Creational Design Patterns in Python
- 1.1. Abstract Factory Pattern
- 1.2. Builder Pattern
- 1.3. Factory Method Pattern
- 1.4. Prototype Pattern
- 1.5. Singleton Pattern
This chapter is from the book
This chapter is from the book
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
的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, andWindowsWidgetFactory, 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 genericcreate_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 inFigure 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 1.1The plain text and SVG diagrams
We will begin by looking at the code common to both versions, starting with themain()function.
defmain(): ... txtDiagram = create_diagram(DiagramFactory())txtDiagram.save(textFilename) svgDiagram = create_diagram(SvgDiagramFactory())
svgDiagram.save(svgFilename)
First we create a couple of filenames (not shown). Next, we create a diagram using the plain text (default) factory (
), which we then save. Then, we create and save the same diagram, only this time using an SVG factory (
).
defcreate_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)returndiagram
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 themake_...()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):
classDiagramFactory:defmake_diagram(self, width, height):returnDiagram(width, height)defmake_rectangle(self, x, y, width, height, fill="white", stroke="black"):returnRectangle(x, y, width, height, fill, stroke)defmake_text(self, x, y, text, fontsize=12):returnText(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 theDiagramFactoryclass.
Here are the first few lines of the SVG diagram factory:
classSvgDiagramFactory(DiagramFactory):defmake_diagram(self, width, height):returnSvgDiagram(width, height) ...
The only difference between the twomake_diagram()methods is that theDiagram-Factory.make_diagram()method returns aDiagramobject and theSvgDiagramFactory.make_diagram()method returns anSvgDiagramobject. This pattern applies to the two other methods in theSvgDiagramFactory(which are not shown).
We will see in a moment that the implementations of the plain textDiagram,Rectangle, andTextclasses are radically different from those of theSvgDiagram,SvgRectangle, andSvgTextclasses—although every class provides the same interface (i.e., bothDiagramandSvgDiagramhave the same methods). This means that we can’t mix classes from different families (e.g.,RectangleandSvgText)—and this is a constraint automatically applied by the factory classes.
Plain textDiagramobjects hold their data as a list of lists of single character strings where the character is a space or+,|,-, and so on. The plain textRectangleandTextand 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).
classText:def__init__(自我,x, y,文本,佛ntsize): self.x = x self.y = y self.rows = [list(text)]
This is the completeTextclass. For plain text we simply discard thefontsize.
classDiagram: ...defadd(self, component):fory, rowin enumerate(component.rows):forx, charin enumerate(row): self.diagram[y + component.y][x + component.x] = char
Here is theDiagram.add()method. When we call it with aRectangleorTextobject (thecomponent), 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. TheDiagram.__init__()method (not shown) has already ensured that itsself.diagram空格字符列表的列表(投入en width and height) whenDiagram(width, height)is called.
SVG_TEXT ="""font-family="sans-serif" font-size="{fontsize}">{text}"""SVG_SCALE =20classSvgText:def__init__(自我,x, y,文本,佛ntsize): x *= SVG_SCALE y *= SVG_SCALE fontsize *= SVG_SCALE //10self.svg = SVG_TEXT.format(**locals())
This is the completeSvgTextclass and the two constants it depends on.*Incidentally, using**locals()saves us from having to writeSVG_TEXT。格式(x = x, y = y =文本,文本字体字形大小=size). From Python 3.2 we could writeSVG_TEXT。for-mat_map(locals())instead, since thestr.format_map()method does the mapping unpacking for us. (See the “Sequence and Mapping Unpacking” sidebar,
13.)
classSvgDiagram: ...defadd(self, component): self.diagram.append(component.svg)
For theSvgDiagramclass, each instance holds a list of strings inself.diagram, each one of which is a piece of SVG text. This makes adding new components (e.g., of typeSvgRectangleorSvgText) really easy.
1.1.2. A More Pythonic Abstract Factory
TheDiagramFactoryand itsSvgDiagramFactorysubclass, 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 forSvgDiagramFactoryis almost identical to that ofDiagramFactory—the only difference being that it returnsSvgTextrather thanTextinstances, 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., usingSvgRectanglerather thanRectangle)为了避免名称冲突,这是不整洁了。(一个溶胶ution 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 indiagram2.py.)
The first change we will make is to nest theDiagram,Rectangle, andTextclasses inside theDiagramFactoryclass. This means that these classes must now be accessed asDiagramFactory.Diagramand so on. We can also nest the equivalent classes inside theSvgDiagramFactoryclass, 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 nowmain(),create_diagram(),DiagramFactory, andSvgDiagramFactory.
classDiagramFactory: @classmethoddefmake_diagram(Class, width, height):returnClass.Diagram(width, height) @classmethoddefmake_rectangle(Class, x, y, width, height, fill="white", stroke="black"):returnClass.Rectangle(x, y, width, height, fill, stroke) @classmethoddefmake_text(Class, x, y, text, fontsize=12):returnClass.Text(x, y, text, fontsize) ...
Here is the start of our newDiagramFactoryclass. Themake_...()methods are now all class methods. This means that when they are called the class is passed as their first argument (rather likeselfis passed for normal methods). So, in this case a call toDiagramFactory.make_text()will mean thatDiagramFactoryis passed as theClass, and aDiagramFactory.Textobject will be created and returned.
This change also means that theSvgDiagramFactorysubclass that inherits fromDiagramFactorydoes not need any of themake_...()methods at all. If we call, say,SvgDiagramFactory.make_rectangle(), sinceSvgDiagramFactorydoesn’t have that method the base classDiagramFactory.make_rectangle()method will be called instead—but theClasspassed will beSvgDiagramFactory. This will result in anSvgDiagramFactory.Rectangleobject being created and returned.
defmain(): ... txtDiagram = create_diagram(DiagramFactory) txtDiagram.save(textFilename) svgDiagram = create_diagram(SvgDiagramFactory) svgDiagram.save(svgFilename)
These changes also mean that we can simplify ourmain()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.
classSvgDiagramFactory(DiagramFactory): ...classText:def__init__(自我,x, y,文本,佛ntsize): x *= SvgDiagramFactory.SVG_SCALE y *= SvgDiagramFactory.SVG_SCALE fontsize *= SvgDiagramFactory.SVG_SCALE //10self.svg = SvgDiagramFactory.SVG_TEXT.format(**locals())
Here is theSvgDiagramFactory’s nestedText类(相当于diagram1.py’sSvgTextclass), which shows how the nested constants must be accessed.