Extensions


Version 1.9.0.1 for version 1.9 of the CF conventions.

The cfdm package has been designed to be subclassed, so that the creation of a new implementation of the CF data model, based on cfdm, is straight forward. For example:

Create a new implementation with a new field construct class.
import cfdm

class my_Field(cfdm.Field):
    def info(self):
        return 'I am a {!r} instance'.format(
            self.__class__.__name__)

The interpretation of CF-netCDF files that is encoded within the cfdm.read and cfdm.write functions is also inheritable, so that an extended data model implementation need not recreate the complicated mapping of CF data model constructs to, and from, CF-netCDF elements. This is made possible by the bridge design pattern, that decouples the implementation of the CF data model from the CF-netCDF encoding so that the two can vary independently.

Define an implementation that is the same as cfdm, but which uses the my_Field class to represent field constructs
>>> my_implementation = cfdm.implementation()
>>> my_implementation.set_class('Field', my_Field)
Define new functions that can read into my_Field instances, and write write my_Field instances to datasets on disk.
>>> import functools
>>> my_read = functools.partial(cfdm.read,
...                             _implementation=my_implementation)
>>> my_write = functools.partial(cfdm.write,
...                              _implementation=my_implementation)
Read my_field constructs from ‘file.nc’, the netCDF file used in the tutorial, using the new my_read function. Inspect a field construct read from the dataset, demonstrating that it is a my_Field instance from the new implementation that has the inherited functionality of a cfdm.Field instance.
>>> q, t = my_read('file.nc')
>>> print(type(q))
<class '__main__.my_Field'>
>>> print(q.info())
I am a 'my_Field' instance
>>> print(repr(q))
<my_Field: specific_humidity(latitude(5), longitude(8)) 1>
>>> print(q.data.array)
[[0.007 0.034 0.003 0.014 0.018 0.037 0.024 0.029]
 [0.023 0.036 0.045 0.062 0.046 0.073 0.006 0.066]
 [0.11  0.131 0.124 0.146 0.087 0.103 0.057 0.011]
 [0.029 0.059 0.039 0.07  0.058 0.072 0.009 0.017]
 [0.006 0.036 0.019 0.035 0.018 0.037 0.034 0.013]]
Write the my_field constructs to a netCDF file using the new my_write function.
>>> my_write([q, t], 'new_file.nc')

Note that, so far, we have only replaced the field construct class in the new implementation, and not any of the metadata constructs or other component classes:

Demonstrate that the metadata construct classes of within the my_Field instance are still cfdm classes.
>>> print(type(q))
<class '__main__.my_Field'>
>>> print(type(q.construct('latitude')))
<class 'cfdm.dimensioncoordinate.DimensionCoordinate'>

If the API of the new implementation is changed such that a given cfdm functionality has a different API in the new implementation, then the new read-from-disk and write-to-disk functions defined above can still be used provided that the new implementation is created from a subclass of cfdm.CFDMImplementation, with the new API being applied in overridden methods.

Create an implementation with a different API.
class my_Field_2(cfdm.Field):
   def my_coordinates(self):
       """Get coordinate constructs with a different API."""
       c = self.coordinates
       if not c:
           return {}
       return c

class my_CFDMImplementation(cfdm.CFDMImplementation):
   def get_coordinates(self, field):
       """Get coordinate constructs from a my_Field_2 instance,
       using its different API.
       """
       return field.my_coordinates()

my_implementation_2 = my_CFDMImplementation(
   cf_version=cfdm.CF(),

   Field=my_Field_2,

   AuxiliaryCoordinate=cfdm.AuxiliaryCoordinate,
   CellMeasure=cfdm.CellMeasure,
   CellMethod=cfdm.CellMethod,
   CoordinateReference=cfdm.CoordinateReference,
   DimensionCoordinate=cfdm.DimensionCoordinate,
   DomainAncillary=cfdm.DomainAncillary,
   DomainAxis=cfdm.DomainAxis,
   FieldAncillary=cfdm.FieldAncillary,

   Bounds=cfdm.Bounds,
   InteriorRing=cfdm.InteriorRing,
   CoordinateConversion=cfdm.CoordinateConversion,
   Datum=cfdm.Datum,

   List=cfdm.List,
   Index=cfdm.Index,
   Count=cfdm.Count,
   NodeCountProperties=cfdm.NodeCountProperties,
   PartNodeCountProperties=cfdm.PartNodeCountProperties,

   Data=cfdm.Data,
   GatheredArray=cfdm.GatheredArray,
   NetCDFArray=cfdm.NetCDFArray,
   RaggedContiguousArray=cfdm.RaggedContiguousArray,
   RaggedIndexedArray=cfdm.RaggedIndexedArray,
   RaggedIndexedContiguousArray=cfdm.RaggedIndexedContiguousArray,
)

As all classes are required for the initialisation of the new implementation class, this demonstrates explicitly that, in the absence of subclasses of the other classes, the cfdm classes may be used.

Read the file into ‘my_Field_2’ instances.
>>> my_read_2 = functools.partial(cfdm.read,
...                               _implementation=my_implementation2)
>>> q, t = my_read_2('file.nc')
>>> print(repr(q))
<my_Field_2: specific_humidity(latitude(5), longitude(8)) 1>

Finally, the mapping of CF data model constructs from CF-netCDF elements, and vice versa, may be modified where desired, leaving all other aspects it unchanged

Modify the mapping of netCDF elements to CF data model instances.
class my_NetCDFRead(cfdm.read_write.netcdf.NetCDFRead):
    def read(self, filename):
        """Read my fields from a netCDF file on disk or from
        an OPeNDAP server location, using my modified mapping
        from netCDF to the CF data model.
        """
        print("Reading dataset using my modified mapping")
        return super().read(filename)
Create a new read-from-disk function that uses the modified mapping.
my_netcdf = my_NetCDFRead(my_implementation_2)
def my_read_3(filename, ):
    """Read my field constructs from a dataset."""
    return my_netcdf.read(filename)
Read the file from disk into ‘my_Field_2’ instances, demonstrating that the modified mapping is being used.
>>> q, t = my_read_3('~/cfdm/docs/source/sample_files/file.nc')
Reading dataset using my modified mapping
>>> print(repr(q))
<my_Field_2: specific_humidity(latitude(5), longitude(8)) 1>

In the same manner, cfdm.read_write.netcdf.NetCDFWrite may be subclassed, and a new write-to-disk function defined, to override aspects of the mapping from CF data model constructs to netCDF elements in a dataset.

The _custom dictionary

All cfdm classes have a _custom attribute that contains a dictionary meant for use in external subclasses.

It is intended for the storage of extra objects that are required by an external subclass, yet can be transferred to copied instances using the inherited cfdm infrastructure. The _custom dictionary is shallow copied, rather than deep copied, when using the standard cfdm deep copy method techniques (i.e. the copy method, initialisation with the source parameter, or applying the copy.deepcopy function) so that subclasses of cfdm are not committed to potentially expensive deep copies of the dictionary values, of which cfdm has no knowledge. Note that calling copy.deepcopy on a cfdm (sub)class simply invokes its copy method. The cfdm library itself does not use the _custom dictionary, other than to pass on a shallow copy of it to copied instances.

The consequence of this shallow-copy behaviour is that if an external subclass stores a mutable object within its custom dictionary then, by default, a deep copy will contain the identical mutable object, to which in-place changes will affect both the original and copied instances.

To account for this, the external subclass can either simply commit to never updating such mutables in-place (which is can be acceptable for private quantities which are tightly controlled); or else include extra code that does deep copy such mutables when any deep copy (or equivalent) operation is called. The latter approach should be implemented in the subclass’s __init__ method, similarly to this:

If desired, ensure that the _custom dictionary ‘x’ value is deep copied when a deep copy of an instance is requested.
import copy

class my_Field_3(cfdm.Field):
    def __init__(self, properties=None, source=None, copy=True,
                 _use_data=True):
        super().__init__(properties=properties, source=source,
                         copy=copy, _use_data=_use_data)
        if source and copy:
            # Deep copy the custom 'x' value
            try:
                self._custom['x'] = copy.deepcopy(source._custom['x'])
            except (AttributeError, KeyError):
                pass

Documentation

The cfdm package uses a “docstring rewriter” that allows commonly used parts of class and class method docstrings to be written once in a central location, and then inserted into each class at import time. In addition, parts of a docstring are modified to reflect the appropriate package and class names. This functionality extends to subclasses of cfdm classes. New docstring substitutions may also be defined for the subclasses.

See cfdm.core.meta.DocstringRewriteMeta for details on how to add to create new docstring substitutions for extensions, and how to modify the substitutions defined in the cfdm package.

A complete example

See cf-python for a complete example of extending the cfdm package in the manner described above.

cf-python adds more flexible inspection, reading and writing; and provides metadata-aware analytical processing capabilities such as regridding and statistical calculations.

It also has a more sophisticated data class that subclasses cfdm.Data, but allows for larger-than-memory manipulations and parallel processing.

cf-python strictly extends the cfdm API, so that a cfdm command will always work on its cf-python counterpart.