Writing a Geometric Solver in Python - Part 3: Fixing our line

So far we have a nice strong framework for expressing geometric constraints as Constraint objects, and we have a clean way of applying these to our geometric objects. We have clear output when we ask the various objects to repr themselves, and we have clear error messages when things don't quite go to plan. Our constraints now support being reciprocal, and if we constraint one object to another that will magically constraint the other object back to the original if we so wish. Where we left it last time is that we needed to update our ConincidentConstraint object so that it implemented this reciprocal feature, and to do that we needed to upgrade our Line object so that it could take constraints. The code from the previous article can be found in this gist.

Upgrading our line.

To get us started in the last article we created a very simple line object:

#... 

class Line:
    def __init__(self, x1, y1, x2, y2):
        self.x1 = x1
        self.y1 = y1
        self.x2 = x2
        self.y2 = y2

    def __repr__(self):
        return f"Line<({self.x1},{self.y1}),({self.x2},{self.y2})>"

Let's look at how we might upgrade that Line to use ConstraintSet so that we can apply constraints to it. How's this for starters:

class Line(ConstraintSet):
    def __init__(self, name="", start=None, end=None):
        super().__init__(name=name)
        self._start = start if start is not None else Point(name + ".start")
        self._end = end if end is not None else Point(name + ".end")

    @property
    def start(self):
        return self._start

    @property
    def end(self):
        return self._end

    def __repr__(self):
        return f"Line({self._name})<{repr(self.start)},{repr(self.end)}>"

l = Line("l")
print(repr(l))

Line(l)<Point(),Point()>

This is great stuff, we have a line which starts at a point and ends at a point. Let's try and play around with it a bit.

l = Line("l")
p = Point(1,2)
l.start = p
print(repr(l))

AttributeError: can't set attribute

Disaster! The issue here is that we didn't define setters for our start and end parameters. But we didn't have to do that when we defined out Point object, so what is going on? Let's revisit that definition of Point:

class Point(ConstraintSet):
    x = ConstrainedValue()
    y = ConstrainedValue()

    def __init__(self, name="", x=None, y=None):
        super().__init__(self)
        self._name = name
        if x is not None:
            self.x = x
        if y is not None :
            self.y = y

    @property
    def name(self):
        return self._name

So we used ConstrainedValue() at the class level when we defined our Point and that looks very neat. Can we do something similar with Line to specify our Point parameters? We could define a ConstrainedPoint object, but this would get very tiresome once we have a large collection of objects. Let's instead modify ConstrainedValue so that it accepts a subclass of ConstraintSet and then use that to specify the parameters of Line:

class ConstrainedValue:
    """An object which can be passed around to represent a value."""

    def __init__(self, constraint_set_class):
        self._constraint_set_class = constraint_set_class
    #... 
    def __get__(self, instance, typ=None):
        # grab the ConstraintSet from the instance
        constraint_set = getattr(instance, self.private_name, None)

        # If the instance didn't have an initialized ConstraintSet then
        # give it one
        if constraint_set is None:
            constraint_set = self._constraint_set_class(name=f"{instance.name}.{self.public_name}")
            setattr(instance, self.private_name, constraint_set)
        return constraint_set
    #... 

class Line(ConstraintSet):
    start = ConstrainedValue(Point)
    end = ConstrainedValue(Point)

    def __init__(self, name="", start=None, end=None):
        super().__init__(name=name)
        if start is not None:
            self.start = start
        if end is not None:
            self.end = end

    def __repr__(self):
        return f"Line({self._name})<{repr(self.start)},{repr(self.end)}>"

And see how it flies:

l = Line("l")
p = Point('p',1,2)
l.start = p
print(repr(l))

AttributeError: 'Line' object has no attribute 'name'

Whoops! Strangely we chose to define the name property on the Point class even though _name is implemented in ConstraintSet. Let's extract that up to ConstraintSet, which will simplify Point a little.

class ConstraintSet:
    #... 
    @property
    def name(self):
        return self._name
    #... 

    def validate_object(self, instance):
        if not isinstance(instance, ConstraintSet):
            raise InvalidConstraintException(f"{self.__class__.__name__} can only"
            f" be applied to `ConstraintSet`, it cannot be applied to `{instance.__class__.__name__}`")

    def apply_reciprocal_constraint(self, instance):
        pass

    def cascade_constraints(self, instance):
        pass
    #... {{% /skip %}}
class Point(ConstraintSet):
    x = ConstrainedValue(ConstraintSet)
    y = ConstrainedValue(ConstraintSet)

    def __init__(self, name="", x=None, y=None):
        super().__init__(self)
        self._name = name
        if x is not None:
            self.x = x
        if y is not None :
            self.y = y

And have another go at defining Line:

class Line(ConstraintSet):
    start = ConstrainedValue(Point)
    end = ConstrainedValue(Point)

    def __init__(self, name="", start=None, end=None):
        super().__init__(name=name)
        if start is not None:
            self.start = start
        if end is not None:
            self.end = end

    def __repr__(self):
        return f"{self.__class__.__name__}({self.name})<{repr(self.start)},{repr(self.end)}>"

l = Line('l')
p = Point('p', 1,2)
l.start = p
print(repr(l))

Line(l)<Point(
    LinkedValueConstraint<p>
),Point()>

Woohoo! This is exactly what we were after.

Returning to the Constraint

So now we must upgrade our CoincidentConstraint so that it provides a reciprocal constraint and implements the three methods with introduced to the Constraint class in the last article. A question we must first answer is what should we call our reciprocal constraint. It seems to me to be equally acceptable to say that a line is coincident with a point as it is to say that a point is coincident with a line. So our natural language tells us that we should use the same constraint object for both the constraint and its reciprocal, which means we need to make our CoincidentConstraint object apply to a Line as well as a Point.

There are other circumstances where this kind of relationship is appropriate. For example if a line is tangent to a circle, then it is equally acceptable to say that the circle is tangent to a line, so it feels like this is a pattern which we may well use again.

Let's remind ourselves what our existing CoincidentConstraint class looks like:

class CoincidentConstraint(Constraint):
    def __init__(self, line):
        self._line = line

    @property
    def line(self):
        return self._line

    def __repr__(self):
        return f"{self.__class__.__name__}<{self.line}>"

    def constraint_callback(self, point):
        if not isinstance(point, Point):
            raise InvalidConstraintException(f"{self.__class__.__name__} can only"
            f" be applied to `Point`, it cannot be applied to `{point.__class__.__name__}`")
        point.x.constrain_with(InfluencedConstraint(self))
        point.y.constrain_with(InfluencedConstraint(self))

Let's first of all add the new methods, without considering the addition of the Line functionality, and we'll leave the apply_reciprocal_constraint method empty for the moment.

class CoincidentConstraint(Constraint):
    #... 

    def validate_object(self, instance):
        if not isinstance(point, Point):
            raise InvalidConstraintException(f"{self.__class__.__name__} can only"
            f" be applied to `Point`, it cannot be applied to `{instance.__class__.__name__}`")

    def apply_reciprocal_constraint(self, instance):
        pass

    def cascade_constraints(self, instance):
        point.x.constrain_with(InfluencedConstraint(self))
        point.y.constrain_with(InfluencedConstraint(self))

We also need to define an __eq__ method to avoid repeating our stack overflow woes from last time, so let's go ahead and do that now.

class CoincidentConstraint(Constraint):
    #... 
    def __eq__(self, other):
        return type(other) == type(self) and self.line == other.line

In order for our CoincidentConstraint to apply to a Line we must allow it to accept a Point. That's a little more messy that it might at first appear, as we'll need to adapt our shiny new __eq__ method to consider this possibility, and the same for our __repr__ method:

class CoincidentConstraint(Constraint):
    def __init__(self, object):
        self._line = object if type(object) == Line else None
        self._point = object if type(object) == Point else None
    #... 
    @property
    def point(self):
        return self._point

    def __repr__(self):
        if self.line is not None:
            return f"{self.__class__.__name__}<{self.line}>"
        return f"{self.__class__.__name__}<{self.point}>"

    #... 
    def __eq__(self, other):
        return type(other) == type(self) and self.line == other.line and self.point == other.point

And now let's modify the validate_object method so that it checks for the correct object type, and tweak cascade_constraints so that it cascades as well to a Line as it does to a Point:

class CoincidentConstraint(Constraint):
    #... 

    def validate_object(self, instance):
        if self._line is not None:
            if not isinstance(instance, Point):
                raise InvalidConstraintException(f"{self.__class__.__name__} which has been"
                " assigned a Line can only be applied to Point, it cannot be applied to"
                f" `{instance.__class__.__name__}`")
        else:
            if not isinstance(instance, Line):
                raise InvalidConstraintException(f"{self.__class__.__name__} which has been"
                " assigned a Point can only be applied to Line, it cannot be applied to"
                f" `{instance.__class__.__name__}`")

    #...

p = Point('p')
c = CoincidentConstraint(p)
l = Line('l')
l.constrain_with(c)
print(repr(l))

Line(l)<Point(
    InfluencedConstraint<CoincidentConstraint<p>>
),Point(
    InfluencedConstraint<CoincidentConstraint<p>>
)>

Aha! Let's alter our Line __repr__ method so that it includes the constraints at the Line level:

class Line(ConstraintSet):
    #... 
    def __repr__(self):
        repr_string = f"{self.__class__.__name__}({self.name})<{repr(self.start)},{repr(self.end)}>"
        if len(self._constraints) == 0:
            return repr_string + "()"
        repr_string += "(\n"
        for constraint in self._constraints:
            repr_string += f"    {constraint}\n"
        repr_string += ")"
        return repr_string

p = Point('p')
c = CoincidentConstraint(p)
l = Line('l')
l.constrain_with(c)
print(repr(l))
m = Line('m')
d = CoincidentConstraint(m)
q = Point('q')
q.constrain_with(d)
print(repr(q))

Line(l)<Point(
    InfluencedConstraint<CoincidentConstraint<p>>
),Point(
    InfluencedConstraint<CoincidentConstraint<p>>
)>(
    CoincidentConstraint<p>
)
Point(
    CoincidentConstraint<m>
)

Now we can see the constraints to which the parameters are bound, as well as those to which the object is bound, or we can for the Line object at least. The Point object is a lot less forthcoming, and on reflection its kind of annoying having to implement __repr__ on every object we define. Is there a way we can do this at the ConstraintSet level and just forget about __repr__ for the rest of time. We can if we make an assumption. I'm pretty sure that the assumption is safe, but I'm also pretty sure that I've been bitten by every other assumption I've ever made. Let's live dangerously and implement a generic __repr__ method. If it bites us we can re-implement it later.

A More Generic Representation

To write something generic, we need a list of the ConstrainedValue attributes in our class. Let's tweak our ConstrainedValue class so that it keeps track:

```python {hl_lines=[4-5,14]}
class ConstrainedValue:
"""An object which can be passed around to represent a value."""
#...
def set_name(self, owner, name):
self.public_name = name
self.private_name = f"_{name}"
# append the name to the list of ConstrainedSets on the class
# creating that list if it doesn't exist
try:
constraint_sets = owner._constraint_sets
except AttributeError:
constraint_sets = []
owner._constraint_sets = constraint_sets
finally:
owner._constraint_sets.append(self.public_name)
#...

Now we can iterate through that list in our `__repr__` method. For the avoidance of doubt, the assumption here being that the only attributes we're printing out are the ones that are participating in our model, and all of those will be of type `ConstraintSet`. Let's see how that goes:

<!--phmdoctest-share-names-->


```python class ConstraintSet: #... 
    def __repr__(self):
        retval = f"{self.name}: {self.__class__.__name__}"
        try:
            num_constraint_sets = len(self._constraint_sets)
        except AttributeError:
            num_constraint_sets = 0
        if num_constraint_sets == 0:
            retval += "()"
        else:
            constraint_set_string = ""
            for constraint_set_name in self._constraint_sets:
                constraint_set_string += repr(getattr(self,constraint_set_name))
            retval += "(\n"
            retval += "    " + "    ".join([l for l in constraint_set_string.splitlines(True)])
            retval += ")\n"

        if len(self._constraints) == 0:
            retval += "<>\n"
        else:
            retval += "<\n"
            for constraint in self._constraints:
                retval += "    " + "    ".join([l for l in repr(constraint).splitlines(True)])
            retval += "\n>\n"
        return retval
    #...

And redefine Point and Line without the __repr__ method:

class Point(ConstraintSet):
    x = ConstrainedValue(ConstraintSet)
    y = ConstrainedValue(ConstraintSet)

    def __init__(self, name="", x=None, y=None):
        super().__init__(self)
        self._name = name
        if x is not None:
            self.x = x
        if y is not None :
            self.y = y

class Line(ConstraintSet):
    start = ConstrainedValue(Point)
    end = ConstrainedValue(Point)

    def __init__(self, name="", start=None, end=None):
        super().__init__(name=name)
        if start is not None:
            self.start = start
        if end is not None:
            self.end = end

#...

These are starting to look super clean! That apparently duplicated code in the __init__ methods hasn't escaped my attention, but I don't want to get distracted. Let's check that that last change worked:

p = Point('p')
c = CoincidentConstraint(p)
l = Line('l')
l.constrain_with(c)
print(repr(l))
m = Line('m')
d = CoincidentConstraint(m)
q = Point('q')
q.constrain_with(d)
print(repr(q))

l: Line(
    l.start: Point(
        l.start.x: ConstraintSet()<>
        l.start.y: ConstraintSet()<>
    )
    <
        InfluencedConstraint<CoincidentConstraint<p>>
    >
    l.end: Point(
        l.end.x: ConstraintSet()<>
        l.end.y: ConstraintSet()<>
    )
    <
        InfluencedConstraint<CoincidentConstraint<p>>
    >
)
<
    CoincidentConstraint<p>
>

q: Point(
    q.x: ConstraintSet()<
        InfluencedConstraint<CoincidentConstraint<m>>
    >
    q.y: ConstraintSet()<
        InfluencedConstraint<CoincidentConstraint<m>>
    >
)
<
    CoincidentConstraint<m>
>

This is a really great output. We can really see how the hierarchy of our constraints is building up. So what about that initializer? Well it turns out we can play a pretty similar trick.

Generic Initialization

Firstly let's describe the behavior we're after, using our Point object as an example:

# Firstly we should be able to define a point specifying nothing at all:
p = Point()

# The it would be great to have a simple constructor which can accept some values:
p = Point(1,2)

# Finally we should be able to specify parameters by nae:
p = Point(x=1, y=2, name='p')

In order to achieve this, in particular the 2nd example, we need to be able to provide a default name. Something like Point1, when no name is specified, but obviously unique for each point created. Let's add a generate_name method to ConstraintSet which can do this for us:

class ConstraintSet:
    #... 
    @classmethod
    def _generate_name(cls):
        try:
            index = cls._counter
        except AttributeError:
            index = 0
        cls._counter = index + 1
        return f"{cls.__name__}{index}"
    #...

It's worth reflecting on how this method works, as its a little subtle. Firstly this method is decorated with @classmethod This means that the class is passed in as the first parameter instead of the instance. By convention this is called cls to distinguish from self in a normal method where the instance is passed in. By using a class method, and the value of cls we can have a different counter for Point and Line. Next try and assign the the value of _counter to index, if we've not yet initialized it this will throw an AttributeError, we catch that and set index to zero. This is an example of "EAFP" or "Easier to ask forgiveness than permission" coding. This rule is not part of PEP20, but it is well established python coding style. Now we set the value of _counter to one more than index, which will initialize counter if its not already, and finally construct our default name out of type(self).__name__ and our index. Let's give it a quick test. The _ prefix by convention means that this is a private method which shouldn't be called from outside of the class, but python does nothing to enforce that, so our test is nice and easy to write:

l = Line()
print(l._generate_name())
print(l._generate_name())
p = Point()
print(p._generate_name())
print(p._generate_name())

Line0
Line1
Point0
Point1

Perfect. Now let's write a generic initializer on ConstraintSet which means we don't have to write initializers on all our objects. Let's remind ourselves of our target behavior:

# Firstly we should be able to define a point specifying nothing at all:
p = Point()

# The it would be great to have a simple constructor which can accept some values:
p = Point(1,2)

# Finally we should be able to specify parameters by nae:
p = Point(x=1, y=2, name='p')

The final behavior is the easiest to implement. We will iterate through the _constraint_sets we built for our __repr__ method and assign values if matching kwargs have been passed. We pop them off kwargs and then pass the remaining kwargs to super().__init__. This last part is important as it preserves our ability to subclass. To pluck an example out of thing air, we might want to create a "DoubleLine" class which draw two lines right next to each other, and takes the distance between the two lines as a parameter:

class DoubleLine(Line):
    distance = ConstrainedValue(ConstraintSet)

dl = DoubleLine(start=Point(1,2), end=Point(2,3), distance=0.1)

If we miss out the "pop and pass" part of our initializer, then Line will never be sent the correct values, and start and end would never be set. A first stab at this generic initializer looks like this:

class ConstraintSet:
    def __init__(self, *args, **kwargs):
        self._constraints = []
        try:
            constraint_sets = self._constraint_sets
        except AttributeError:
            # _constraint_sets is not set if `self` is a top level ConstraintSet
            # so this is not an error, there's just nothing to do.
            pass
        else:
            for constraint_set_name in self._constraint_sets:
                try:
                    setattr(self, constraint_set_name, kwargs.pop(constraint_set_name))
                except KeyError:
                    # Not a problem if a value for _constraint_set_name has not been provided.
                    pass
        super().__init__(*args, **kwargs)

name is not a ConstraintSet so won't appear in our list, so we must explicitly handle that. It's tempting to write something like this:

class ConstraintSet:
    def __init__(self, *args, **kwargs):
        self._constraints = []
        name = kwargs.pop('name') if 'name' in kwargs else self._generate_name
        self._name = name
        # ... ConstraintSet code
    # ...

However if we did write this, then the call to super().__init__() would not have a name parameter, so that call would immediately overwrite our name with a _generate_name value. To prevent this we must check to see if _name is already defined, and only provide a default value if it is not.

class ConstraintSet:
    def __init__(self, *args, **kwargs):
        self._constraints = []
        try:
            if not hasattr(self._name):
                self._name = kwargs.pop('name')
        except IndexError: # 'name' has not been provided as a kwargs, so provide a default value
            self._name = self._generate_name()
        super().__init__(*args, **kwargs)
    #... ConstraintSet code

Lastly we need to consider args. To maximize code reuse, the best thing to do is to work out which ConstraintSet each arg belongs to, and add it explicitly to **kwargs. The initializers are called from the most specific subclass up to the most general. Our DoubleLine for example would have its initializer called first, and then the initializer for Line and finally the initializer for ConstraintSet. We therefore want to start at the end of our list and work towards the start, so that more specific args go at the end of the call. In order to make this work we'll need to split out arg processing into a separate method so that we can call it further up the initializer. Also, by default, args is passed to us a tuple which is immutable, so we'll need to change it to something we can remove values from. A List will do:

def __init__(self, *args, **kwargs):
        self._constraints = []
        # converts args to a list, so that we can mutate it
        args = list(args)
    #... name code
        kwargs |= self._process_args(args)
    #... ConstraintSet code 

    def _process_args(self, args):
        # give our parent a chance to nab the arguments before us:
        try:
            super().__process_args(args)
        except AttributeError:
            # not a problem if `super() doesn't have `__process_args`
            # just means we're near the top of the tree
            pass

        retval = dict()
        # iterate backwards through our constraints, and add a
        # dictionary entry for each arg whilst one exists
        try:
            for constraint_set_name in self._constraint_sets[::-1]:
                retval[constraint_name] = args.pop()
        except IndexError:
            # just means we got to the end of the list
            pass
        return retval

Putting these together gives us a mammoth initializer, so I've broken out the kwargs processing into a separate method too, to give anyone reading this code half a chance of following it:

class ConstraintSet:

    def __init__(self, *args, **kwargs):
        self._constraints = []
        # convert args to a list, so that we can mutate it
        args = list(args)
        """give ourselves a sensible name unless one is provided."""
        try:
            if not hasattr(self, '_name'):
                self._name = kwargs.pop('name')
        except KeyError: # 'name' has not been provided as a kwargs, so provide a default value
            self._name = self._generate_name()
        """copy args into the appropriate place in kwargs"""
        kwargs |= self._process_args(args)
        """assign each kwarg to its matching ConstraintSet"""
        self._process_kwargs(kwargs)
        super().__init__(*args, **kwargs)

    def _process_args(self, args):
        # give our parent a chance to nab the arguments before us:
        try:
            super().__process_args(args)
        except AttributeError:
            # not a problem if `super() doesn't have `__process_args`
            # just means we're near the top of the tree
            pass

        retval = dict()
        # iterate backwards through our constraints, and add a
        # dictionary entry for each arg whilst one exists
        try:
            for constraint_set_name in self._constraint_sets[::-1]:
                retval[constraint_set_name] = args.pop()
        except (IndexError, AttributeError):
            # just means we got to the end of the list
            pass

        return retval

    def _process_kwargs(self, kwargs):
        try:
            constraint_sets = self._constraint_sets
        except AttributeError:
            # _constraint_sets is not set if `self` is a top level ConstraintSet
            # so this is not an error, there's just nothing to do.
            pass
        else:
            for constraint_set_name in self._constraint_sets:
                try:
                    setattr(self, constraint_set_name, kwargs.pop(constraint_set_name))
                except KeyError:
                    # Not a problem if a value for _constraint_set_name has not been provided.
                    pass

    #... 

    def validate_object(self, instance):
        if not isinstance(instance, ConstraintSet):
            raise InvalidConstraintException(f"{self.__class__.__name__} can only"
            f" be applied to `ConstraintSet`, it cannot be applied to `{instance.__class__.__name__}`")

    def apply_reciprocal_constraint(self, instance):
        pass

    def cascade_constraints(self, instance):
        pass

class ConstrainedValue:
    """An object which can be passed around to represent a value."""

    def __init__(self, constraint_set_class):
        self._constraint_set_class = constraint_set_class

    def __set_name__(self, owner, name):
        self.public_name = name
        self.private_name = f"_{name}"
        # append the name to the list of ConstrainedSets on the class
        # creating that list if it doesn't exist
        try:
            constraint_sets = owner._constraint_sets
        except AttributeError:
            constraint_sets = []
            owner._constraint_sets = constraint_sets
        finally:
            owner._constraint_sets.append(self.public_name)

    def __get__(self, instance, typ=None):
        # grab the ConstraintSet from the instance
        constraint_set = getattr(instance, self.private_name, None)

        # If the instance didn't have an initialized ConstraintSet then
        # give it one
        if constraint_set is None:
            constraint_set = self._constraint_set_class(name=f"{instance.name}.{self.public_name}")
            setattr(instance, self.private_name, constraint_set)
        return constraint_set

    def __set__(self, instance, value):
        # Grab the ConstraintSet from the instance
        constraint_set = self.__get__(instance, None)

        constraint_set.reset_constraints()
        # if the value we've been asked to assign is a ConstraintSet
        # then add a LinkedValueConstraint:
        if isinstance(value, ConstraintSet):
            constraint_set.constrain_with(LinkedValueConstraint(value))
            return

        # otherwise use a FixedValueConstraint to constrain to the provided
        # value
        constraint_set.constrain_with(FixedValueConstraint(value))
    #... {{% /skip %}}

Redefine Point and Line without their initializers...

class Point(ConstraintSet):
    x = ConstrainedValue(ConstraintSet)
    y = ConstrainedValue(ConstraintSet)

class Line(ConstraintSet):
    start = ConstrainedValue(Point)
    end = ConstrainedValue(Point)

#...

Let's test this by revisiting our behaviors:

# Firstly we should be able to define a point specifying nothing at all:
p = Point()
print(repr(p))

# The it would be great to have a simple constructor which can accept some values:
p = Point(1,2)
print(repr(p))

# Finally we should be able to specify parameters by nae:
p = Point(x=1, y=2, name='p')
print(repr(p))

Point0: Point(
    Point0.x: ConstraintSet()<>
    Point0.y: ConstraintSet()<>
)
<>

Point1: Point(
    Point1.x: ConstraintSet()<
        FixedValueConstraint<1>
    >
    Point1.y: ConstraintSet()<
        FixedValueConstraint<2>
    >
)
<>

p: Point(
    p.x: ConstraintSet()<
        FixedValueConstraint<1>
    >
    p.y: ConstraintSet()<
        FixedValueConstraint<2>
    >
)
<>

This is pretty incredible stuff. We've managed to abstract all our functionality into our ConstrainedValue and ConstraintSet classes so that we have this super clean and easy to use developer interface. The only thing which bugs me slightly. Have another look at this listing:

class Point(ConstraintSet):
    x = ConstrainedValue(ConstraintSet)
    y = ConstrainedValue(ConstraintSet)

class Line(ConstraintSet):
    start = ConstrainedValue(Point)
    end = ConstrainedValue(Point)

To my mind, ConstrainedValue(Point) is intuitive. It clearly says that we'd like a constrained value and that it should be of type Point. ConstrainedValue(ConstraintSet) doesn't have the same feel however, it really isn't clear what type the value is. Let's fix that by adding an alias clas to ConstrainedSet which has a more appropriate name for the end user:

class Scalar(ConstraintSet):
    """Alias for ConstraintSet to provide more intuitive name"""
    pass

class Point(ConstraintSet):
    x = ConstrainedValue(Scalar)
    y = ConstrainedValue(Scalar)

#... 
p = Point(1,2)
print(repr(p))

Point0: Point(
    Point0.x: Scalar()<
        FixedValueConstraint<1>
    >
    Point0.y: Scalar()<
        FixedValueConstraint<2>
    >
)
<>

Let's just quickly check that we've not broken anything:

#... 
p = Point()
c = CoincidentConstraint(p)
l = Line()
l.constrain_with(c)
q = Point()
l.start = q
print(repr(l))

Line0: Line(
    Line0.start: Point(
        Line0.start.x: Scalar()<>
        Line0.start.y: Scalar()<>
    )
    <
        LinkedValueConstraint<Point1>
    >
    Line0.end: Point(
        Line0.end.x: Scalar()<>
        Line0.end.y: Scalar()<>
    )
    <
        InfluencedConstraint<CoincidentConstraint<Point0>>
    >
)
<
    CoincidentConstraint<Point0>
>

Small bug, looks like the LinkedValueConstraint has nuked the InfluencedConstraint from Line0.end. This is because our __set__ method in ConstrainedValue calls reset_constraints which probably seemed like a good idea at the time, but now looks more like a bug. Let's remove that call:

class ConstrainedValue:
    """An object which can be passed around to represent a value."""
    #... 
    def __set__(self, instance, value):
        # Grab the ConstraintSet from the instance
        constraint_set = self.__get__(instance, None)

        # if the value we've been asked to assign is a ConstraintSet
        # then add a LinkedValueConstraint:
        if isinstance(value, ConstraintSet):
            constraint_set.constrain_with(LinkedValueConstraint(value))
            return

        # otherwise use a FixedValueConstraint to constrain to the provided
        # value
        constraint_set.constrain_with(FixedValueConstraint(value))

#...

#... 
p = Point()
c = CoincidentConstraint(p)
l = Line()
l.constrain_with(c)
q = Point()
l.start = q
print(repr(l))

Line0: Line(
    Line0.start: Point(
        Line0.start.x: Scalar()<>
        Line0.start.y: Scalar()<>
    )
    <
        InfluencedConstraint<CoincidentConstraint<Point0>>
        LinkedValueConstraint<Point1>
    >
    Line0.end: Point(
        Line0.end.x: Scalar()<>
        Line0.end.y: Scalar()<>
    )
    <
        InfluencedConstraint<CoincidentConstraint<Point0>>
    >
)
<
    CoincidentConstraint<Point0>
>

Phew! We made it! We now have a super powerful little framework with which we can model constraints. It's really worth reflecting on what we've achieved here. Not only have we implemented all that power, but if we compare the implementation of our Point with the naive implementation we started with at the beginning of part 1, you'll see its actually simpler to implement using our framework than without.

In the next article we'll look at adding some more constraints into the mix. In particular we'll look at how we can do some simple arithmetic operations with out ConstraintSet.

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