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Creational Design Patterns in Python

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In this chapter from Python in Practice: Create Better Programs Using Concurrency, Libraries, and Patterns, Mark Summerfield discusses creational design patterns in Python, including the Abstract Factory, Builder, Factory Method, Prototype, and Singleton Patterns.
This chapter is from the book

Creational design patterns are concerned with how objects are created. Normally we create objects by calling their constructor (i.e., calling their class object with arguments), but sometimes we need more flexibility in how objects are created—which is why the creational design patterns are useful.

For Python programmers, some of these patterns are fairly similar to each other—and some of them, as we will note, aren’t really needed at all. This is because the original design patterns were primarily created for the C++ language and needed to work around some of that language’s limitations. Python doesn’t have those limitations.

1.1. Abstract Factory Pattern

The Abstract Factory Pattern is designed for situations where we want to create complex objects that are composed of other objects and where the composed objects are all of one particular “family”.

For example, in a GUI system we might have an abstract widget factory that has three concrete subclass factories: MacWidgetFactory, XfceWidgetFactory, and WindowsWidgetFactory, all of which provide methods for creating the same objects (make_button(), make_spinbox(), etc.), but that do so using the platform-appropriate styling. This allows us to create a generic create_dialog() function that takes a factory instance as argument and produces a dialog with the OS X, Xfce, or Windows look and feel, depending on which factory we pass it.

1.1.1. A Classic Abstract Factory

To illustrate the Abstract Factory Pattern we will review a program that produces a simple diagram. Two factories will be used: one to produce plain text output, and the other to produce SVG (Scalable Vector Graphics) output. Both outputs are shown in Figure 1.1. The first version of the program we will look at, diagram1.py, shows the pattern in its pure form. The second version, diagram2.py, takes advantage of some Python-specific features to make the code slightly shorter and cleaner. Both versions produce identical output.*

Figure 1.1

Figure 1.1 The plain text and SVG diagrams

We will begin by looking at the code common to both versions, starting with the main() function.

def main():
    ...
    txtDiagram = create_diagram(DiagramFactory())  black-circle_1.jpg
    txtDiagram.save(textFilename)

    svgDiagram = create_diagram(SvgDiagramFactory())  black-circle_2.jpg
    svgDiagram.save(svgFilename)

First we create a couple of filenames (not shown). Next, we create a diagram using the plain text (default) factory (black-circle_1.jpg), which we then save. Then, we create and save the same diagram, only this time using an SVG factory (black-circle_2.jpg).

def create_diagram(factory):
    diagram = factory.make_diagram(30, 7)
    rectangle = factory.make_rectangle(4, 1, 22, 5, "yellow")
    text = factory.make_text(7, 3, "Abstract Factory")
    diagram.add(rectangle)
    diagram.add(text)
    return diagram

This function takes a diagram factory as its sole argument and uses it to create the required diagram. The function doesn’t know or care what kind of factory it receives so long as it supports our diagram factory interface. We will look at the make_...() methods shortly.

Now that we have seen how the factories are used, we can turn to the factories themselves. Here is the plain text diagram factory (which is also the factory base class):

class DiagramFactory:

    def make_diagram(self, width, height):
        return Diagram(width, height)

    def make_rectangle(self, x, y, width, height, fill="white",
            stroke="black"):
        return Rectangle(x, y, width, height, fill, stroke)     

    def make_text(self, x, y, text, fontsize=12):
        return Text(x, y, text, fontsize)

Despite the word “abstract” in the pattern’s name, it is usual for one class to serve both as a base class that provides the interface (i.e., the abstraction), and also as a concrete class in its own right. We have followed that approach here with the DiagramFactory class.

Here are the first few lines of the SVG diagram factory:

class SvgDiagramFactory(DiagramFactory):

    def make_diagram(self, width, height):
        return SvgDiagram(width, height)
    ...

The only difference between the two make_diagram() methods is that the Diagram-Factory.make_diagram() method returns a Diagram object and the SvgDiagramFactory.make_diagram() method returns an SvgDiagram object. This pattern applies to the two other methods in the SvgDiagramFactory (which are not shown).

We will see in a moment that the implementations of the plain text Diagram, Rectangle, and Text classes are radically different from those of the SvgDiagram, SvgRectangle, and SvgText classes—although every class provides the same interface (i.e., both Diagram and SvgDiagram have the same methods). This means that we can’t mix classes from different families (e.g., Rectangle and SvgText)—and this is a constraint automatically applied by the factory classes.

Plain text Diagram objects hold their data as a list of lists of single character strings where the character is a space or +, |, -, and so on. The plain text Rectangle and Text and a list of lists of single character strings that are to replace those in the overall diagram at their position (and working right and down as necessary).

class Text:

    def __init__(self, x, y, text, fontsize):
        self.x = x
        self.y = y
        self.rows = [list(text)]

This is the complete Text class. For plain text we simply discard the fontsize.

class Diagram:
    ...

    def add(self, component):
        for y, row in enumerate(component.rows):
            for x, char in enumerate(row):
                self.diagram[y + component.y][x + component.x] = char

Here is the Diagram.add() method. When we call it with a Rectangle or Text object (the component), this method iterates over all the characters in the component’s list of lists of single character strings (component.rows) and replaces corresponding characters in the diagram. The Diagram.__init__() method (not shown) has already ensured that its self.diagram is a list of lists of space characters (of the given width and height) when Diagram(width, height) is called.

SVG_TEXT = """<text x="{x}" y="{y}" text-anchor="left" \
font-family="sans-serif" font-size="{fontsize}">{text}</text>"""

SVG_SCALE = 20

class SvgText:

    def __init__(self, x, y, text, fontsize):
        x *= SVG_SCALE
        y *= SVG_SCALE
        fontsize *= SVG_SCALE // 10
        self.svg = SVG_TEXT.format(**locals())

This is the complete SvgText class and the two constants it depends on.* Incidentally, using **locals() saves us from having to write SVG_TEXT.format(x=x, y=y, text=text, fontsize=fontsize). From Python 3.2 we could write SVG_TEXT.for-mat_map(locals()) instead, since the str.format_map() method does the mapping unpacking for us. (See the “Sequence and Mapping Unpacking” sidebar, right-arrow.jpg 13.)

class SvgDiagram:
    ...

    def add(self, component):
        self.diagram.append(component.svg)

For the SvgDiagram class, each instance holds a list of strings in self.diagram, each one of which is a piece of SVG text. This makes adding new components (e.g., of type SvgRectangle or SvgText) really easy.

1.1.2. A More Pythonic Abstract Factory

The DiagramFactory and its SvgDiagramFactory subclass, and the classes they make use of (Diagram, SvgDiagram, etc.), work perfectly well and exemplify the design pattern.

Nonetheless, our implementation has some deficiencies. First, neither of the factories needs any state of its own, so we don’t really need to create factory instances. Second, the code for SvgDiagramFactory is almost identical to that of DiagramFactory—the only difference being that it returns SvgText rather than Text instances, and so on—which seems like needless duplication. Third, our top-level namespace contains all of the classes: DiagramFactory, Diagram, Rectangle, Text, and all the SVG equivalents. Yet we only really need to access the two factories. Furthermore, we have been forced to prefix the SVG class names (e.g., using SvgRectangle rather than Rectangle) to avoid name clashes, which is untidy. (One solution for avoiding name conflicts would be to put each class in its own module. However, this approach would not solve the problem of code duplication.)

In this subsection we will address all these deficiencies. (The code is in diagram2.py.)

The first change we will make is to nest the Diagram, Rectangle, and Text classes inside the DiagramFactory class. This means that these classes must now be accessed as DiagramFactory.Diagram and so on. We can also nest the equivalent classes inside the SvgDiagramFactory class, only now we can give them the same names as the plain text classes since a name conflict is no longer possible—for example, SvgDiagramFactory.Diagram. We have also nested the constants the classes depend on, so our only top-level names are now main(), create_diagram(), DiagramFactory, and SvgDiagramFactory.

class DiagramFactory:

    @classmethod
    def make_diagram(Class, width, height):
        return Class.Diagram(width, height)

    @classmethod
    def make_rectangle(Class, x, y, width, height, fill="white",
            stroke="black"):
        return Class.Rectangle(x, y, width, height, fill, stroke)

    @classmethod
    def make_text(Class, x, y, text, fontsize=12):
        return Class.Text(x, y, text, fontsize)

    ...

Here is the start of our new DiagramFactory class. The make_...() methods are now all class methods. This means that when they are called the class is passed as their first argument (rather like self is passed for normal methods). So, in this case a call to DiagramFactory.make_text() will mean that DiagramFactory is passed as the Class, and a DiagramFactory.Text object will be created and returned.

This change also means that the SvgDiagramFactory subclass that inherits from DiagramFactory does not need any of the make_...() methods at all. If we call, say, SvgDiagramFactory.make_rectangle(), since SvgDiagramFactory doesn’t have that method the base class DiagramFactory.make_rectangle() method will be called instead—but the Class passed will be SvgDiagramFactory. This will result in an SvgDiagramFactory.Rectangle object being created and returned.

def main():
    ...
    txtDiagram = create_diagram(DiagramFactory)
    txtDiagram.save(textFilename)

    svgDiagram = create_diagram(SvgDiagramFactory)
    svgDiagram.save(svgFilename)

These changes also mean that we can simplify our main() function since we no longer need to create factory instances.

The rest of the code is almost identical to before, the key difference being that since the constants and non-factory classes are now nested inside the factories, we must access them using the factory name.

class SvgDiagramFactory(DiagramFactory):
    ...
    class Text:

        def __init__(self, x, y, text, fontsize):
            x *= SvgDiagramFactory.SVG_SCALE
            y *= SvgDiagramFactory.SVG_SCALE
            fontsize *= SvgDiagramFactory.SVG_SCALE // 10
            self.svg = SvgDiagramFactory.SVG_TEXT.format(**locals())

Here is the SvgDiagramFactory’s nested Text class (equivalent to diagram1.py’s SvgText class), which shows how the nested constants must be accessed.

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