Cover by Mark Lutz

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Chapter 18. Arguments

Chapter 17 explored the details behind Python’s scopes—the places where variables are defined and looked up. As we learned, 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 we’ll see, arguments (a.k.a. parameters) are assigned to names in a function, but they have more to do with object references than with variable scopes. We’ll also find that Python provides extra tools, such as keywords, defaults, and arbitrary argument collectors, that allow for wide flexibility in the way arguments are sent to a function.

Argument-Passing Basics

Earlier in this part of the book, I noted that arguments are passed by assignment. This has a few ramifications that aren’t always obvious to beginners, which I’ll expand on in this section. Here is a rundown of the key points in passing arguments to functions:

  • Arguments are passed by automatically assigning objects to local variable names. Function arguments—references to (possibly) shared objects sent by the caller—are just another instance of Python assignment at work. Because references are implemented as pointers, all arguments are, in effect, passed by pointer. Objects passed as arguments are never automatically copied.

  • Assigning to argument names inside a function does not affect the caller. Argument names in the function header become new, local names ...

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