indexedproperty 0.1.4
Properties with item-style access
License: MIT
Keywords: properties

==================
Indexed Properties
==================

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

Overview
========

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:

.. code:: python

    >>> 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
    5
    >>> x.iprop['Gilliam'] = 'Terry'
    Setting iprop Gilliam=Terry
    >>> x.iprop['Gilliam']
    'Terry'
    
Decorator Functions
===================

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

    indexedproperty
        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
        
    containerproperty(base)
        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:

.. code:: python

    class C:
        @indexedproperty
        def x(self, key):
            ...
            
        @x.__iter__
        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                            |
+===============================+=================================+
| ::                            | ::                              |
|                               |                                 |
|   x[5]                        |   return x.getter(self, 5)      |
+-------------------------------+---------------------------------+
| ::                            | ::                              |
|                               |                                 | 
|   x[5,10,15]                  |   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] = [              |     x.setter(self, 5, 'Larry')  | 
|       "Larry", "can't",       |     x.setter(self, 10, "can't") |
|       "spel"                  |     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.

Subclasses
==========

ContainerProperty
-----------------

ContainerProperty is an IndexedProperty linked to a *collections.abc.Container*,
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 *collections.abc.Collection*,
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``.

.. code:: python

    >>> 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
-------------

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:

.. code:: python

    >>> 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
    Uint32(0x22222222)
    >>> 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()
    8
    >>> x.bit.lowest()
    1
    >>> x.bit.highest()
    29
    >>> (list(x.bit))[::-1] == list(reversed(x.bit))
    True
    
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:

.. code:: python

    >>> 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']
    3.14
    >>> x.constant['PI']
    3.14
    >>> 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)]