{"id":1150,"date":"2010-12-12T14:19:34","date_gmt":"2010-12-12T19:19:34","guid":{"rendered":"http:\/\/cemclinux1.math.uwaterloo.ca\/~cscircles\/wordpress\/"},"modified":"2010-12-12T14:19:34","modified_gmt":"2010-12-12T19:19:34","slug":"10-def","status":"publish","type":"page","link":"https:\/\/olescs.hkmu.edu.hk\/python\/10-def\/","title":{"rendered":"10: def"},"content":{"rendered":"\n

In this lesson we show you the most important idea in programming: defining your own functions! Functions let your program become shorter, better-organized, easier to read, easier to debug, and more reusable. In later lessons we will see other benefits like recursion<\/em>.<\/p>\n

Defining a Function<\/h1>\n

We'll start with an example. Remember the function abs<\/code>? It takes one argument (a number x<\/em>), and returns its absolute value (which is x<\/em> when x <\/em>\u2265 0, and -x<\/em> when x<\/em> < 0). The way to define this function in Python is:<\/p>\n

def abs(x):     # define a function named 'abs' of one argument named 'x'\n  if x >= 0:    # function body starts here\n    return x\n  else:\n    return -x   # function body ends here<\/pre>(Click here<\/a> to test this code in the console.)<\/p>\n
The Two Parts of a Function Definition<\/h2>\n
The first line in a function definition specifies the name<\/em> of the function and the arguments<\/em> of the function. It should look like<\/p>\n
def \u00abfunction-name\u00bb(\u00abargument-list\u00bb):<\/code><\/p>\n
In the example above, abs<\/code> was the function name, and the argument list was x<\/code>. It is called an argument list<\/em> because there can be multiple arguments such as x, y, z<\/code>; it's also possible to have zero arguments, i.e. an empty list. The arguments are the input(s) to the function.<\/strong><\/p>\n
Everything after the first line of a function definition is the body<\/em> of the function. The body is an indented block of code. Whenever the function is called, the body is executed on those arguments\/inputs. Finally, when we reach this line in the function body,<\/p>\n
return \u00abvalue\u00bb<\/code><\/p>\n
the function stops running and returns \u00abvalue\u00bb<\/code> as its output. That is, <\/strong>the return value is the output of the function.<\/strong>\u00a0As the abs<\/code> example shows, the body may contain several return<\/code> statements; but only the first one executed has an effect since the function stops executing afterwards.<\/p>\n
Example: Defining and Calling a Function<\/h2>\n
The following program contains a function definition and some other statements that are executed after the function is defined. Can you guess what the output will be?<\/p>\n