advanced-features-functions/script.rst
author Puneeth Chaganti <punchagan@fossee.in>
Mon, 11 Oct 2010 00:57:10 +0530
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Slides for advanced features of functions LO.
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.. Objectives
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.. ----------
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.. At the end of this tutorial, you will be able to 
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.. 1. Assign default values to arguments, when defining functions
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.. 2. Define and call functions with keyword arguments. 
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.. 3. Also, you will get a glimpse of the plethora of functions
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.. available, in Python standard library and the scientific computing
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.. libraries. 
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.. Prerequisites
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.. -------------
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..   1. getting started with ipython
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..   #. getting started with functions
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.. Author              : Puneeth 
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   Internal Reviewer   : 
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   External Reviewer   :
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   Checklist OK?       : <put date stamp here, if OK> [2010-10-05]
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Script
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------
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{{{ Show the slide containing title }}}
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Welcome to the tutorial on advanced feature of functions. 
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{{{ Show the outline slide }}}
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In this tutorial we shall be looking at specifying default arguments
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to functions when defining them and calling functions using keyword
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arguments. We shall also, look at some of the built-in functions
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available in the standard library of Python and the scientific
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computing libraries. 
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{{{ switch to terminal }}}
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We have an ``ipython`` terminal open, which we shall be using through
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out this session. 
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Let's use the ``round`` function as an example to understand what a
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default value of an argument means. Let's type the following
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expressions in the terminal. 
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::
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  round(2.484)
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  round(2.484, 2)
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Both the first expression and the second are calls to the ``round``
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function, but the first calls it with only one argument and the second
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calls it with two arguments. By observing the output, we can guess
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that the first one is equivalent to call with the second argument
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being 0. 0 is the default value of the argument. 
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{{{ show a slide with examples of functions showing default values }}}
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::
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  s.strip() # strips on spaces. 
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  s.strip('@') # strips the string of '@' symbols.
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  plot(x, y) # plots with x vs. y using default line style. 
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  plot(x, y, 'o') # plots x vs. y with circle markers. 
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  linspace(0, 2*pi, 100) # returns 100 points between 0 and 2pi
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  linspace(0, 2*pi) # returns 50 points between 0 and 2pi
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.. #[punch: all above content goes on to a slide]
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{{{ switch back to ipython }}}
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Let's now define a simple function that uses default arguments. We
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define a simple function that prints a welcome message to a person,
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given a greeting and his/her name.
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::
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  def welcome(greet, name="World"):
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      print greet, name
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Let us first call the function with two arguments, one for ``greet``
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and other for ``name``.
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  welcome("Hi", "Guido")          
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We get the expected welcome message, "Hi Guido". 
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Now let us call the function with just one argument "Hello". 
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  welcome("Hello")
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"Hello" is treated as the ``greet`` and we get "Hello World" as
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the output. "World" is the default value for the argument ``name``. 
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Following is an (are) exercise(s) that you must do. 
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%%1%% Redefine the function ``welcome``, by interchanging it's
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arguments. Place the ``name`` argument with it's default value of
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"World" before the ``greet`` argument.
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Please, pause the video here. Do the exercise and then continue. 
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::
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  def welcome(name="World", greet):
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      print greet, name
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We get an error that reads ``SyntaxError: non-default argument follows
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default argument``. When defining a function all the argument with
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default values should come at the end. 
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Following is an exercise that you must do. 
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%%2%% See the definition of linspace using ``?`` and observe how all
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the arguments with default values are towards the end.
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Please, pause the video here. Do the exercise and then continue. 
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::
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  linspace?
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Following is an exercise that you must do. 
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%%3%% Redefine the function ``welcome`` with a default value of
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"Hello" to the ``greet`` argument. Then, call the function without any
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arguments. 
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Please, pause the video here. Do the exercise and then continue. 
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::
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  def welcome(greet="Hello", name="World"):
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      print greet, name
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  welcome()
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Let us now learn what keyword arguments are. 
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{{{ show a slide with examples using keyword arguments. }}}
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::
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  legend(['sin(2y)'], loc = 'center')
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  plot(y, sin(y), 'g', linewidth = 2)
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  annotate('local max', xy = (1.5, 1))
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  pie(science.values(), labels = science.keys())
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When you are calling functions in Python, you don't need to remember
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the order in which to pass the arguments. Instead, you can use the
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name of the argument to pass it a value. This slide shows a few
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function calls that use keyword arguments. ``loc``, ``linewidth``,
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``xy`` and ``labels`` are being called with keyword arguments. 
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{{{ switch to ipython terminal }}}
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Let us try and understand this better using the ``welcome`` function
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that we have been using all along. Let us call it in different ways
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and observe the output to see how keyword arguments work. 
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::
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  welcome()
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  welcome("Hello", "James")
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  welcome("Hi", name="Guido")
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When no keyword is specified, the arguments are allotted based on
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their position. So, "Hi" is the value of the argument ``greet`` and
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name is passed the value "Guido". 
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::
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  welcome(name="Guido", greet="Hey! ")
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When keyword arguments are used, the arguments can be called in any
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order. 
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::
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  welcome(name="Guido", "Hey")
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This call returns an error that reads, ``non keyword arg after keyword
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arg``. Python expects all the keyword to be present towards the end. 
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That brings us to the end of what we wanted to learn about ``keyword``
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arguments. 
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{{{ switch to a slide showing variety of functions with uses }}}
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Before defining a function of your own, make sure that you check the
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standard library, for a similar function. Python is popularly called a
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"Batteries included" language, for the huge library that comes along
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with it. 
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::
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  Math functions - abs, sin, ....
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.. #[punch: Need to decide, exactly what to put here. Reviewer comments
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..  welcome.] 
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{{{ switch to slide showing classes of functions in pylab, scipy }}}
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Apart from the standard library there are other libraries like ``pylab``,
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``scipy``, etc which have a huge collection of functions for scientific
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purposes. 
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::
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  pylab
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    plot, bar, contour, boxplot, errorbar, log, polar, quiver, semilog
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  scipy (modules)
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    fftpack, stats, linalg, ndimage, signal, optimize, integrate
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{{{ Show summary slide }}}
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That brings us to the end of this tutorial. In this tutorial we have
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learnt how to use functions with default values and keyword
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arguments. We also looked at the range of functions available in the
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Python standard library and the Scientific Computing related
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packages. 
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{{{ Show the "sponsored by FOSSEE" slide }}}
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This tutorial was created as a part of FOSSEE project, NME ICT, MHRD India
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Hope you have enjoyed and found it useful.
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Thank you!