Learning Python - Mark Lutz [258]
Test Your Knowledge: Answers
The output here is '1 2 5', because 1 and 2 are passed to a and b by position, and c is omitted in the call and defaults to 5.
The output this time is '1 2 3': 1 is passed to a by position, and b and c are passed 2 and 3 by name (the left-to-right order doesn’t matter when keyword arguments are used like this).
This code prints '1 (2, 3)', because 1 is passed to a and the *pargs collects the remaining positional arguments into a new tuple object. We can step through the extra positional arguments tuple with any iteration tool (e.g., for arg in pargs: ...).
This time the code prints '1, {'c': 3, 'b': 2}', because 1 is passed to a by name and the **kargs collects the remaining keyword arguments into a dictionary. We could step through the extra keyword arguments dictionary by key with any iteration tool (e.g., for key in kargs: ...).
The output here is '1 5 6 4': 1 matches a by position, 5 and 6 match b and c by *name positionals (6 overrides c’s default), and d defaults to 4 because it was not passed a value.
Functions can send back results with return statements, by changing passed-in mutable arguments, and by setting global variables. Globals are generally frowned upon (except for very special cases, like multithreaded programs) because they can make code more difficult to understand and use. return statements are usually best, but changing mutables is fine, if expected. Functions may also communicate with system devices such as files and sockets, but these are beyond our scope here.
Chapter 19. Advanced Function Topics
This chapter introduces a collection of more advanced function-related topics: recursive functions, function attributes and annotations, the lambda expression, and functional programming tools such as map and filter. These are all somewhat advanced tools that, depending on your job description, you may not encounter on a regular basis. Because of their roles in some domains, though, a basic understanding can be useful; lambdas, for instance, are regular customers in GUIs.
Part of the art of using functions lies in the interfaces between them, so we will also explore some general function design principles here. The next chapter continues this advanced theme with an exploration of generator functions and expressions and a revival of list comprehensions in the context of the functional tools we will study here.
Function Design Concepts
Now that we’ve had a chance to study function basics in Python, let’s begin this chapter with a few words of context. When you start using functions in earnest, you’re faced with choices about how to glue components together—for instance, how to decompose a task into purposeful functions (known as cohesion), how your functions should communicate (called coupling), and so on. You also need to take into account concepts such as the size of your functions, because they directly impact code usability. Some of this falls into the category of structured analysis and design, but it applies to Python code as to any other.
We introduced some ideas related to function and module coupling in the Chapter 17 when studying scopes, but here is a review of a few general guidelines for function beginners:
Coupling: use arguments for inputs and return for outputs. Generally, you should strive to make a function independent of things outside of it. Arguments and return statements are often the best ways to isolate external dependencies to a small number of well-known places in your code.
Coupling: use global variables only when truly necessary. Global variables (i.e., names in the enclosing module) are usually a poor way for functions to communicate. They can create dependencies and timing issues that make programs difficult to debug and change.
Coupling: don’t change mutable arguments unless the caller expects it. Functions can change parts of passed-in mutable objects, but (as with global variables) this creates lots of coupling between the caller and callee, which can make a function