Implementing a Data Model or Type System

Contributed by: David Beazley and Brian K. Jones

Difficulty level: Easy

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Problem

You want to define various kinds of data structures and want to enforce constraints on the values that are allowed to be assigned to certain attributes.

Solution

In this problem, you are basically faced with the task of placing checks or assertions on the setting of certain instance attributes. To do this, you need to customize the setting of attributes on a per-attribute basis; you should use descriptors.

The following code illustrates the use of descriptors to implement a system type and value checking framework:

# Base class. Uses a descriptor to set a value
class Descriptor(object):
    def __init__(self, name=None, **opts):
        self.name = name
        for key, value in opts.items():
            setattr(self, key, value)

    def __set__(self, instance, value):
        instance.__dict__[self.name] = value

# Descriptor for enforcing types
class Typed(Descriptor):
    expected_type = type(None)

    def __set__(self, instance, value):
        if not isinstance(value, self.expected_type):
            raise TypeError('expected ' + str(self.expected_type))
        super().__set__(instance, value)

# Descriptor for enforcing values
class Unsigned(Descriptor):
    def __set__(self, instance, value):
        if value < 0:
            raise ValueError('Expected >= 0')
        super().__set__(instance, value)

class MaxSized(Descriptor):
    def __init__(self, name ...