Field encapsulation pattern in Python, part 4 - Field objects

published on 2015-03-07

Here we are in the fourth portion of our trek to create a field encapsulation pattern in Python! For previous legs of our journey, you can try here, here, or here. Otherwise, let’s kick it off!

Where are we?

By the end of our last meeting, we had made pretty good progress. We’re able to type fields on an object, mark them as required so we must provide their values when creating said object, and mark attributes as read-only so we can’t change them after the object has been created.

But, before we go any further with my requirements, let’s take a step sideways. See, while the pattern we’ve created thus far is very flexible, it’s extremely rigid in its flexibility.

The problem with typing

An example is in order. Currently, we can make a field be virtually any type that you can define. This is pretty awesome, because the same code will handle an int, a str, a uuid.UUID, etc. However, what happens if we want to get more specific than that? Let’s say, instead of a str object, I want a URL? It’ll still be represented as a string, but now it has to adhere to specific factors. It has to have http, or maybe ftp, but perhaps https. Then a “://”, some other combination of letters, numbers dashes, dots, etc. After that, it could be .com, .net, .info, .wed, or any of another several hundred (810, to be exact endings. Then, it could be followed by a slash with MORE letters, numbers, characters, etc. Or not.

Now, how can we add a field which is is a URL and be sure that it’s a URL? Maybe we can create a URL object which will handle all that for you, and set the type of that field to be URL. Great… until we want to ensure it’s a URL which points to an FTP site (starting with “ftp://”). Ah. So… maybe extend URL to make a class FTPURL? What if we want to limit the urls to only FTP sites with a top-level-domain of “.net”. Are we going to be making a FTPURLDotNet class?

This is not the only problem with typing. IMO, it’s completely reasonable to have different types for the same value passed into MyClass. Example: 1425797027 instead of a datetime object referring to 6:43:47 AM on Mar. 8, 2015 in UTC. Another example, the string ‘aadac851-3453-4484-b598-6b0e06f0a960’ instead of a uuid.UUID object wrapping this string. You see, we could get data from json, from a database, built on the fly, etc. If a value can be translated into a valid type, I think that it should be accepted. This doesn’t mean that I should accept the string ‘goat’ where a uuid.UUID object is expected, but I should accept something which will properly construct a uuid.UUID object.

Code repetition

Another issue which is less problematic but more pernicious is the fact that we have a dictionary with all the attributes of a field. This means that every time I want to define a field, I have to write down all the attributes for that field. When it’s just ‘type’, no prob. Now, it’s ‘type’, ‘required’, and ‘read-only’. I’ll be adding at least two more. That’s a lot of code which is going to be repeated, often unnecessarily.

Now, I WILL admit that we can just have a default value in places where we use those dictionary attributes:

>>> # instead of this
>>> if attribute_details[field_name]['required']:
...     # do something
>>> # we can do this
>>> if attribute_details[field_name].get('required', False):
...     # if the required attribute isn't set for this field,
...     # assume it's not required; same as having 'required': False

The main issue I have with this is it’s more clunky; I’d rather be sure the value is there than have to go and check for its existence every time I want to access it.

So what are you saying?

Well, it feels that there are enough issues to warrant a refactor. To me, the most logical place to make a split is going to be making a Field object. In this way, we can immediately address the code repetition concern I had, and we will eventually address the typing concern. So, what is the field object going to look like?

>>> class Field(object):
...     def __init__(self, name, type, required=False, read_only=False):
... = name
...         self.type = type
...         self.required = required
...         self.read_only = read_only

Nothing fancy, just a collection of the same attributes in the dictionary. I went ahead and added the name of the field so it can know what it’s a representation of.

Now, I know you’re asking this: why are we not doing all the protection here as we are in the main class? The main reason is that this is going to be essentially internal to the object we’re actually using. When we import MyClass, we probably aren’t going to be importing the Field object as well. Sure, that doesn’t prevent us from modifying the field once it’s on an instance of MyClass. Well, in fact, MyClass is going to do that for us. Remember the __setattr__ and __getattr__ functions on MyClass only deal with specific fields; we just have to be sure we don’t include the fields list as one which can be modified externally!

Yes, we’re getting tricky, so let me show you what goes on here; we’ll take MyClass from the end of last time and update it to use the Field object.

from types import *
import uuid
import datetime

class Field(object):
    def __init__(self, name, type, required=False, read_only=False): = name
        self.type = type
        self.required = required
        self.read_only = read_only

class MyClass:
    attribute_details = {
        'my_uuid': Field('my_uuid', uuid.UUID, required=True),
        'your_uuid': Field('your_uuid', uuid.UUID, required=True),
        'uuid_a': Field('uuid_a', uuid.UUID, read_only=True),
        'uuid_b': Field('uuid_b', uuid.UUID, read_only=True),
        'created_datetime': Field('created_datetime', datetime.datetime, required=True, read_only=True),
        'updated_datetime': Field('updated_datetime', datetime.datetime),
        'target_datetime': Field('target_datetime', datetime.datetime),
        'name': Field('name', StringType, read_only=True),
        'location': Field('location', StringType),
        'num_actions_taken': Field('num_actions_taken', IntType)

    def __init__(self, **kwargs):
        self.__dict__['_in_init'] = True
        for field_name, details in self.attribute_details.iteritems():
            if details.required and field_name not in kwargs:
                raise KeyError('%s is required' % field_name)
            elif field_name in kwargs:
                self.__setattr__(field_name, kwargs[field_name])
        self.__dict__['_in_init'] = False

    def __setattr__(self, attr_name, value):
        attr_detail = self.attribute_details.get(attr_name, None)
        if not attr_detail:
            raise AttributeError('no attribute %s in object of type %s' % (attr_name, self.__class__.__name__))
        if attr_detail.read_only and not self.__dict__['_in_init']:
            raise AttributeError('%s is read-only' % attr_name)
        if type(value) is not attr_detail.type:
            raise ValueError('%s expected to be a %s' % (attr_name, attr_detail.type.__name__))
        self.__dict__['_%s' % attr_name] = value

    def __getattr__(self, attr_name):
        if attr_name not in self.attribute_details:
            raise AttributeError('no attribute "%s" in object of type %s' % (attr_name, self.__class__.__name__))
        return self.__dict__['_%s' % attr_name]

Not a large change so far, not scary, taking it easy… cool. Now, this frees us up to get a leeeeetle more creative.

Shifting the responsibility

So, one of the core tenets of encapsulation (and, indeed, object-oriented programming as a whole) is putting responsibility where it belongs. You wouldn’t expect me to inform the DMV that I’d paid my speeding ticket at the court, especially since every other ticket I’d paid was reported for me; that should be the court’s responsibility, not mine. (Note: in real life, shit happens. Sometimes the court doesn’t tell the DMV that you’d paid a ticket and this results in a suspension of your license. Sometimes you come across badly encapsulated code).

I digress. The point I was trying to make is that we shouldn’t make MyClass responsible for determining if a value is acceptable to the Field. Remember, MyClass told the Field object what type it should look for; we may as well make the Field do something with it.

class Field(object):
    def clean(self, value):
        if type(value) is not self.type:
            raise ValueError('%s expected to be a %s' % (, self.type.__name__)
        return value

class MyClass(object):
    def __setattr__(self, attr_name, value):
        # check that attr_name exists
        # check if read-only
        self.__dict__['_s' % attr_name] = attr_detail.clean(value)

In the example, we’ve added a function called clean. Its main responsibility is to take a value, make sure that it matches the type the Field object expects, raise a ValueError if it doesn’t, and return it if it does.

Now, we’re splitting up responsibilities; MyClass is in charge of wrangling together a bunch of fields (plus some other functionality which we haven’t yet defined and which is outside the scope of this set of articles), and the Field objects will take control over what that field’s value can do and can be. The two will work together to make everything all nice and cozy.

Getting cray-cray

So, going back to the earlier issue I raised with typing. As mentioned, I could create different classes for the types, but that gets pretty cumbersome pretty quickly. However, now that I have a different object for a field and a function which I call to clean up and validate the value of the field, I can shift gears. What if we give a function which knows how to clean and validate a field to the Field object and then call it in the clean function? By this count, we can get rid of limiting a Field object to a type and rely on what’s called “duck-typing”.

Also, while we’re at it, why not be allowed to pass a list of cleaning and/or validating functions? In our earlier example, where we needed to see that the URL is an FTP url in the top-level-domain .net, we could have three separate functions:

  1. assure_value_is_url
  2. assure_url_is_ftp
  3. assure_url_is_dot_net

By doing this, we can mix and match the validation functions across any number of Fields across any number of classes using this paradigm. Each of them is going to be pretty simple by itself, but by combining functions, you can get quite a lot of customization!

Doing this will give us a lot of breathing room. First, I don’t have to create a new object for every arbitrary value, as I just covered. Second, we don’t have to ensure the values are of the correct types wherever we’re setting/constructing MyClass objects; as long as we pass something which will pass validation, we should be good. Third, we can easily share logic all over the place, not just in Field objects. Want to enforce that a value is a url? Run it through assure_value_is_url!

Nuff talking; let’s write some code. We’ll be creating a new module called to contain all the… validators. Next, we’ll write validators for all our fields (we’ll add a little logic later). Finally, we’ll update the declaration of the dictionary with all the Field objects. Oh, one last thing: we’ll pull the required check into the Field also; I mean, it has all the information it needs, right?

# A collection of functions we'll use to ensure values meet certain requirements. Some notes:
# - we should make a reasonable effort to convert a bad value to a good one if 
#       a: it makes sense to do so (i.e. a uuid string -> a UUID object)
#       b: we don't lose the meaning of the object
#       c: we do so in a predictable fashion (i.e. if a value to check is None, we shouldn't create a new arbitrary value)
# - if validation is not successful, raise a ValueError
# - the function should only return the cleaned value. Returning the same value as passed in is acceptable, if it makes sense
# - functions should start with 'ensure', because we are ensuring that, given a value, we'll give back an acceptable
#   value, raising an error if it doesn't.
import datetime
import time
import uuid
from types import Type

def ensure_value_is_uuid(value):
    if type(value) is StringType:
        value = uuid.UUID(value) # this will raise ValueError if it doesn't translate to a UUID
    if type(value) is not uuid.UUID:
        raise ValueError('expected type uuid.UUID, received %s (%s)' % (value, type(value)))
    return value

def ensure_value_is_datetime(value):
    if type(value) is IntType:
        value = datetime.datetime.fromtimestamp(value) # this will raise ValueError if invalid
    elif type(value) is TimeType:
        value = datetime.datetime.fromtimestamp(value.time())
    if type(value) is not datetime.datetime:
        # I could try string types here, but it's just ridiculous to do so
        raise ValueError('expected type datetime.datetime, received %s (%s)' % (value, type(value)))

def ensure_value_is_string(value):
    return value if type(value) is StringType else str(value)

def ensure_value_is_int(value):
    return value if type(value) is IntType else int(value)
import validators as val

class Field(object):
    def __init__(self, name, clean_funcs=[], required=False, read_only=False): = name
        self.clean_funcs = clean_funcs
        self.required = required
        self.read_only = read_only
    def clean(self, value):
        # reducing the array is also feasible, though less readable
        if value is None and self.required:
            raise ValueError('%s is a required field' % (
        for f in self.clean_funcs:
            value = f(value)
        return value

class MyClass:
    fields = {
        'my_uuid': Field('my_uuid', [val.ensure_value_is_uuid], required=True),
        'your_uuid': Field('your_uuid', [val.ensure_value_is_uuid], required=True),
        'uuid_a': Field('uuid_a', [val.ensure_value_is_uuid], read_only=True),
        'uuid_b': Field('uuid_b', [val.ensure_value_is_uuid], read_only=True),
        'created_datetime': Field('created_datetime', [val.ensure_value_is_datetime], required=True, read_only=True),
        'updated_datetime': Field('updated_datetime', [val.ensure_value_is_datetime]),
        'target_datetime': Field('target_datetime', [val.ensure_value_is_datetime]),
        'name': Field('name', [val.ensure_value_is_string], read_only=True),
        'location': Field('location', [val.ensure_value_is_string]),
        'num_actions_taken': Field('num_actions_taken', [val.ensure_value_is_int])

    def __init__(self, **kwargs):
        self.__dict__['_in_init'] = True
        for fields in self.fields.items():
            if field.required and not in kwargs:
                raise KeyError('%s is required' %
            elif in kwargs:
                self.__setattr__(, kwargs[])
        self.__dict__['_in_init'] = False

    def __setattr__(self, attr_name, value):
        field = self.fields.get(attr_name, None)
        if not field:
            raise AttributeError('no attribute %s in object of type %s' % (attr_name, self.__class__.__name__))
        if field.read_only and not self.__dict__['_in_init']:
            raise AttributeError('%s is read-only' % attr_name)
        self.__dict__['_%s' % attr_name] = field.clean(value)

    def __getattr__(self, attr_name):
        if attr_name not in self.fields:
            raise AttributeError('no attribute "%s" in object of type %s' % (attr_name, self.__class__.__name__))
        return self.__dict__['_%s' % attr_name]

Man, made a few changes, added a couple abstractions, but doesn’t it feel good? You may say that it’s more complex than it needs to be. It’s my duty to prove to you that this is not the case. In the next installment, I’ll hammer through the rest of my requirements and demonstrate how helpful this really is. I’ll also add some arbitrary limitations on the fields, possibly adding more, to show just how easy it is to change how the validation works.

For now, however, I bid you adieu!