Composable, inheritance-free data modeling in Python

composable_data_modeling.py

"""
Goals:
 - Extensible data modeling in python
 - Does _not_ use inheritance as a mechanism for model extension
 - Lightweight / simple infrastructure
 - Data objects have reasonably nice user API
"""

class Model:
    """Base class providing infrastructure for composable data modeling
    """
    def __init__(self, attachments=()):
        self._attachments = {}
        self._attached_types = set()
        for a in attachments:
            self.attach(a)
            
    def attach(self, attachment):
        name = attachment._attaches_to[type(self)]
        assert name not in self._attachments
        self._attachments[name] = attachment
        self._attached_tyes.add(type(attachment))
        
    def __getattr__(self, name):
        return self._attachments[name]

    def has(self, attachment_type):
        return attachment_type in self._attached_types


### Data model classes

class Image(Model):
    data = None
    
class AcquiredImageMeta(Model):
    _attaches_to = {Image: 'acq'}
    timestamp = None
    transform = None
    device = None

class CameraImageMeta(Model):
    _attaches_to = {AcquiredImageMeta: 'camera'}
    binning = None
    region = None
    exposure = None

class LaserScanImageMeta(Model):
    _attaches_to = {AcquiredImageMeta: 'laser_scan'}
    pixel_dwell_time = None
    laser_power = None
    laser_wavelength = None


### Use example

# one-shot construction:
img = Image(data, attachments=[
    AcquiredImageMeta(timestamp, transform, attachments=[
        CameraImageMeta(region, binning, exposure)
    ])
])

# piecewise construction (because sometimes all of the pieces can't be
# easily instantiated at the same time)
img = Image(data)
img.attach(AcquiredImageMeta(timestamp, transform))
img.acq.attach(CameraImageMeta(region, binning, exposure))


# introspection
assert img.has(AcquiredImageMeta)
assert img.acq.has(CameraImageMeta)

# with modifications to the Model class, we could also support this kind of introspection:
# (all Image instances have an `acq` attribute, but it is set to None if the attachment is not present)
assert img.acq is not None


# data access
assert img.acq.timestamp == timestamp
assert img.acq.camera.binning == binning