Learning Python, 6th Edition

Chapter 18. Arguments

The preceding chapter explored Python’s scopes—the places where variables are defined and looked up. As we saw, the place where a name is defined in our code determines much of its meaning. This chapter continues the function story by studying the concepts in Python argument passing—the way that objects are sent to functions as inputs. As you’ll see, arguments (a.k.a. parameters) are assigned to names in a function, but have more to do with object references than with variable scopes. You’ll also find that Python provides extra tools, such as keywords, defaults, and argument collectors and extractors, that allow arguments to be sent to functions flexibly.

Argument-Passing Basics

Earlier in this part of the book, we learned that def and lambda are function definitions, and both include argument-list headers that name variables which receive values passed by calls. These arguments are used in function bodies, and may be matched between call and header by position, name, and other means we’ll explore later in this chapter.

More fundamentally, though, it was also noted that all Python arguments are passed by assignment—which means object reference. This has some subtle ramifications that aren’t always obvious to newcomers. Let’s start our arguments adventure by exploring how this works. Here is a rundown of the key points in this model:

• Arguments are passed by automatically assigning objects to local variable names. Function arguments are just another instance ...