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{{{ show the welcome slide }}}

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.

{{{ 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. 

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

::

  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``. 

E%% %% Pause the video here and redefine the function ``welcome``, by

interchanging it's arguments. Place the ``name`` argument with it's

default value of "Hello" before the ``greet`` argument. Then, resume

the video. 

::

  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. 

E%% %% Pause the video here and type ``linspace?`` to see the

definition of the command and notice how all the arguments with

default values are towards the end.

::

  linspace?

E%% %% Pause the video here and redefine the function ``welcome`` with

a default value of "Hello" to the ``greet`` argument. Then, call the

function without any arguments. Then, resume the video. 

::

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

      print greet, name

  welcome()

Let us now learn what keyword arguments are. 

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

::

  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 }}}

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 }}}

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

{{{ switch slide to summary slide }}}

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. 

Thank You!