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Functions, or subroutines, are a very common form of reusable code, so let's look

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at how functions are done in Python.

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We'll make a working copy of syntax.py.

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We'll call this syntax-functions.py.

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Go ahead and open our working copy.

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We'll notice, of course, that main here is a function.

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And the purpose of that is not so much reusability as it is modularity in

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allowing us to create more functions, and be able to run them from main.

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So let's go ahead and create a function here.

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And I'll just call it func.

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And we'll print out a range of numbers.

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And in my print statement I am going to say end= ' '.

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And that way instead of putting each number on a separate line, we'll put them

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all in the same line, separated by spaces.

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And I'll put another print after it, an empty one to print a new line.

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And so I'll save that, and I'll call the function from in main.

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And we'll just call it instead of this print function here, we'll just say func.

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And that's all we need to do to call our function.

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So we'll save it and run.

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And we see we are printing out this sequence of 10 numbers.

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And this is, in fact, how the range function works.

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It prints out everything not including the number of the range.

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So it's 10 numbers staring at 0.

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Now if I wanted to call the function again, I would just say func again, and I can

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do that several times.

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And each time it will run this same block of code.

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So I save it and I run it, and that's how a function works.

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So let's look at how we define the function.

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The def keyword is for defining the function.

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That means that what comes next is going to be the name of the function, so

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there is the name, and then there is a set of parentheses where we would put

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arguments, or parameters.

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And the colon says that this is going to be followed by a block of code, or a

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suite of codes, as it's called in Python.

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And then we have the suite of code itself.

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The suite of code is indented under the definition of the function, and in this

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case of course, we're using the traditional four spaces of indenting, like that.

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Inside the parentheses we could put arguments.

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So for example, if I put an argument called a there, then I could say a, 10.

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So it would start at the number a in its range here, and so here I could use a 1,

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and here I could use a 3, and here I could use a 5.

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And it would use that as an argument to the range function.

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What that does is it defines a starting point for the range.

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And so if I save that and run it, then we see we have one that starts with 1, one

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that starts with 3, and one that starts with 5.

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And so this argument in the parentheses is used inside the function in this way.

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We can give the argument a default value by saying a=0.

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Well, let's give it something different so we can see. Let's say a=7.

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And if in this middle one I don't put an argument, then that will be the default

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argument that's used.

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It'll default to 7.

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So if I save this and run it, you see that the middle one starts at 7.

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So there are a lot of other things we can do with functions and with function

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arguments, and we'll cover those later in our chapter on functions.

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But this is, in a nutshell, how you define a function in Python, and how you

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call and use that function, and how you can use arguments when you call and

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use that function.