...         return self.fooddict[idx]
...     @lunch.setter
...     def lunch(self, idx, value):
...         self.fooddict[idx] = value
>>> x = FoodRestrictions()
>>> x.lunch['apples'] = 'I have an apple'
>>> x.lunch['bread'] = 'But I want bread'
Traceback (most recent call last):
KeyError: 'bread'
>>> x.lunch['pears'] = 5
>>> x.lunch['apples', 'pears']
['I have an apple', 5]
>>> sorted(x.lunch)
['apples', 'bananas', 'pears']


RangeProperty is an IndexedProperty linked to a range of integer values. Much like a list, keys support slicing and negative indices. Also like a list, iteration is considered to be over values rather than over keys. It provides:

  • len(prop) : Returns the number of elements in the property
  • iter(prop) : Iterates over the values in the property from start to stop.
  • reverse(prop) : Allows the reversed function to iterate from stop to start.
  • prop.items() : Iterates over (index, value) pairs from start to stop.
  • prop.range : A read-only range object representing the range of the property.

Here we have both rangeproperty in it’s natural habitat, and an utterly gratuitious use of assigning additional functions to the property:

>>> import indexedproperty as ix
>>> class Uint32:
...     def __init__(self, val=0):
...         self.word = val
...     def __repr__(self):
...         return "{0}(0x{1:x})".format(type(self).__name__, self.word)
...     @ix.RangeProperty(32)
...     def bit(self, b):
...         return self.word & (1 << b)
...     @bit.setter
...     def bit(self, b, val):
...         v = 1 << b
...         self.word |= v
...         if not val:
...             self.word ^= v
...     @bit.count
...     def bit(self):
...         return sum(bool(b) for b in self.bit)
...     @bit.lowest
...     def bit(self):
...         for idx, b in self.bit.items():
...             if b:
...                 return idx
...         return None
...     @bit.highest
...     def bit(self):
...         for idx in reversed(self.bit.range):
...             if self.bit[idx]:
...                 return idx
...         return None
...     def clear(self):
...         self.word = 0
>>> x = Uint32()
>>> x.bit[1::4] = True
>>> x
>>> x.bit[:8]
[0, 2, 0, 0, 0, 32, 0, 0]
>>> x.bit[15:7:-1]
[0, 0, 8192, 0, 0, 0, 512, 0]
>>> x.bit.count()
>>> x.bit.lowest()
>>> x.bit.highest()
>>> (list(x.bit))[::-1] == list(reversed(x.bit))

What’s Under The Hood

When you get a class member defined as an IndexedProperty, what is returned is a subclass of Trampoline. The definition of that class is local to the specific IndexedProperty under discussion, and is updated every time a new member is created by one of the IndexedProperty’s decorators. In the above example, when @lunch.setter is executed it updates the class definition for the lunch Trampoline to include a setter() method.

So when you ask for x.lunch, you get a new instance of that Trampoline subclass that has that setter function, as well as getter, __iter__, __len__, and items, and a .obj pointer to x. The Trampoline the function calls against it back against the functions originally decorated.

Extending IndexedProperty

New types of indexed properties (such as RangeProperty) can be created by subclassing IndexedProperty. This can be a bit tricky, because the class does some of the work and the Trampoline subclass does the rest.

See the source code for ContainerProperty and RangeProperty for examples of how this is done. Start with ContainerProperty, it’s the more straightforward of the two.

The important logic to follow is

1) The IndexedProperty subclass has a ._Trampoline member, which is a subclass of Trampoline. Class methods for the trampoline that are not specific to a given instance of the IndexedProperty can be defined here. In these methods, the object that the property is a member of is available as self.obj.

2) For class methods (and properties) that are instance specific, the IndexedProperty subclass has a .tdict member, which is the class dictionary for the Trampoline.

3) Having modified the .tdict (probably in __init__), a call to updatetrampoline() will recreate the instance’s ._trampolinecls, which is a subclass of the IndexedProperty ._Trampoline with overloading defined by .tdict. This is what puts the methods, such as the getter, setter, etc, into the _Trampoline.

Another very common usage is wanting a variant on ContainerProperty that performs some minor transformation on the key before checking it against the container. For all IndexedProperty subclasses, the modindex and moduserindex methods can be overloaded to handle verification and modification of keys. For example, if the key is a string it should be made uppercase:

>>> from indexedproperty import ContainerProperty
>>> class UCProperty(ContainerProperty):
...     """A ContainerProperty that transforms string keys to uppercase."""
...     class _Trampoline(ContainerProperty._Trampoline):
...         def modindex(self, index):
...             index = index.upper()
...             return super().modindex(index)
>>> class TestClass:
...     _indices = {'PI':3.14, 'E':2.718, 'I':(0+1j), 'TAU':6.28}
...     @UCProperty(_indices)
...     def constant(self, key):
...         return self._indices[key]

>>> x = TestClass()
>>> x.constant['pi']
>>> x.constant['PI']
>>> x.constant['PI'] = 5
Traceback (most recent call last):
NotImplementedError: no property setter defined
>>> sorted(x.constant.items())
[('E', 2.718), ('I', 1j), ('PI', 3.14), ('TAU', 6.28)]

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