items 0.6.5
Attribute accessible dicts and collections thereof
License: Apache License 2.0
Keywords: attributes attrs


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Attributes are the most straightforward and convenient to access composite data
in many situations. ``item.name`` is neater, more readable, and more concise
than the indexing style ``item['name']`` typical of dictionaries. Having
attribute access often is the difference between being able to easily
de-reference a component of ``item`` directly and deciding to store that
attribute in a completely separate variable for clarity (``item_name =
item['name']``).

In traversing data structures from XML, JSON, and other typically-nested data
sources, concise direct access can clean up code considerably.

Items
-----

``items`` therefore provides ``Item``, a convenient attribute-accessible ``dict`` subclass,
plus helper functions.

``itemize``, for example, helps iterate through a sequence of dictionaries, as often found
in JSON processing: Each record is handed back as an ``Item`` rather than a Python
``dict``.

A typical progression would be from:

.. code-block:: python

    for item in data:
        item_name = item['name']
        # ...
        print(item_name)

to

.. code-block:: python

    from items import itemize

    for item in itemize(data):
        # ...
        print(item.name)

To process a sequence wholesale, returning a ``list``:

.. code-block:: python

    from items import itemize_all

    itemize_all(data)

If you're iterating over a sequence of tuples (or lists) rather than
dictionaries, you can still use ``itemize`` by providing the field names
you wnat assigned.

.. code-block:: python

    parser = ...
    for item in itemize(parser, fields='prefix token value'):
        if item.prefix is None and item.token == 'start_array':
            ...

Here each result returned by ``parser`` (typically a Python generator)
is converted from a tuple (or list) into an ``Item``.  Now you have several values
conveniently packaged in a name-accessible way without having to create
a separate ``namedtuple`` for this result type, and without any need for
tuple positional indexing.

You can even do this for a scalar sequence:

    for item in itemize('aeiou', fields='vowel'):
        item.value = 20 if item.vowel == 'e' else 15

Beyond graceful handling of single-valued sequences, this example demonstrates
the mutability of each ``Item``. ``namedtuples`` are grand as return types,
but they cannot be easily extended or annotated by subsequent processing...a
common requirement for many algorithms.

Diving Deeper
-------------

``Item`` subclasses ``collections.OrderedDict``, so keys are ordered the same
as when your program first encountered them. The performance overhead of
ordered mappings is minimal in most development contexts, especially in
exploratory and data-cleanup tasks. Whatever overhead there is is more than
made up for by the programming and debugging clarity of not having keys occur
in seemingly randomized order.

``Item`` s are also permissive, in a way that ``dict`` and its variants usually
are not: If you access ``item.some_attribute`` where the attribute does
not exist, you do **not** raise a ``KeyError``, unlike typical
Python dictionaries. Instead you get ``Empty``, a designated, false-y
value similar to, but distinct from, ``None``. This is convenient for
processing data which is irregular or not uniformly filled-in, because you do
not need the constant "guard conditions"--``if`` statements or
``try``/``except KeyError`` blocks--to protect against cases where this data
value or that is missing. Using ``Empty`` instead of ``None`` preserves your
ability to use ``None`` in cases where it's semantically important. For
example, in parsing JSON, ``None`` is returned from JSON's ``null`` value.

``Empty`` objects are infinitely dereferenceable. No matter how many levels of
indirection, they always just hand back themselves--the same gentle "nothing
here, no exceptions raised" behavior. You can also iterate over an
``Empty``--it will simply iterate zero times. This neatly avoids the common
``TypeError: 'NoneType' object is not iterable`` error messages in instances
where a value can be a list--or ``None`` if the list is not present.

.. code-block:: python

    from items import Empty

    e = Empty
    assert e[1].method().there[33][0].no.attributes[99].here is Empty
    for x in Empty:
        print('hey!')     # never prints, because no such iterations occur

For more on the background of ``Empty``, see the `nulltype <https://pypi.org/project/nulltype/>`_
module. A typical use would be:

.. code-block:: python

    for item in itemize(data):
        if item.name:
            process(item)

Items that lack names are simply not processed.

The more nested, complex, and irregular your data structures, the
more valuable this becomes.

Serialization and Deserialization
=================================

Be careful importing data. Popular Python modules for reading JSON, YAML, and
other formats do not believe mappings are (or should be) ordered. Historically
and officially, they're not, no matter how ordered they look, no matter that
other languages such as JavaScript take a different approach, and no matter how
many Stack Overflow questions demonstrate that ordered input and output is
strongly and broadly desired. Therefore stock input/output modules can cause
dislocation as data is parsed. Take steps to return ordered mappings from them.

.. code-block:: python

    # YAML module that will load into OrderedDict instances, which can then
    # be easily converted to Item instances; based on default PyYAML
    import oyaml as yaml
    data = itemize_all(yaml.load(rawyaml))

    # modified call to json.load or json.loads to preserve order by instantiating
    # Item instances rather than dict
    import json
    data = json.loads(rawjson, object_pairs_hook=Item)

Cycles
======

Not currently organized for handling cyclic data structures. Those do not
appear in processing JSON, XML, and other common data formats, but might
be a nice future extension.

Installation
============

To install or upgrade to the latest version::

    pip install -U items

Sometimes Python installations have different names for ``pip`` (e.g. ``pip``,
``pip2``, and ``pip3``), and on systems with multiple versions of Python, which
``pip`` goes with which Python interpreter can become confusing. In those
cases, try running ``pip`` as a module of the Python version you want to
install under. This can reduce conflicts and confusion::

    python3.6 -m pip install -U items

On Unix, Linux, and macOS you may need to prefix these with ``sudo`` to authorize
installation. In environments without super-user privileges, you may want to
use ``pip``'s ``--user`` option, to install only for a single user, rather
than system-wide.

Testing
=======

If you wish to run the module tests locally, you'll need to install
``pytest`` and ``tox``.  For full testing, you will also need ``pytest-cov``
and ``coverage``. Then run one of these commands::

    tox                # normal run - speed optimized
    tox -e py37        # run for a specific version only
    tox -c toxcov.ini  # run full coverage tests

Notes
=====

* Does not work on Python 2. Should work on Python 3 prior to 3.6, but not guaranteed.
  Testing more difficult given different dictionary ordering rules prior to 3.6.
  Since items intended as forward-looking module, not currently worth energy to retrofit
  to archaic dialects.