items 0.5.3

Attribute accessible dicts and collections thereof

Attribute-accessible dictonaries are the most convenient way to access dictionaries and other mappings in many algorithms. item.name is more readable and concise than item['name']. 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 to make working with Item s.

itemize, for example, helps iterate of a list 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:

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

to

from items import itemize

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

To process a list wholesale:

from items import itemize_all

itemize_all(data)

Item objects are subclasses of collections.OrderedDict, so that keys are ordered the same as when yoor program first encountered them. The performance or 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 random locations.

Item``s are also permissive, in a way that ``dict and its variants often are not: If you access item.arbitary_attribute where the attribute does not exist, you do not raise a KeyError as you might expect from normal Python dictionaries. Instead you get back Empty, a designated, false-y value similar to, but distinct from, None. This is convenient for processing data which is not irregular and not uniformly filled-in, because you do not need the constant “gaurd conditions”–either 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 semanticailly 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, but 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.

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:

for item in itemize(data):
    if item.name:
        # if there is a name attribute, it's processed here
    # if not, no problem; processing just continues here

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

Serialization and Deserialization

Be careful importing data from files. Popular Python modules for reading JSON, YAML, and other formats do not believe mappings are 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 import is stronly and broadly desired. Therefore stock input/output modules can cause dislocation as data is parsed. Take steps to return ordered mappings from them.

# 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
items = itemize_all(yaml.load(data))

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

Recursion

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

Alternate Construction

The constructor for Item is intended to take an existing mapping, or a set of keywords. Unlike most dict subtypes, it will not directly convert a primitive list of two-element tuples (a historical analog of dictionaries in Python, from before the advent of OrderedDict) directly. If you need to convert such a typle list into an Item, use the class method Item.from_tuples(...).

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 conflects 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 py27        # run for a specific version only (e.g. py27, py34)
tox -c toxcov.ini  # run full coverage tests