Powerful Python

Chapter 3. Advanced Functions

In this chapter, we go beyond the basics of using functions. I’ll assume you can write functions with default argument values:

>>> def foo(a, b, x=3, y=2):
...     return (a+b)/(x+y)
...
>>> foo(5, 0)
1.0
>>> foo(10, 2, y=3)
2.0
>>> foo(b=4, x=8, a=1)
0.5

Notice the way foo() is called the last time, with arguments out of order, and everything specified by key-value pairs. Not everyone knows that you can call most Python functions this way. So long as the value of each argument is unambiguously specified, Python doesn’t care how you call the function (and this case, we specify b, x, and a out of order, letting y be its default value). We will leverage this flexibility later.

This chapter’s topics are useful and valuable on their own. And they are important building blocks for some extremely powerful patterns, which you will learn in later chapters. Let’s get started!

Accepting and Passing Variable Arguments

The foo() function above can be called with two, three, or four arguments. Sometimes you want to define a function that can take any number of arguments—zero or more, in other words. In Python, it looks like this:

# Note the asterisk. That's the magic part
def takes_any_args(*args):
    print("Type of args: " + str(type(args)))
    print("Value of args: " + str(args))

Look carefully at the syntax here. takes_any_args() is just like a regular function, except you put an asterisk right before the argument args. Within the function, args is a tuple:

>>> ...