Metadata

Metadata information associated with a data model is accessed through its meta member. For example, to access the date that an observation was made:

print(model.meta.observation.start_time)

Metadata values are automatically type-checked against the schema when they are set. Therefore, setting a attribute which expects a number to a string will raise an exception.

>>> from roman_datamodels import datamodels as rdm
>>> from roman_datamodels.maker_utils import mk_datamodel

>>> model = mk_datamodel(rdm.ImageModel)
>>> model.meta.target.ra = "foo"  
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/ddavis/miniconda3/envs/rcal_dev/lib/python3.9/site-packages/roman_datamodels/stnode.py", line 183, in __setattr__
    if schema is None or _validate(key, value, schema, self.ctx):
  File "/Users/ddavis/miniconda3/envs/rcal_dev/lib/python3.9/site-packages/roman_datamodels/stnode.py", line 97, in _validate
    return _value_change(attr, tagged_tree, schema, False, strict_validation, ctx)
  File "/Users/ddavis/miniconda3/envs/rcal_dev/lib/python3.9/site-packages/roman_datamodels/stnode.py", line 68, in _value_change
    raise jsonschema.ValidationError(errmsg)
jsonschema.exceptions.ValidationError: While validating ra the following error occurred:
'foo' is not of type 'number'

Failed validating 'type' in schema:
    {'$schema': 'http://stsci.edu/schemas/asdf-schema/0.1.0/asdf-schema',
     'archive_catalog': {'datatype': 'float',
                         'destination': ['ScienceCommon.ra']},
     'sdf': {'source': {'origin': 'PSS:fixed_target.ra_computed'},
             'special_processing': 'VALUE_REQUIRED'},
     'title': 'Target RA at mid time of exposure',
     'type': 'number'}

On instance:
    'foo'

The set of available metadata elements is defined in a YAML Schema that is installed with roman_datamodels from the RAD (Roman Attribute Dictionary).

There is also a utility method for finding the schema associated with a given model.

>>> raw_science = mk_datamodel(rdm.ScienceRawModel)  # Create a model of the desired type
>>> print(raw_science.schema_uri)  # find the associated Schema
asdf://stsci.edu/datamodels/roman/schemas/wfi_science_raw-1.0.0

An alternative method to get and set metadata values is to use a dot-separated name as a dictionary lookup. This is useful for databases, such as CRDS, where the path to the metadata element is most conveniently stored as a string. The following two lines are equivalent

>>> print(raw_science.meta['visit']['start_time'])
2020-01-01T00:00:00.000
>>> print(raw_science.meta.visit.start_time)
2020-01-01T00:00:00.000

In addition the times are stored as Astropy time objects and so the date can be displayed using various formats

>>> print(raw_science.meta.visit.start_time.iso)
2020-01-01 00:00:00.000
>>> print(raw_science.meta.visit.start_time.mjd)
58849.0
>>> print(raw_science.meta.visit.start_time.yday)
2020:001:00:00:00.000