soln7_3.md

Exercise 7.3 - Solution

Here's a full version of the structure.py file.

# structure.py

from validate import Validator, validated

class Structure:
    _fields = ()
    _types = ()

    def __setattr__(self, name, value):
        if name.startswith('_') or name in self._fields:
            super().__setattr__(name, value)
        else:
            raise AttributeError('No attribute %s' % name)

    def __repr__(self):
        return '%s(%s)' % (type(self).__name__,
                           ', '.join(repr(getattr(self, name)) for name in self._fields))

    @classmethod
    def from_row(cls, row):
        rowdata = [ func(val) for func, val in zip(cls._types, row) ]
        return cls(*rowdata)

    @classmethod
    def create_init(cls):
        '''
        Create an __init__ method from _fields
        '''
        args = ','.join(cls._fields)
        code = f'def __init__(self, {args}):\n'
        for name in cls._fields:
            code += f'    self.{name} = {name}\n'
        locs = { }
        exec(code, locs)
        cls.__init__ = locs['__init__']

    @classmethod
    def __init_subclass__(cls):
        # Apply the validated decorator to subclasses
        validate_attributes(cls)

def validate_attributes(cls):
    '''
    Class decorator that scans a class definition for Validators
    and builds a _fields variable that captures their definition order.
    '''
    validators = []
    for name, val in vars(cls).items():
        if isinstance(val, Validator):
            validators.append(val)

        # Apply validated decorator to any callable with annotations
        elif callable(val) and val.__annotations__:
            setattr(cls, name, validated(val))

    # Collect all of the field names
    cls._fields = tuple([v.name for v in validators])

    # Collect type conversions. The lambda x:x is an identity
    # function that's used in case no expected_type is found.
    cls._types = tuple([ getattr(v, 'expected_type', lambda x: x)
                   for v in validators ])

    # Create the __init__ method
    if cls._fields:
        cls.create_init()
    
    return cls

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