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Why Factories?

So what good is the factory function (besides providing an excuse to illustrate class objects in this book)? Unfortunately, it’s difficult to show applications of this design pattern without listing much more code than we have space for here. In general, though, such a factory might allow code to be insulated from the details of dynamically configured object construction.

For instance, recall the processor example presented in the abstract in Chapter 25, and then again as a composition example in this chapter. It accepts reader and writer objects for processing arbitrary data streams. The original version of this example manually passed in instances of specialized classes like FileWriter and SocketReader to customize the data streams being processed; later, we passed in hardcoded file, stream, and formatter objects. In a more dynamic scenario, external devices such as configuration files or GUIs might be used to configure the streams.

In such a dynamic world, we might not be able to hardcode the creation of stream interface objects in our scripts, but might instead create them at runtime according to the contents of a configuration file.

For example, the file might simply give the string name of a stream class to be imported from a module, plus an optional constructor call argument. Factory-style functions or code might come in handy here because they would allow us to fetch and pass in classes that are not hardcoded in our program ahead of time. Indeed, those classes might not even have existed at all when we wrote our code:

classname = ...parse from config file...
classarg  = ...parse from config file...

import streamtypes                           # Customizable code
aclass = getattr(streamtypes, classname)     # Fetch from module
reader = factory(aclass, classarg)           # Or aclass(classarg)
processor(reader, ...)

Here, the getattr built-in is again used to fetch a module attribute given a string name (it’s like saying obj.attr, but attr is a string). Because this code snippet assumes a single constructor argument, it doesn’t strictly need factory or apply—we could make an instance with just aclass(classarg). They may prove more useful in the presence of unknown argument lists, however, and the general factory coding pattern can improve the code’s flexibility.

[71] Actually, this syntax can invoke any callable object, including functions, classes, and methods. Hence, the factory function here can also run any callable object, not just a class (despite the argument name). Also, as we learned in Chapter 18, Python 2.6 has an alternative to aClass(*args): the apply(aClass, args) built-in call, which has been removed in Python 3.0 because of its redundancy and limitations.