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Class allowing for data models equivalently represented as Python dictionaries, JSON, and XML

Project Description
DataModelDict
*************


Introduction
============

The DataModelDict class is used for handling data models that have
equivalent representations in XML, JSON, and Python. Constructing
data models in this way is convenient as it supports compatibility
across different software tools, such as different types of databases.

The DataModelDict class:

* is a child of OrderedDict,

* has methods for converting to/from XML and JSON,

* has methods for searching through elements, and

* has methods that help with constructing and interacting with
compliant data models.


Setup
=====

The code has no requirements that limit which systems it can be used
on, i.e. it should work on Linux, Mac and Windows computers.

The latest release can be installed using pip:

::

pip install DataModelDict

The code and all documentation is hosted on GitHub and can be directly
downloaded at https://github.com/usnistgov/DataModelDict.


Conversions
===========

Some considerations need to be taken into account for designing data
models that allow for exact reversible transformations between the
three formats:

* Valid, full XML requires that there is exactly one root element. In
other words, the top-level DataModelDict of a data model can have
only one key.

* Do not use lists of lists for representing data. The XML
conversions are only reversible for lists of values or lists of
dictionaries. Future updates may allow this.

* Avoid using XML attributes if possible. While the XML conversions
do reversibly handle attributes, it complicates the Python and JSON
representations.

* Embedded XML content, i.e. "text with <embed>embedded</embed>
content", might not be reversible:

..
* If this is in a Python/JSON value, converting to XML gives
"text with &amp;lt;embed&amp;gt;embedded&amp;lt;/embed&amp;gt;
content". This is reversible.

* If this is an XML text field, parsing to Python pulls the
embedded elements out of the text, which is not reversible!

* XML subelements of the same name within an element should be given
consecutively. When parsed, all values of subelements of the same
name are collected together in a list. This will alter the original
order of subelements if matching names were not originally
consecutive.


Conversion from Python to JSON
------------------------------

The Python-JSON conversions use the standard Python JSON library. In
converting from Python to JSON, all elements of the DataModelDict must
be an instance of a supported data type (with unicode and long being
specific to Python 2).

+------------------+-----------+
| Python | JSON |
+==================+===========+
| dict | object |
+------------------+-----------+
| list, tuple | array |
+------------------+-----------+
| str, unicode | string |
+------------------+-----------+
| int, long, float | number |
+------------------+-----------+
| True | true |
+------------------+-----------+
| False | false |
+------------------+-----------+
| None | null |
+------------------+-----------+
| np.nan | NaN |
+------------------+-----------+
| np.inf | Infinity |
+------------------+-----------+
| -np.inf | -Infinity |
+------------------+-----------+

As DataModelDict is a child of OrderedDict, it registers as being an
instance of dict. Any other objects would first need to be converted
to one of these types, e.g. a numpy array would need to be converted
to a list.


Conversion from Python to XML
-----------------------------

The Python-XML conversions use the xmltodict Python package. The XML
content is constructed based on the Python data types.

+------------------+------------------+
| Python | XML |
+==================+==================+
| dict | subelement |
+------------------+------------------+
| list, tuple | repeated element |
+------------------+------------------+
| str, unicode | text |
+------------------+------------------+
| int, long, float | text (from repr) |
+------------------+------------------+
| True | text = True |
+------------------+------------------+
| False | text = False |
+------------------+------------------+
| None | empty text field |
+------------------+------------------+
| np.nan | text = NaN |
+------------------+------------------+
| np.inf | text = Infinity |
+------------------+------------------+
| -np.inf | text = -Infinity |
+------------------+------------------+

Some characters in the XML text fields will also be converted to avoid
conflicts.

* XML limited characters such as <, > and & are converted to their
HTML entities.

* n, t, r are converted to \n, \t, and \r

Any dictionary keys starting with '@' will be converted into XML
attributes, and the dictionary key '#text' is interpreted as the text
value of the element.


Conversion from JSON to Python
------------------------------

The Python-JSON conversions use the standard Python JSON library. In
converting from JSON to Python, the conversions of types is
straight-forward.

+---------------+---------------+---------------+
| JSON | Python 2 | Python 3 |
+===============+===============+===============+
| object | DataModelDict | DataModelDict |
+---------------+---------------+---------------+
| array | list | list |
+---------------+---------------+---------------+
| string | unicode | str |
+---------------+---------------+---------------+
| number (int) | long | int |
+---------------+---------------+---------------+
| number (real) | float | float |
+---------------+---------------+---------------+
| true | True | True |
+---------------+---------------+---------------+
| false | False | False |
+---------------+---------------+---------------+
| null | None | None |
+---------------+---------------+---------------+
| NaN | np.nan | np.nan |
+---------------+---------------+---------------+
| Infinity | np.inf | np.inf |
+---------------+---------------+---------------+
| -Infinity | -np.inf | -np.inf |
+---------------+---------------+---------------+


Conversion from XML to Python
-----------------------------

The Python-XML conversions use the xmltodict Python package. The text
fields will be interpreted based on the following sequential tests:

+------------------+----------+----------+
| XML text | Python 2 | Python 3 |
+==================+==========+==========+
| == 'True' | True | True |
+------------------+----------+----------+
| == 'False' | False | False |
+------------------+----------+----------+
| == '' | None | None |
+------------------+----------+----------+
| == 'NaN' | np.nan | np.nan |
+------------------+----------+----------+
| == 'Infinity' | np.inf | np.inf |
+------------------+----------+----------+
| == '-Infinity' | -np.inf | -np.inf |
+------------------+----------+----------+
| try: int(text) | long | int |
+------------------+----------+----------+
| try: float(text) | float | float |
+------------------+----------+----------+
| otherwise | unicode | str |
+------------------+----------+----------+

The reverse conversions are done for the special characters mentioned
in the Conversion from Python to XML section above.

Any 'attr' attribute fields are converted to elements named '@attr'
and corresponding '#text' elements are created if needed.


Class Documentation
===================

Provides the DataModelDict class for creating, accessing and
manipulating json/xml data structures.

**class DataModelDict.DataModelDict(*args, **kwargs)**

Bases: ``collections.OrderedDict``, ``object``

Class for handling json/xml equivalent data structures.

**append(key, value)**

Adds a value for element key by either adding key to the
dictionary or appending the value as a list to any current
value.

:Parameters:
* **key** -- The dictionary key.

* **value** -- The value to add to the dictionary key. If
key exists, the element is converted to a list if needed
and value is appended.

**aslist(key)**

Gets the value of a dictionary key as a list. Useful for
elements whose values may or may not be lists.

:Parameters:
**key** -- Dictionary key

:Returns:
The dictionary's element value or [value] depending on if it
already is a list.

:Return type:
list

**find(key, yes={}, no={})**

Return the value of a subelement at any level uniquely
identified by the specified conditions.

:Parameters:
* **key** -- Dictionary key to search for.

* **yes** (*dict*) -- Key-value terms which the subelement
must have to be considered a match.

* **no** (*dict*) -- Key-value terms which the subelement
must not have to be considered a match.

:Returns:
The value of the uniquely identified subelement.

:Return type:
any

:Raises:
``ValueError`` -- If exactly one matching subelement is not
identified.

**finds(key, yes={}, no={})**

Finds the values of all subelements at any level identified by
the specified conditions.

:Parameters:
* **key** -- Dictionary key to search for.

* **yes** (*dict*) -- Key-value terms which the subelement
must have to be considered a match.

* **no** (*dict*) -- Key-value terms which the subelement
must not have to be considered a match.

:Returns:
The values of any matching subelements.

:Return type:
list

**iteraslist(key)**

Iterates through the values of a dictionary key. Useful for
elements whose values may or may not be lists.

:Parameters:
**key** -- Dictionary key

:Yields:
*any* -- The dictionary's value or each element in value if
value is a list.

**iterfinds(key, yes={}, no={})**

Iterates over the values of all subelements at any level
identified by the specified conditions.

:Parameters:
* **key** -- Dictionary key to search for.

* **yes** (*dict*) -- Key-value terms which the subelement
must have to be considered a match.

* **no** (*dict*) -- Key-value terms which the subelement
must not have to be considered a match.

:Yields:
*any* -- The values of any matching subelements.

**iterpaths(key, yes={}, no={})**

Iterates over the path lists to all elements at any level
identified by the specified conditions.

:Parameters:
* **key** -- Dictionary key to search for.

* **yes** (*dict*) -- Key-value terms which the subelement
must have to be considered a match.

* **no** (*dict*) -- Key-value terms which the subelement
must not have to be considered a match.

:Yields:
*list of str* -- The path lists to any matching subelements.

**json(fp=None, indent=None, separators=(u', ', u': '))**

Converts the DataModelDict to JSON content.

:Parameters:
* **fp** (*file-like object** or **None**, **optional*) -- An
open file to write the content to. If None (default), then
the content is returned as a str.

* **indent** (*int** or **None**, **optional*) -- Number of
spaces to indent lines. If None (default), the content
will be inline.

* **separators** (*tuple of str**, **optional*) -- Allows for
item_separator and dict_separator) to be changed. Default
is (', ', ': ').

:Returns:
The JSON content (only returned if fp is None).

:Return type:
str, optional

**load(model, format=None)**

Read in values from a json/xml string or file-like object.

:Parameters:
* **model** (*str** or **file-like object*) -- The XML or
JSON content to read. This is allowed to be either a file
path, a string representation, or an open file-like object
in byte mode.

* **format** (*str** or **None**, **optional*) -- Allows for
the format of the content to be explicitly stated ('xml' or
'json'). If None (default), will try to determine which
format based on if the first character of model is '<' or
'{'.

:Raises:
``ValueError`` -- If format is None and unable to identify
XML/JON content, or if format is not equal to 'xml' or
'json'.

**path(key, yes={}, no={})**

Return the path list of a subelement at any level uniquely
identified by the specified conditions. Issues an error if
either no match, or multiple matches are found.

:Parameters:
* **key** -- Dictionary key to search for.

* **yes** (*dict*) -- Key-value terms which the subelement
must have to be considered a match.

* **no** (*dict*) -- Key-value terms which the subelement
must not have to be considered a match.

:Returns:
The subelement path list to the uniquely identified
subelement.

:Return type:
list of str

:Raises:
``ValueError`` -- If exactly one matching subelement is not
identified.

**paths(key, yes={}, no={})**

Return a list of all path lists of all elements at any level
identified by the specified conditions.

:Parameters:
* **key** -- Dictionary key to search for.

* **yes** (*dict*) -- Key-value terms which the subelement
must have to be considered a match.

* **no** (*dict*) -- Key-value terms which the subelement
must not have to be considered a match.

:Returns:
The path lists for any matching subelements.

:Return type:
list

**xml(fp=None, indent=None, full_document=True)**

Return the DataModelDict as XML content.

:Parameters:
* **fp** (*file-like object** or **None**, **optional*) -- An
open file to write the content to. If None (default), then
the content is returned as a str.

* **indent** (*int** or **None**, **optional*) -- Number of
spaces to indent lines. If None (default), the content
will be inline.

* **full_document** (*bool**, **otional*) -- Indicates if the
output is associated with a full xml model. If True
(default), the content can have only one root, and a header
is added.

:Returns:
The XML content (only returned if fp is None).

:Return type:
str, optional


Release History

Release History

This version
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0.9.2

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0.9.1

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