pvl

Python implementation of a PVL (Parameter Value Language) library.

• Free software: BSD license
• Documentation: http://pvl.readthedocs.org.
• Support for Python 3.6 and higher (avaiable via pypi and conda).
• PlanetaryPy Affiliate Package.

PVL is a markup language, like JSON or YAML, commonly employed for entries in the Planetary Data System used by NASA to archive mission data, among other uses. This package supports both encoding and decoding a variety of PVL 'flavors' including PVL itself, ODL, NASA PDS 3 Labels, and USGS ISIS Cube Labels.

Installation

Can either install with pip or with conda.

To install with pip, at the command line:

$ pip install pvl

Directions for installing with conda-forge:

Installing pvl from the conda-forge channel can be achieved by adding conda-forge to your channels with:

conda config --add channels conda-forge

Once the conda-forge channel has been enabled, pvl can be installed with:

conda install pvl

It is possible to list all of the versions of pvl available on your platform with:

conda search pvl --channel conda-forge

Basic Usage

pvl exposes an API familiar to users of the standard library json module.

Decoding is primarily done through pvl.load() for file-like objects and pvl.loads() for strings:

>>> import pvl
>>> module = pvl.loads("""
...     foo = bar
...     items = (1, 2, 3)
...     END
... """)
>>> print(module)
PVLModule([
  ('foo', 'bar')
  ('items', [1, 2, 3])
])
>>> print(module['foo'])
bar

There is also a pvl.loadu() to which you can provide the URL of a file that you would normally provide to pvl.load().

You may also use pvl.load() to read PVL text directly from an image that begins with PVL text:

>>> import pvl
>>> label = pvl.load('tests/data/pattern.cub')
>>> print(label)
PVLModule([
  ('IsisCube',
   {'Core': {'Dimensions': {'Bands': 1,
                            'Lines': 90,
                            'Samples': 90},
             'Format': 'Tile',
             'Pixels': {'Base': 0.0,
                        'ByteOrder': 'Lsb',
                        'Multiplier': 1.0,
                        'Type': 'Real'},
             'StartByte': 65537,
             'TileLines': 128,
             'TileSamples': 128}})
  ('Label', PVLObject([
    ('Bytes', 65536)
  ]))
])
>>> print(label['IsisCube']['Core']['StartByte'])
65537

Similarly, encoding Python objects as PVL text is done through pvl.dump() and pvl.dumps():

>>> import pvl
>>> print(pvl.dumps({
...     'foo': 'bar',
...     'items': [1, 2, 3]
... }))
FOO   = bar
ITEMS = (1, 2, 3)
END
<BLANKLINE>

pvl.PVLModule objects may also be pragmatically built up to control the order of parameters as well as duplicate keys:

>>> import pvl
>>> module = pvl.PVLModule({'foo': 'bar'})
>>> module.append('items', [1, 2, 3])
>>> print(pvl.dumps(module))
FOO   = bar
ITEMS = (1, 2, 3)
END
<BLANKLINE>

A pvl.PVLModule is a dict-like container that preserves ordering as well as allows multiple values for the same key. It provides similar semantics to a list of key/value tuples but with dict-style access:

>>> import pvl
>>> module = pvl.PVLModule([
...     ('foo', 'bar'),
...     ('items', [1, 2, 3]),
...     ('foo', 'remember me?'),
... ])
>>> print(module['foo'])
bar
>>> print(module.getall('foo'))
['bar', 'remember me?']
>>> print(module.items())
ItemsView(PVLModule([
  ('foo', 'bar')
  ('items', [1, 2, 3])
  ('foo', 'remember me?')
]))
>>> print(pvl.dumps(module))
FOO   = bar
ITEMS = (1, 2, 3)
FOO   = 'remember me?'
END
<BLANKLINE>

However, there are some aspects to the default pvl.PVLModule that are not entirely aligned with the modern Python 3 expectations of a Mapping object. If you would like to experiment with a more Python-3-ic object, you could instantiate a pvl.collections.PVLMultiDict object, or import pvl.new as pvl in your code to have the loaders return objects of this type (and then easily switch back by just changing the import statement). To learn more about how PVLMultiDict is different from the existing OrderedMultiDict that PVLModule is derived from, please read the new PVLMultiDict documentation.

The intent is for the loaders (pvl.load(), pvl.loads(), and pvl.loadu()) to be permissive, and attempt to parse as wide a variety of PVL text as possible, including some kinds of 'broken' PVL text.

On the flip side, when dumping a Python object to PVL text (via pvl.dumps() and pvl.dump()), the library will default to writing PDS3-Standards-compliant PVL text, which in some ways is the most restrictive, but the most likely version of PVL text that you need if you're writing it out (this is different from pre-1.0 versions of pvl).

You can change this behavior by giving different parameters to the loaders and dumpers that define the grammar of the PVL text that you're interested in, as well as custom parsers, decoders, and encoders.

For more information on custom serilization and deseralization see the full documentation.

Contributing

Feedback, issues, and contributions are always gratefully welcomed. See the contributing guide for details on how to help and setup a development environment.