.. Objectives

.. ----------

.. At the end of this tutorial, you will be able to 

.. 1. Assign default values to arguments, when defining functions

.. 2. Define and call functions with keyword arguments. 

.. 3. Also, you will get a glimpse of the plethora of functions

.. available, in Python standard library and the scientific computing

.. libraries. 

.. Prerequisites

.. -------------

..   1. getting started with ipython

..   #. getting started with functions

.. Author              : Puneeth 

   Internal Reviewer   : Anoop Jacob Thomas<anoop@fossee.in>

   External Reviewer   :

   Checklist OK?       : <put date stamp here, if OK> [2010-10-05]

Script

------

{{{ Show the slide containing title }}}

Welcome to the tutorial on advanced feature of functions. 

{{{ Show the outline slide }}}

In this tutorial we shall be looking at specifying default arguments

to functions when defining them and calling functions using keyword

arguments. We shall also, look at some of the built-in functions

available in the standard library of Python and the scientific

computing libraries. 

{{{ switch to terminal }}}

We have an ``ipython`` terminal open, which we shall be using through

out this session. 

Let's use the ``round`` function as an example to understand what a

default value of an argument means. Let's type the following

expressions in the terminal. 

::

  round(2.484)

  round(2.484, 2)

Both the first expression and the second are calls to the ``round``

function, but the first calls it with only one argument and the second

calls it with two arguments. By observing the output, we can guess

that the first one is equivalent to call with the second argument

being 0. 0 is the default value of the argument. 

.. #[[Anoop: It will be good if we show ``round??`` and tell them the

   optional argument ndigits, or it could be given as an

   exercise(solved) asking them to find the name of the argument in

   the function round]]

{{{ show a slide with examples of functions showing default values }}}

.. #[[Anoop: I think the slide is not there]]

::

  s.strip() # strips on spaces. 

  s.strip('@') # strips the string of '@' symbols.

  plot(x, y) # plots with x vs. y using default line style. 

  plot(x, y, 'o') # plots x vs. y with circle markers. 

  linspace(0, 2*pi, 100) # returns 100 points between 0 and 2pi

  linspace(0, 2*pi) # returns 50 points between 0 and 2pi

.. #[punch: all above content goes on to a slide]

{{{ switch back to ipython }}}

Let's now define a simple function that uses default arguments. We

define a simple function that prints a welcome message to a person,

given a greeting and his/her name.

::

  def welcome(greet, name="World"):

      print greet, name

Let us first call the function with two arguments, one for ``greet``

and other for ``name``.

::

  welcome("Hi", "Guido")          

We get the expected welcome message, "Hi Guido". 

Now let us call the function with just one argument "Hello". 

::

  welcome("Hello")

"Hello" is treated as the ``greet`` and we get "Hello World" as

the output. "World" is the default value for the argument ``name``. 

Following is an (are) exercise(s) that you must do. 

{{{ switch to next slide, containing problem statement of 

    question 1 }}}

%%1%% Redefine the function ``welcome``, by interchanging it's

arguments. Place the ``name`` argument with it's default value of

"World" before the ``greet`` argument.

Please, pause the video here. Do the exercise and then continue. 

{{{ switch to next slide, containing the solution to problem 1 }}}

::

  def welcome(name="World", greet):

      print greet, name

We get an error that reads ``SyntaxError: non-default argument follows

default argument``. When defining a function all the argument with

default values should come at the end. 

.. #[[Anoop: In the slide, "when defining a function all the default

   arguments must be defined at the end" has to be emphasized"]]

Following is an exercise that you must do. 

{{{ switch to next slide, containing the problem statement of 

    question 2 }}}

%%2%% See the definition of linspace using ``?`` and observe how all

the arguments with default values are towards the end.

Please, pause the video here. Do the exercise and then continue. 

{{{ switch to next slide, containing solution to problem 2 }}}

::

  linspace?

Following is an exercise that you must do. 

{{{ switch to next slide, problem statement }}}

%%3%% Redefine the function ``welcome`` with a default value of

"Hello" to the ``greet`` argument. Then, call the function without any

arguments. 

Please, pause the video here. Do the exercise and then continue. 

{{{ switch to next slide, solution }}}

::

  def welcome(greet="Hello", name="World"):

      print greet, name

  welcome()

Let us now learn what keyword arguments or named arguments are. We

shall refer to them as keyword arguments, henceforth. 

{{{ show a slide with examples using keyword arguments. }}}

.. #[[Anoop: slide is missing]]

::

  legend(['sin(2y)'], loc = 'center')

  plot(y, sin(y), 'g', linewidth = 2)

  annotate('local max', xy = (1.5, 1))

  pie(science.values(), labels = science.keys())

When you are calling functions in Python, you don't need to remember

the order in which to pass the arguments. Instead, you can use the

name of the argument to pass it a value. This slide shows a few

function calls that use keyword arguments. ``loc``, ``linewidth``,

``xy`` and ``labels`` are being called with keyword arguments. 

{{{ switch to ipython terminal }}}

Let us try and understand this better using the ``welcome`` function

that we have been using all along. Let us call it in different ways

and observe the output to see how keyword arguments work. 

::

  welcome()

  welcome("Hello", "James")

  welcome("Hi", name="Guido")

When no keyword is specified, the arguments are allotted based on

their position. So, "Hi" is the value of the argument ``greet`` and

name is passed the value "Guido". 

::

  welcome(name="Guido", greet="Hey! ")

When keyword arguments are used, the arguments can be called in any

order. 

::

  welcome(name="Guido", "Hey")

This call returns an error that reads, ``non keyword arg after keyword

arg``. Python expects all the keyword to be present towards the end. 

That brings us to the end of what we wanted to learn about ``keyword``

arguments. 

{{{ switch to a slide showing variety of functions with uses }}}

.. #[[Anoop: slide missing]]

Before defining a function of your own, make sure that you check the

standard library, for a similar function. Python is popularly called a

"Batteries included" language, for the huge library that comes along

with it. 

::

  Math functions - abs, sin, ....

.. #[punch: Need to decide, exactly what to put here. Reviewer comments

..  welcome.] 

{{{ switch to slide showing classes of functions in pylab, scipy }}}

.. #[[Anoop: slide missing]]

Apart from the standard library there are other libraries like ``pylab``,

``scipy``, etc which have a huge collection of functions for scientific

purposes. 

::

  pylab

    plot, bar, contour, boxplot, errorbar, log, polar, quiver, semilog

  scipy (modules)

    fftpack, stats, linalg, ndimage, signal, optimize, integrate

{{{ Show summary slide }}}

.. #[[Anoop: add range of functions available in python standard

   library]]

That brings us to the end of this tutorial. In this tutorial we have

learnt how to use functions with default values and keyword

arguments. We also looked at the range of functions available in the

Python standard library and the Scientific Computing related

packages. 

{{{ Show the "sponsored by FOSSEE" slide }}}

This tutorial was created as a part of FOSSEE project, NME ICT, MHRD India

Hope you have enjoyed and found it useful.

Thank you!