Python-Old-Version/C4/Functions/English
Welcome friends.
In this tutorial we shall be looking at Functions in Python. We already have looked at the basics of functions in the tutorial on solving equations. We shall first review these basics. Then we shall move on to other details such as doc-strings, default arguments and keyword arguments.
First let's start IPython by typing ipython in the terminal.
Let's write a simple function that prints a Hello message, upon accepting a name.
def welcome(name):
print "Hello", name
You would recall that def is a keyword that indicates the function definition. 'welcome' is the name of the function and 'name' is the lone argument to the function. Note that the function is defined within an indented block, just like to any other block. Our function welcome just has one line in it's definition.
We can call our function, as follows -
welcome("World")
(all arguments are local to a function)
In general functions should be accompanied by documentation on how to use them. Python provides a convenient way of writing this within the function itself, using what are called doc strings. They were mentioned in the tutorial on strings. Let's look at how to write them here.
Let us add a simple doc string to our welcome function.
def welcome(name):
""" Prints a hello message given a name,
passed as argument. """
print "Hello", name
Notice that the doc string uses triple quotes. If the doc-string exceeds one line, we can use new line characters in it. Also, as expected the doc-string is indented as is required for anything within a block. Now that we have written the documentation, how do we access it? IPython provides the question mark feature that we have seen in the previous tutorials. welcome? will display the docstring that we have just written.
We shall now look at default arguments. [show slide with examples of functions with default arguments] The split function has been used in two different ways in the given example - one for splitting on spaces and the other for splitting on commas.
The function split is being called with no arguments and one argument, respectively. In the first case, white space is being used as a default value. Let's now edit our function, welcome, to use default values. (For convenience sake, we have dropped the doc-string)
def welcome(name="World!"):
print "Hello", name
Now, we call the function 'welcome' without passing any arguments to it.
welcome()
As you can see the output is "Hello World!". Here "World!" is used as a default argument, when no name argument is passed to 'welcome'.
Let's now look at the use of keyword arguments. [show slide with examples of functions with keyword arguments] We have already looked at functions and keyword arguments in these examples. loc, linewidth, xy, labels are all keywords.
Let's now edit our function so that it displays a custom greeting message as well.
def welcome( greet = 'Hello', name = 'World!'):
print greet, name
Let's now see, how we can call our updated 'welcome' function, using keyword arguments. We can call the function in a variety of ways.
welcome("Hello", "James")
welcome("Hi", name="Guido")
welcome(name="Guido", greet="Hello")
Keyword arguments allow us to call functions by passing arguments in any order and removes the need to remember the order of arguments in the function definition.
Let's now write a new function
def per_square(n):
i = 1
while ( i*i < n ):
i += 1
return i*i == n, i
What does this function do? It checks if the given number is a perfect square. If it is, then the function returns True along with the square root of the given number. If the number is not a perfect square it returns False and the square root of the next perfect square.
Please observe that this function returns 2 values. In Python there is no restriction on the number of values returned by a function. Whenever a function has to return more than one value, the multiple values are packed into one single tuple and that single tuple is returned.
With this we come to the end of this tutorial on functions. In this tutorial we have learnt about functions in a greater detail. We looked at how to define functions, calling them, default and keyword arguments.
